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The Nuts and Bolts of Implantable Device Therapy Pacemakers

The Nuts and Bolts of Implantable Device TherapyPacemakersTom Kenny FHRS CCDSDirector of Education PrepMD Braintree MA USA

This edition first published 2015 copy 2015 by John Wiley amp Sons Ltd

Registered Office John Wiley amp Sons Ltd The Atrium Southern Gate Chichester West Sussex PO19 8SQ UK

Editorial Offices 9600 Garsington Road Oxford OX4 2DQ UK111 River Street Hoboken NJ 07030-5774 USA

For details of our global editorial offices for customer services and for information about how to apply for permission to reuse the copyright material in this book please see our website at wwwwileycomwiley-blackwell

The right of the author to be identified as the author of this work has been asserted in accordance with the UK Copyright Designs and Patents Act 1988

All rights reserved No part of this publication may be reproduced stored in a retrieval system or transmitted in any form or by any means electronic mechanical photocopying recording or otherwise except as permitted by the UK Copyright Designs and Patents Act 1988 without the prior permission of the publisher

Designations used by companies to distinguish their products are often claimed as trademarks All brand names and product names used in this book are trade names service marks trademarks or registered trademarks of their respective owners The publisher is not associated with any product or vendor mentioned in this book It is sold on the understanding that the publisher is not engaged in rendering professional services If professional advice or other expert assistance is required the services of a competent professional should be sought

The contents of this work are intended to further general scientific research understanding and discussion only and are not intended and should not be relied upon as recommending or promoting a specific method diagnosis or treatment by health science practitioners for any particular patient The publisher and the author make no representations or warranties with respect to the accuracy or completeness of the contents of this work and specifically disclaim all warranties including without limitation any implied warranties of fitness for a particular purpose In view of ongoing research equipment modifications changes in governmental regulations and the constant flow of information relating to the use of medicines equipment and devices the reader is urged to review and evaluate the information provided in the package insert or instructions for each medicine equipment or device for among other things any changes in the instructions or indication of usage and for added warnings and precautions Readers should consult with a specialist where appropriate The fact that an organization or Website is referred to in this work as a citation andor a potential source of further information does not mean that the author or the publisher endorses the information the organization or Website may provide or recommendations it may make Further readers should be aware that Internet Websites listed in this work may have changed or disappeared between when this work was written and when it is read No warranty may be created or extended by any promotional statements for this work Neither the publisher nor the author shall be liable for any damages arising herefrom

Library of Congress Cataloging-in-Publication Data

Kenny Tom 1954ndash author The nuts and bolts of implantable device therapy pacemakers Tom Kenny p cm Includes index ISBN 978-1-118-67067-5 (pbk) I Title [DNLM 1 Cardiac Pacing Artificialndashmethods 2 Pacemaker Artificial WG 168] RC684P3 6174prime120645ndashdc23

2014027299

A catalogue record for this book is available from the British Library

Cover images copy iStockcomangelhel copy iStockcomRASimon

Wiley also publishes its books in a variety of electronic formats Some content that appears in print may not be available in electronic books

Set in 9512pt Minion by SPi Publisher Services Pondicherry India

1 2015

The Nuts and Bolts of Implantable Device Therapy Pacemakers

The Nuts and Bolts of Implantable Device TherapyPacemakersTom Kenny FHRS CCDSDirector of Education PrepMD Braintree MA USA

This edition first published 2015 copy 2015 by John Wiley amp Sons Ltd

Registered Office John Wiley amp Sons Ltd The Atrium Southern Gate Chichester West Sussex PO19 8SQ UK

Editorial Offices 9600 Garsington Road Oxford OX4 2DQ UK111 River Street Hoboken NJ 07030-5774 USA

For details of our global editorial offices for customer services and for information about how to apply for permission to reuse the copyright material in this book please see our website at wwwwileycomwiley-blackwell

The right of the author to be identified as the author of this work has been asserted in accordance with the UK Copyright Designs and Patents Act 1988

All rights reserved No part of this publication may be reproduced stored in a retrieval system or transmitted in any form or by any means electronic mechanical photocopying recording or otherwise except as permitted by the UK Copyright Designs and Patents Act 1988 without the prior permission of the publisher

Designations used by companies to distinguish their products are often claimed as trademarks All brand names and product names used in this book are trade names service marks trademarks or registered trademarks of their respective owners The publisher is not associated with any product or vendor mentioned in this book It is sold on the understanding that the publisher is not engaged in rendering professional services If professional advice or other expert assistance is required the services of a competent professional should be sought

The contents of this work are intended to further general scientific research understanding and discussion only and are not intended and should not be relied upon as recommending or promoting a specific method diagnosis or treatment by health science practitioners for any particular patient The publisher and the author make no representations or warranties with respect to the accuracy or completeness of the contents of this work and specifically disclaim all warranties including without limitation any implied warranties of fitness for a particular purpose In view of ongoing research equipment modifications changes in governmental regulations and the constant flow of information relating to the use of medicines equipment and devices the reader is urged to review and evaluate the information provided in the package insert or instructions for each medicine equipment or device for among other things any changes in the instructions or indication of usage and for added warnings and precautions Readers should consult with a specialist where appropriate The fact that an organization or Website is referred to in this work as a citation andor a potential source of further information does not mean that the author or the publisher endorses the information the organization or Website may provide or recommendations it may make Further readers should be aware that Internet Websites listed in this work may have changed or disappeared between when this work was written and when it is read No warranty may be created or extended by any promotional statements for this work Neither the publisher nor the author shall be liable for any damages arising herefrom

Library of Congress Cataloging-in-Publication Data

Kenny Tom 1954ndash author The nuts and bolts of implantable device therapy pacemakers Tom Kenny p cm Includes index ISBN 978-1-118-67067-5 (pbk) I Title [DNLM 1 Cardiac Pacing Artificialndashmethods 2 Pacemaker Artificial WG 168] RC684P3 6174prime120645ndashdc23

2014027299

A catalogue record for this book is available from the British Library

Cover images copy iStockcomangelhel copy iStockcomRASimon

Wiley also publishes its books in a variety of electronic formats Some content that appears in print may not be available in electronic books

Set in 9512pt Minion by SPi Publisher Services Pondicherry India

1 2015

The Nuts and Bolts of Implantable Device TherapyPacemakersTom Kenny FHRS CCDSDirector of Education PrepMD Braintree MA USA

This edition first published 2015 copy 2015 by John Wiley amp Sons Ltd

Registered Office John Wiley amp Sons Ltd The Atrium Southern Gate Chichester West Sussex PO19 8SQ UK

Editorial Offices 9600 Garsington Road Oxford OX4 2DQ UK111 River Street Hoboken NJ 07030-5774 USA

For details of our global editorial offices for customer services and for information about how to apply for permission to reuse the copyright material in this book please see our website at wwwwileycomwiley-blackwell

The right of the author to be identified as the author of this work has been asserted in accordance with the UK Copyright Designs and Patents Act 1988

All rights reserved No part of this publication may be reproduced stored in a retrieval system or transmitted in any form or by any means electronic mechanical photocopying recording or otherwise except as permitted by the UK Copyright Designs and Patents Act 1988 without the prior permission of the publisher

Designations used by companies to distinguish their products are often claimed as trademarks All brand names and product names used in this book are trade names service marks trademarks or registered trademarks of their respective owners The publisher is not associated with any product or vendor mentioned in this book It is sold on the understanding that the publisher is not engaged in rendering professional services If professional advice or other expert assistance is required the services of a competent professional should be sought

The contents of this work are intended to further general scientific research understanding and discussion only and are not intended and should not be relied upon as recommending or promoting a specific method diagnosis or treatment by health science practitioners for any particular patient The publisher and the author make no representations or warranties with respect to the accuracy or completeness of the contents of this work and specifically disclaim all warranties including without limitation any implied warranties of fitness for a particular purpose In view of ongoing research equipment modifications changes in governmental regulations and the constant flow of information relating to the use of medicines equipment and devices the reader is urged to review and evaluate the information provided in the package insert or instructions for each medicine equipment or device for among other things any changes in the instructions or indication of usage and for added warnings and precautions Readers should consult with a specialist where appropriate The fact that an organization or Website is referred to in this work as a citation andor a potential source of further information does not mean that the author or the publisher endorses the information the organization or Website may provide or recommendations it may make Further readers should be aware that Internet Websites listed in this work may have changed or disappeared between when this work was written and when it is read No warranty may be created or extended by any promotional statements for this work Neither the publisher nor the author shall be liable for any damages arising herefrom

Library of Congress Cataloging-in-Publication Data

Kenny Tom 1954ndash author The nuts and bolts of implantable device therapy pacemakers Tom Kenny p cm Includes index ISBN 978-1-118-67067-5 (pbk) I Title [DNLM 1 Cardiac Pacing Artificialndashmethods 2 Pacemaker Artificial WG 168] RC684P3 6174prime120645ndashdc23

2014027299

A catalogue record for this book is available from the British Library

Cover images copy iStockcomangelhel copy iStockcomRASimon

Wiley also publishes its books in a variety of electronic formats Some content that appears in print may not be available in electronic books

Set in 9512pt Minion by SPi Publisher Services Pondicherry India

1 2015

This edition first published 2015 copy 2015 by John Wiley amp Sons Ltd

Registered Office John Wiley amp Sons Ltd The Atrium Southern Gate Chichester West Sussex PO19 8SQ UK

Editorial Offices 9600 Garsington Road Oxford OX4 2DQ UK111 River Street Hoboken NJ 07030-5774 USA

For details of our global editorial offices for customer services and for information about how to apply for permission to reuse the copyright material in this book please see our website at wwwwileycomwiley-blackwell

The right of the author to be identified as the author of this work has been asserted in accordance with the UK Copyright Designs and Patents Act 1988

All rights reserved No part of this publication may be reproduced stored in a retrieval system or transmitted in any form or by any means electronic mechanical photocopying recording or otherwise except as permitted by the UK Copyright Designs and Patents Act 1988 without the prior permission of the publisher

Designations used by companies to distinguish their products are often claimed as trademarks All brand names and product names used in this book are trade names service marks trademarks or registered trademarks of their respective owners The publisher is not associated with any product or vendor mentioned in this book It is sold on the understanding that the publisher is not engaged in rendering professional services If professional advice or other expert assistance is required the services of a competent professional should be sought

The contents of this work are intended to further general scientific research understanding and discussion only and are not intended and should not be relied upon as recommending or promoting a specific method diagnosis or treatment by health science practitioners for any particular patient The publisher and the author make no representations or warranties with respect to the accuracy or completeness of the contents of this work and specifically disclaim all warranties including without limitation any implied warranties of fitness for a particular purpose In view of ongoing research equipment modifications changes in governmental regulations and the constant flow of information relating to the use of medicines equipment and devices the reader is urged to review and evaluate the information provided in the package insert or instructions for each medicine equipment or device for among other things any changes in the instructions or indication of usage and for added warnings and precautions Readers should consult with a specialist where appropriate The fact that an organization or Website is referred to in this work as a citation andor a potential source of further information does not mean that the author or the publisher endorses the information the organization or Website may provide or recommendations it may make Further readers should be aware that Internet Websites listed in this work may have changed or disappeared between when this work was written and when it is read No warranty may be created or extended by any promotional statements for this work Neither the publisher nor the author shall be liable for any damages arising herefrom

Library of Congress Cataloging-in-Publication Data

Kenny Tom 1954ndash author The nuts and bolts of implantable device therapy pacemakers Tom Kenny p cm Includes index ISBN 978-1-118-67067-5 (pbk) I Title [DNLM 1 Cardiac Pacing Artificialndashmethods 2 Pacemaker Artificial WG 168] RC684P3 6174prime120645ndashdc23

2014027299

A catalogue record for this book is available from the British Library

Cover images copy iStockcomangelhel copy iStockcomRASimon

Wiley also publishes its books in a variety of electronic formats Some content that appears in print may not be available in electronic books

Set in 9512pt Minion by SPi Publisher Services Pondicherry India

1 2015

v

Contents

Preface vi

Acknowledgments vii

1 Cardiovascular anatomy and physiology 1

2 Cardiac conduction system 15

3 The cardiac cycle and hemodynamics 21

4 Heart disease 29

5 Cardiac medications related to cardiac rhythm management devices 37

6 The basics of eCg and rhythm interpretation 48

7 Arrhythmia analysis 58

8 electricity 101 78

9 Pacing 101 84

10 Indications for pacing 104

11 Pacemaker implantation 116

12 Connecting the leads to the pulse generator 131

13 Pacemaker modes and codes 140

14 Single-chamber timing cycles 153

15 Introduction to dual-chamber timing cycles 169

16 Dual-chamber timing cycles the atrial channel 179

17 Dual-chamber timing cycles the ventricular channel 195

18 Paced eCg and egM analysis 205

19 Upper-rate behavior 219

20 Advanced dual-chamber timing 233

21 Rate-responsive pacing 242

22 Special features 260

23 Automatic capture algorithms 278

24 Pacemaker follow-up 297

25 Follow-up and troubleshooting 310

Key answer 316

Index 320

vi

Preface

Cardiac pacing has changed dramatically in the past decades and yet certain things have not changed at all While technology has advanced cardiac pacing to provide benefits to more and more patients while streamlining device function and downsizing the pulse generators cardiac pacing is still a field that can be difficult to learn and slow to master Those who become cardiac pacing specialists are those who take the time to learn the concepts work sys-tematically through follow-up and listen carefully to their patients It has been more than three decades since I entered the cardiac rhythm management field and two decades since I wrote my first work-book on pacing to assist those trying to learn the fieldmdashand while everyone talks about how much cardiac pacing has changed in the past decades it strikes me how much has stayed the same

After all it is still about a battery a wire in the heart and timing tiny electrical pulses to fire at precisely the right split-second It is still about knowing rate and mode and being able to find a pacing spike on an ECG And it is still about restoring as near-to-normal a cardiac rhythm as possible to our patients

For those embarking on a career in cardiac pacing or clinicians who just need to know more about device-based cardiac rhythm management therapy it is my hope that this book will help

break down some of the complexities of this cardiac pacing Cardiac pacing can seem overwhelmingly complex because it involves the simultaneous inter-play of many different factors Somebody once told me that there are more than a million pos-sible device parameter combinations available in todayrsquos most advanced pacemakers So how does a busy clinician find that one-in-a-million pacing prescription The number of options may have changed but it is still about the basics You find the right mode The right rate You program an AV delay You just work through the programming one simple step at a time For those willing to learn the concepts and approach things systematically cardiac pacing can be mastered

It has always been about the patient and how that patient interacts with the pacemaker Our patients come to the clinicmdashoften nervous about their new ldquobattery-poweredrdquo statusmdashand expect to find in us clinicians both expertise and reas-surance I hope this book conveys my sentiment that the greatest expert is one who not only knows what to do but also knows how to talk warmly frankly and helpfully to each and every patient

Tom KennyNovember 2014

vii

Acknowledgments

Nobody writes a book alone even if there is just one name on the front cover First I must acknowl-edge with deep gratitude my students past and pre-sent over the years From these men and women I have learned a great deal about the kind of person who seeks a career in device-based therapy and how to break down difficult concepts to make them not just understandable but logical I think every time one of my students had an ldquoah-hardquo moment I had one too I hope that my instruction my expla-nations my anecdotes and my method of teaching serve you well in your future clinical work I would also like to thank my mentors that have taught me so much over the years I know much of what I know because of people like Orlando Maytin Michael Chizner Kathy King Barbara Perra and Eliot Ostrow Thank you for sharing your clinical expertise and sparking my interest in cardiology

Second I want to thank my friends and family who supported me in the writing of this book

I owe a great debt to my publishers who not only believed in this project but who went the extra mile to make it a reality

Third I must thank my editor Jo Ann LeQuang who assisted in all of the work that goes into turning a course into a manuscript She worked with me on my very first workbook when I worked in industry and she was a key contributor to this project as well I want to thank Matt OrsquoNeal Bob Matioli and all of the folks at PrepMD where I work today for their support and encouragement with this project

Finally I want to thank the many people who encouraged me to write about cardiac pacing No matter how much I teach this subject lecture on it or write about itmdashI still have a lot to say That shows the depth of this subject and also my passion for it It is my wish that this book not just informs future pacing experts but inspires them as well

1

The Nuts and Bolts of Implantable Device Therapy Pacemakers First Edition Tom Kenny copy 2015 John Wiley amp Sons Ltd Published 2015 by John Wiley amp Sons Ltd

Chapter 1

Cardiovascular anatomy and physiology

Introduction

An encyclopedia could be written on the anatomy and physiology of the human heart and that is not our purpose Device clinicians must understand the cardiovascular system to understand arrhyth-mias and device therapy This chapter will intro-duce the important concepts of cardiac anatomy and physiology necessary for an understanding of cardiac rhythm management To that end this chapter will describe the chambers valves and major vessels of the heart and how these control the flow of blood in the body Although we think of the heartmdashrightlymdashas a pump it also possesses a com-plex electrical system The cells of the human heart are unique in many ways and how they produce conduct and dissipate electrical energy is very important particularly to pacing Our goal here is to describe the anatomy and physiology of the

healthy heart and cardiovascular system in terms of what device clinicians need to know

the healthy heart

The human heart is a double pump (right and left) that sits in the middle of the chest slightly to the left and rotated so that the right side is more anterior than the left An average adult human heart is relatively large about 13 by 9 by 6 cm and weighing about 300 g The heart is protected by the rib cage and sits directly behind one of the bodyrsquos thickest bones the sternum The bottom of the heart rests on the diaphragm muscle The heart is encased in this protected but somewhat crowded areamdashit also contains the lungs (three lobes on the right two on the left) the stomach and the intestines

The bottom tip of the heart (called the apex) taps up against the chest when the heart contracts By placing his hands on the chest a physician can feel the place where the apex of the heart makes contact with the chest this place is called the point of max-imal impulse (PMI) Knowing the precise location of the PMI can be very useful in treating cardiology patients because the PMI of a healthy heart occurs slightly to the left while the PMI of a person with an enlarged heart is going to occur much farther to the left even off to the side A healthy heart is roughly the size of the fist but when hearts enlarge such as occurs with disease progression the enlargement occurs toward the left Thus PMI can

1

Learning objectives

bullensp Point out the key landmarks in the human heart relevant to cardiac rhythm management

bullensp Name the four chambers of the heart the four valves and the major vessels

bullensp Describe the flow of blood through the heartbullensp Define AV synchrony and explain why it is

importantbullensp State the difference between the bodyrsquos arterial

versus venous systems

2 The nuts and bolts of implantable device therapy pacemakers

provide a fast noninvasive way of determining if and to what degree the heart has enlarged

The left ventricle composes most of the mass of the heart being by far the largest of the four pump-ing chambers A healthy heart circulates about 4ndash6 l of blood a minutemdashwhich is the entire blood volume of the body That means the entire circulating volume of blood in the body moves around every minute or once per beat

The heart consists of four chambers two upper chambers called atria (singular atrium) and two lower and larger chambers called ventricles To understand the healthy heart it is useful to think of the heart in terms of right side (right atrium and right ventricle) and left side (left atrium and left ventricle) The right side of the heart circulates deoxygenated blood to the lungs (where it can be oxygenated) The left side of the heart pumps oxy-genated blood out to the rest of the body (see Figure 11)

The heart is a muscle and consists of four dis-tinct layers The endocardium is the innermost layer and composes a lining for the interior of the heart The epicardium is the outer layer of the heart Between the endocardium and epicardium lies the myocardiummdashthe thickest layermdashwhich is muscle The entire heart is encased in a

liquid-filled sac called the pericardium which acts like a shock absorber for the heart The pericardial sac contains about 15ndash20 cc of pericardial fluid in a healthy individual In the event that fluid builds up to abnormally high levels in the pericardial sac (such as might occur when a lead or catheter inside the heart perforates the endocardium myocar-dium and epicardium and goes exterior to the heart) this fluid can place pressure on the heart in a condition known as cardiac tamponade Since the heart is contained in a relatively small space this pressure can compromise the heartrsquos ability to fill with blood and pump efficiently During device implantation perforation is an important concern because it can lead to cardiac tamponade In the event that perforation results in cardiac tamponade a needle is inserted into the pericardial sac (through the chest wall) to drain the blood Lead perforation does not always result in cardiac tamponade but it is a serious concern

Blood flow through the heart

The heart is a pump and it is located amid a net-work of vessels that carry deoxygenated blood into the right side of the heart and reoxygenated blood into the left side of the heart The flow is actually

Right atrium

Left atrium

Left pulmonaryarteries

Left pulmonaryveins

Left ventricle

Right ventricle

Aorta

Aortic valve

Mitral valve

Tricuspidvalve

Pulmonaryvalve

SA node

AV node

Right pulmonaryarteries

Right pulmonaryveins

Figure 11 Cross section of the heart showing the chambers

Chapter 1 Cardiovascular anatomy and physiology 3

fairly simple Deoxygenated blood enters the right side of the heart and is pumped over to the lungs via the pulmonary arteries and is returned back (as oxygen-rich blood) to the left side of the heart by way of the pulmonary veins (PV) While both right and left sides of the heart contract at the same time as a single unit the right side is busy pumping deoxygenated blood to the lungs while the left side is pumping reoxygenated blood out to the rest of the body

Deoxygenated blood enters the right side of the heart via the superior vena cava (SVC) but once it has become oxygenated again blood is pumped back out from the left side of the heart into the aorta The aorta is the largest vessel in the body and it forms a U shape at the top of the heart These portions of the aorta are called the ascending the descending and the arch Coming off the aortic arch are three main arteries the left subclavian artery the left common carotid artery and the brachiocephalic trunk

To better understand the blood flow through the heart it is important to review the structure of the heart The atria or upper chambers of the heart are smaller have thinner walls and are smoother on the inside than the ventricles Within the ventri-cles is a network of fibrous strands known as trabeculae These structural differences become important in lead implantation within the heart it is much easier to affix or lodge a lead in the trabeculae of the ventricles than to try to anchor the lead to a smooth atrial wall Historically atrial leads have almost always been active-fixation screw-in-type leads while ventricular leads were almost always passive-fixation leads (fins or tines that lodge in the trabeculae) Today active-fixation leads are often used in both chambers since they facilitate lead removal (Figure 12)

Overall blood flow to the heart is discussed right and left sides although it is important to recognize that what happens in the heart that is systole (contraction) and diastole (relaxation) are happening on both sides at the same time The right atrium of the heart receives blood from the SVC the inferior vena cava (IVC) and the coronary sinus (CS) The CS is technically a vein and it has an opening or ostium (sometimes just called os) at the base of the right atrium slightly posterior The CS delivers oxygen-depleted blood

to the right atrium from the coronary arteries that encircle the exterior of the heart The CS is of interest in cardiac resynchronization therapy (CRT) because the left ventricular lead is passed through the CS (counter to the flow of blood) in order to be placed into the coronary vessels to pace the left ventricle CRT is used in patients with heart failure whose hearts have remodeled that is enlarged and changed shape (It may be said that with heart failure the heart changes from the shape of a football to the shape of a basketball) The CS may be relocated in this remodeling which can be challenging in implanting a CRT lead because the physician must first locate the os of the CS and then navigate through it in order to implant the left ventricular lead

Anatomically the heart is dominated by the large muscle mass of the left ventricle which makes up about two-thirds of the heart in terms of weight and volume This greater size is typically ascribed to the fact that the left ventricle must pump blood throughout the whole body whereas the right ventricle only has to pump blood to the lungs The left and right ventricles pump blood to different destinations but the left ventricle is larger and more muscular for a reasonmdashpressure It is impor-tant to review the pressures against which the heart must work to understand cardiac blood flow (Figure 13)

Deoxygenated blood in the right side of the heart must travel over the lungs to pick up oxygen This means that blood in the right ventricle travels across the pulmonary valve into the pulmonary artery and then out toward the lungs The pulmonary valve opens automatically when pressure from the contracting right ventricle forces it open This occurs when the pressure in the right ventricle exceeds the pressure in the pulmonary artery Pressure gradients are key concepts in understanding blood flow Valves are like gates that open and close in response to pressure In general the pressure in the PV is fairly low around 12 mmHg Thus the right ventricle does not need to create a lot of force to open the pulmonary valve

Meanwhile as the left ventricle contracts it cre-ates pressure on the aortic valve leading to the aorta In order to open the aortic valve and pump blood out into the aorta the heart must overcome the pressure in the aortic valve Pressure in the

4 The nuts and bolts of implantable device therapy pacemakers

Right atrium

Left atrium

Left ventricle

Right ventricle

Figure 12 Note that the atria are smooth walled while the ventricles contain a spongelike fibrous network of trabeculae

Left pulmonaryarteries

Left pulmonaryveins

Aorta

Aortic valve

Pulmonaryvalve

Right pulmonaryarteries

Superiorvena cava

Inferiorvena cava

Right pulmonaryveins

Figure 13 The blood flow within the heart takes oxygen-depleted blood from the body into the right atrium where it flows to the right ventricle and is pumped out over the lungs the reoxygenated blood from the lungs is pumped into the left atrium where it flows to the left ventricle and is pumped out via the aorta to the body

Chapter 1 Cardiovascular anatomy and physiology 5

aorta is high around 120 mmHg or ten times higher than the pressure in the PV The left ventricle must therefore work much harder to pump blood than the right ventricle This requires the left ventricle to be larger and more muscular than the right ven-tricle (see Figure 14)

On the right side of the heart blood travels from the right atrium into the right ventricle via the tricuspid valve The tricuspid valve gets its name from its characteristic shape involving three leaves or cusps Attached to these cusps are cords that anchor into the base of the ventricle known as chordae tendineae they look almost like little parachutes The strands of the chordae tendineae attach to tiny papillary muscles These chords attach to the valve leaves at one end and a papil-lary muscle at the other end On the right side of the heart the tricuspid valve is associated with three papillary muscles The purpose of these chords and muscles is to assure that the valve is

effectively closed and opened at the proper times (Figure 15)

The heart can rightly be thought of as a pump but it must be remembered that the heart is also a muscle and all muscles need a steady supply of oxy-gen-rich blood The heart muscle is supplied with blood through a network of coronary arteries that surround the outside of the heart Blockage in a coronary artery results in ischemia which can lead to death of cardiac muscle including the chordae tendineae and papillary muscles While the patient may survive such an ischemic event the damage to the heart may lead to an incompetent valve that is a valve that is no longer able to function effectively

In tracing the blood flow from the right ventricle to the pulmonary artery it should be clear that the blood has to go from down in the right ventricle to up through the pulmonary valve and into the pulmonary artery The blood is able to make this journey because of the pumping pressure of the heart The route the blood takes as it exits the right ventricle and journeys up toward the pulmonary valve is known as the right ventricular outflow tract (RVOT) On the other side of the heart there is also a corresponding left ventricular outflow tract (LVOT) of approximately the same size Cardiac leads are sometimes fixated in the RVOT (see Figure 16)

Blood pumped out of the right ventricle crosses the pulmonary valve and enters the pulmonary artery which splits into two branches right and left The right pulmonary artery takes blood to the right lung while the left pulmonary artery takes blood to the left lung In this respect the pulmonary artery is unique in the body in that it is an artery but it carries deoxygenated blood Blood travels through the lung to the alveoli where it gains oxygen and loses carbon dioxide Once it is reoxygenated the blood gathers into the PV (which are also unique being the only veins to carry oxygenated blood) There are four PV in total the two right-sided PV take oxygen-ated blood from the right lung while the two left-sided PV take oxygenated blood from the left lung and they all bring this reoxygenated blood to the left atrium

The left atrium is smooth walled like the right atrium and although the left atrium is much

Figure 14 The left ventricular is far more muscular than the right ventricle because it must overcome 10 times the pressure of the right ventricle in order to pump blood out via the aortic valve and into the aorta

6 The nuts and bolts of implantable device therapy pacemakers

Right atrium

Left atrium

Left ventricle

Right ventricle

Figure 15 The leaflets of the valves are attached by chordae tendineae at one end and papillary muscles on the other which assure the effective opening and closing of the valves

Right-ventricularoutflow tract (RVOT)

Figure 16 There are two outflow tracts in the heart one associated with the right ventricle and the other the left ventricle These outflow tracts are roughly the same size The illustration shows the right ventricular outflow tract (RVOT) a preferred location for right ventricular lead fixation The left ventricular outflow tract (LVOT) cannot be seen in this illustration as it is posterior

Chapter 1 Cardiovascular anatomy and physiology 7

smaller than the left ventricle it is larger than the right atrium The blood will travel from the left atrium into the left ventricle by way of the mitral valve The mitral valve gets its name from the miter a bishoprsquos hat The mitral valve has two cusps and papillary muscles connected by chordae tendineae Once blood is in the left ventricle a contraction will force it toward the aorta by way of the LVOT In persons with hypertrophic obstructive cardiomy-opathy (HOCM) the septum or wall in the heart can become thick and enlarged effectively narrow-ing the LVOT In the healthy heart the blood flows up the LVOT across the aortic valve and into the aorta From the aorta blood will be directed upward toward the head downward toward the legs and feet and some blood will be redirected back to the coronary arteries of the heart

The blood flow from the left ventricle to the aorta and back to the coronary arteries is unique in the body in that most of that flow occurs during diastole rather than systole Think of it this way as blood is pumped by the left ventricle during systole upward and downward to the brain and the rest of the body some of that blood flows back during diastole and drains into the coronary arteries Although it may seem counterintuitive it is impor-tant to remember that coronary artery perfusion occurs primarily during diastole

Volume valves and pressure

While it is tempting to think of the heart as simply a pump with an electrical system it is more accu-rate to say that the heart moves blood because of variations in volume and pressure

The heart has four valves two are atrioventricular (AV) valves because they connect the atrium to the ventricle and two are semilunar valves connecting the heart with pulmonary arteries or the aorta The right AV valve is the tricuspid while the left AV valve is the mitral valve The flow or movement of blood is guided and controlled by the heartrsquos muscular action in the form of systole (contraction) or diastole (relaxation)

When blood pours into the right atrium it dis-tends and expands the right atrium When this volume of blood creates sufficient pressure it forces the tricuspid valve open Think of it as volume creating pressure and pressure opening

valves When the tricuspid valve opens the blood dumps into the right ventricle During this period of diastole blood pours into the right ventricle creating volume and expanding it As blood pours in and ventricular diastole nears the end the atria contract while the AV valves are still open in something nicknamed atrial kick Atrial systole occurs at the end of ventricular diastole Atrial kick forces the maximum amount of blood from the atria into the ventricles It is estimated that atrial kick delivers 20ndash30 of cardiac output

The atrial contribution to ventricular filling (atrial kick) is of enormous clinical significance Patients with certain atrial arrhythmias such as atrial fibrillation (AF) lose atrial kick Even milder forms of atrial tachyarrhythmias may compromise atrial kick Moreover patients who do not have AV synchrony (one atrial beat corresponding to one ventricular beat) will lose atrial kick The loss of atrial kick can reduce cardiac output by 20ndash30

Once the atria have contracted and the ventricles are filled with blood the AV valves snap closed The ventricles now start to contract and push open the semilunar valves For example as the right ven-tricle contracts (systole) it opens the PV (The PV is sometimes called the pulmonic valve itrsquos the same thing) The whole process works on volume pressure and contractionrelaxation

Of the four valves in the heart the tricuspid valve is of most interest to device specialists because at least one lead will be placed into the right ventricle This means that pacing and defi-brillation leads typically cross the tricuspid valve These leads may interfere with the proper closing of the tricuspid valve particularly if the lead is large diameter or several leads are passed over the tricuspid valve A single lead through the tricuspid valve is likely to cause minimal to no dysfunction However it is not unusual to see patients with three or more leads in the right ventricle and these leads may hamper the tricuspid valve in its closing leading to tricuspid regurgitation or backflow of blood Patients with multiple leads (such as chil-dren who grow up with pacemakers) may need to have some of them removed to avoid tricuspid regurgitation

Right ventricular leads are typically fixated at the right ventricular apex or in the RVOT and may be

8 The nuts and bolts of implantable device therapy pacemakers

lodged in the trabeculae Trabeculae are a fibrous network that could be described as similar to the pores of a sponge (see Figure 17)

On the left side of the heart the blood flow pattern is similar Blood flows into the left atrium over the open mitral valve and into the left atrium At the end of ventricular diastole as the ventricle is distended and stretched atrial kick delivers more blood into left ventricle The volume of blood cre-ates pressure The mitral valve closes As the left ventricle contracts the growing pressure over-comes the resistance on the aortic valve and it opens allowing blood to flow out into the aorta and beyond

While any valve can become diseased or dys-functional valvular disease more commonly affects the mitral and the aortic valves (left-sided valves) than the right-sided valves (tricuspid and pul-monary) These left valves are more likely to suffer damage or disease because of the high-pressure environment in which they function exposing them to more potential damage Other damage can occur when plaque builds up in the coronary arteries and the aorta The aorta can become

diseased typically in the case of an aortic aneurysm or dissection Carotid arteries in the neck are also frequently a site for atherosclerotic disease

the right atrial appendage

The right atrial appendage (RAA) is an area near the right atrium which is a preferred site for attach-ing active-fixation atrial leads (see Figure 18) The RAA serves no obvious purpose and may be sacri-ficed in certain heart surgeries such as a coronary artery bypass graft (CABG) procedure CABG or bypass surgery involves stopping the heart and running the blood through a machine to reoxygen-ate it during the course of the procedure This is done by attaching a large-diameter hose to the RAA when surgery concludes and this hose is removed it can damage the RAA to the point that it is completely or partially surgically removed In patients who have had a CABG procedure the RAA may not be available for right atrial lead placement For such patients the right atrial lead is often placed against the lateral wall of the right atrium It is sometimes possible to fixate a right

Figure 17 Trabeculae form a fibrous network within the ventricles Passive-fixation ventricular leads are held in place by lodging their tines or fins in the trabeculae

Chapter 1 Cardiovascular anatomy and physiology 9

atrial lead in the remnants of the RAA if a portion of it is preserved

arteries and veins

Any discussion about the heart necessarily involves the vasculature which is why we commonly refer to our circulatory system as the cardiovascular

system As most of us remember from anatomy classes arteries carry blood away from the heart while veins transport blood back to the heart (The exceptions are the PV and arteries described earlier) Arteries carry oxygen-rich blood while veins carry deoxygenated blood What is of importance to device clinicians is that arteries are high-pressure vessels while veins are low-pressure vessels Arteries tend to be more muscular in structure and muscular contractions of the arteries help to move blood outward into the body Veins tend to be less muscular than arteries but unlike arteries they contain a system of tiny interior valves The purpose of these little valves is to maintain a unidirectional flow of blood (see Figure 19)

Some of the most important veins in the body are the SVC and its counterpart the IVC The SVC takes blood from the upper part of the body and delivers it into the right atrium with the IVC doing the same for blood from the lower part of the body The coronary veins networked around the outside of the heart carry blood from the heart tissue and deliver it back to the right atrium they are closely linked to the coronary arteries which carry reoxygenated blood from the left ventricle back to the heart muscle The CS is the site where

Right atrial appendage

Figure 18 The right atrial appendage serves no obvious hemodynamic purpose but is often the site of fixation of atrial leads

Artery Vein

Figure 19 Veins and arteries have different purposes different anatomical structure and different pressures Overall veins are low-pressure systems that move blood by a series of little valves that keep blood flowing in the same direction Arteries are high-pressure systems and are more muscular so that they force blood forward by squeezing

10 The nuts and bolts of implantable device therapy pacemakers

the great middle and small cardiac veins all drain the CS takes this blood and delivers it back to the right atrium

The PV carry reoxygenated blood from the lungs and deliver it to the left atrium so that it can be pumped back out to the body by the left ventricle

The most important artery in the body is by far the aorta The aorta is connected to the left ven-tricle and it is the first and main conduit that takes reoxygenated blood from the left ventricle and sends it out to the body As this reoxygenated blood makes its way out into the body its first two stops are the head (brain) and the heart muscle Inadequate oxygenated blood to the brain can very quickly provoke symptoms This is the reason that depressed cardiac output is associated with symptoms like dizziness light-headedness feeling woozy and fatigue Severe symptoms might include syncope

The coronary arteries arise from the root of the aorta just above the cusps of the aortic valve Coronary arteries receive most blood during dias-tole so blood is more or less pouring back into these vessels The coronary artery system is a networkmdashalmost a meshmdashof middle-sized to very small vessels that cover the exterior surface of the heart (see Figure 110)

Coronary arteries get their name from the Latin word for crown (such as in our word coronation) because this network of vessels sits like a crown atop the heart and encircles it The heart gets blood from outside in as oxygen-rich blood in these coro-nary arteries is delivered to the exterior of the heart muscle Coronary arteries are not within the heart they are epicardial structures The two main coro-nary arteries are the right coronary artery (RCA) and the left coronary artery (LCA) The RCA and LCA come off the aorta and are located in the AV groove an exterior structure below the atria and above the ventricles The RCA runs from the aorta in the right AV groove while the LCA comes off the aorta and quickly branches to the left main The left main is a very short artery (about the size of a thumbnail) which branches quickly into the left anterior descending artery (LAD) and the circum-flex artery The LAD travels down the front of the

left side of the heart The circumflex artery can be found in the left AV groove

The RCA bifurcates to the posterior descending artery The right atrium and right ventricle get their supply of oxygen-rich blood from the RCA Much of the cardiac conduction system is right sided that is it commences with the sinoatrial (SA) node in the high right atrium and travels down to the AV node on the right side of the septum These important electrical structures get their oxygen-rich blood supply from the RCA The electrical system travels down the ventricles to the inferior wall of the left ventricle (located at the bottom of the heart but belonging to the left ventricle) the inferior wall is also fed oxygen-rich blood by the RCA An occlusion of the RCA such as might occur in a myocardial infarction (MI) may result in death of the heart muscle it feeds which includes the right atrium the right ventricle and the inferior wall of the left ventricle and it might affect the conduction system including the SA node and the AV node An inferior wall MI is typically the result of a clogged RCA

RCA

LCA

Aorta

Figure 110 The coronary arteries of the heart branch out to surround the entire heart muscle The two most prominent coronary arteries are the right coronary artery (RCA) and the left coronary artery (LCA)

Chapter 1 Cardiovascular anatomy and physiology 11

For an MI the more distal the occlusion (that is further from the aorta) the less potential damage the heart attack will have In other words the worst occlusions are proximal Whether or not a patient survives an MI comes down to the numbers The loss of over 40 of the left ventricular muscle mass results in death However that 40 is cumulative For example a person can survive an MI that costs him 30 of left ventricular muscle mass but if that heart attack is followed by another that kills 15 of the left ventricular muscle mass the second (milder) heart attack will prove fatal This is the reason many people survive a first heart attack but die of a second or third attack even though their fatal MI may be relatively mild

The LCA branches quickly to the left main which branches almost at once to the LAD The LAD runs on the outside of the heart along the front of the heart along the septum The left main supplies the left atrium and the majority of the left ventricle in other words the major pumping por-tion of the heart An occlusion in the left main can be particularly disastrous because it is so far upstream and affects the heartrsquos left ventricular muscle mass No wonder they call the left main the widow maker

The coronary arteries do not run in straight lines they twist and turn and sometimes make very sharp bends People with coronary artery disease (CAD) typically have plaque buildup in the areas where the coronary arteries bend at sharp angles These are areas where the blood flow creates a lot of turbulence and plaque and other substances can collect and build up It is typically to find blockage and occlusions at these places

The LAD supplies the anterior wall of the left ventricle and the septum with oxygen-rich blood Since part of the heartrsquos conduction system runs through the septum (the right and left bundle branches) an LAD occlusion can damage the heartrsquos ability to conduct electricity properly

The circumflex artery runs along the AV groove laterally and thus supplies oxygen-rich blood to the lateral wall of the left ventricle along with the left atrium The circumflex runs along the AV groove to the left heading toward the back of the heart where it meets with the RCA which

travels along the AV groove to the left toward the back of the heart Both the circumflex and the RCA are responsible for providing oxygenated blood to the posterior wall of the ventricle But which is the more important provider That depends on crux and whether the heart is right or left dominant

The crux refers to an imaginary line drawn exactly down the middle of the ventricular poste-rior wall without any regard to anatomical landmarks If the RCA crosses the crux then the heart is said to be right dominant and the RCA is the greater supplier of oxygenated blood to the ven-tricular posterior wall On the other hand if the circumflex crosses the crux then the heart is said to be left dominant and the circumflex is the more significant provider Roughly 60 of the population is right dominant

While this information about the vasculature is important it is even more important for clini-cians to recognize that there is tremendous interpatient variability in terms of venous anatomy While we can describe major structures and typical venous formations anomalies are very common These anomalies may be minor variations or they can be very pronounced such as missing veins and arteries Clinicians who deal with the cardiovascular system must be prepared for large and small differences in vascular anatomy The cardiovascular system is incredibly resilient and can adapt to some anomalies by building up collateral circulation For example a person with a missing or incompetent vessel may over time build up a network of smaller collateral vessels that compensates by doing the same job It is not unusual to see vessels grow larger or branch out to compensate Sometimes when an artery is blocked collateral circulation will build up and compensate by delivering oxygen-rich blood to the heart muscle

Every artery in the body has a matching or corresponding vein usually located in close proximity This applies to the coronary system as well Note that there are great differences in coro-nary venous anatomy among patients which becomes important when implanting left ventric-ular leads for CRT devices (see Figure 111)

12 The nuts and bolts of implantable device therapy pacemakers

the nuts and bolts of cardiovascular anatomy and physiology

bull The heart consists of four chambers which may be considered the upper chambers (right atrium and left atrium) and lower chambers (right ven-tricle and left ventricle) or may be thought of as the right heart (right atrium and right ventricle) and left heart (left atrium and left ventricle)

bull Deoxygenated blood enters the right atrium flows to the right ventricle and is pumped out via the pulmonary valve to the lungs The blood is reoxygenated in the lungs and then enters the left atrium and flows to the left atrium and out of the aortic valve into the aorta and the rest of the body

bull The heart has two outflow tracts a right ven-tricular outflow tract (RVOT) and a left ven-tricular outflow tract (LVOT)

bull The largest chamber of the heart is the left ven-tricle making up about 23 of the cardiac mass The left ventricle is muscular and massive because it must pump against approximately 10 times the pressure against which the right ven-tricle pumps (120 vs 12 mmHg)

bull The two cardiac valves most susceptible to dysfunction and disease are the mitral and aortic valve because they function in a high-pressure environment

bull The chordae tendineae connect the leaflets of the valves to the papillary muscles and look like little parachutes These help the valves open and close properly

bull In general arteries transport blood away from the heart are muscular and contract to help move the blood on its way and have thicker walls than veins Veins have thinner less muscular walls and carry oxygen-depleted blood back to the heart using tiny interior valves to keep the blood flow moving forward Arteries transport oxygenated blood and can be considered high-pressure systems while veins transported deoxygenated blood in a low-pressure system

bull Cardiac contraction is systole cardiac relaxation is diastole Atrial systole occurs at the very end of ventricular diastole in something nicknamed atrial kick In a healthy individual about 20ndash30 of cardiac output comes from atrial kick

bull The heart is a muscle that is fed with oxygenated blood through a network of coronary arteries Coronary arteries get oxygen-rich blood from the aorta and they perfuse the heart muscle during diastole as blood drains back

bull Although arteries and veins generally take blood away from or to the heart respectively

Rightventricle

Leftventricle

Leftatrium

Aorta

Rightatrium

Rightcoronaryartery

Leftcoronaryartery

Circumflexartery

Superiorvena cava

Inferiorvena cava

Arterial system

Rightventricle

Leftventricle

Greatcardiac vein

Smallcardiac vein

Middlecardiac vein

Leftatrium

Aorta

Rightatrium

Superiorvena cava

Inferiorvena cava

Venous system

Coronarysinus

Figure 111 The coronary artery system is matched by a coronary venous system

Chapter 1 Cardiovascular anatomy and physiology 13

test your knowledge

Fill in these anatomical landmarks of the heart They include the four chambers the four valves and the bodyrsquos main artery

the pulmonary vessels are the exception The pulmonary artery transports deoxygenated blood while the pulmonary veins carry oxygen-ated blood This is because of their special loca-tion and the fact that their work is taking blood from the right side to the left side of the heart

bull The main coronary arteries are the right coro-nary artery and the left coronary artery The left coronary artery branches quickly to a short vessel known as the left main (the widow maker) and the circumflex artery

bull A myocardial infarction can kill the heart muscle The clogged arteries determine the loc ation of the affected muscle A right coro-nary artery occlusion will affect the right atrium and ventricle and the inferior wall of the left ventricle while a left coronary artery occlusion will affect the left atrium and most of the left ventricle By far the most dangerous occlusion is in the left main because it feeds the majority of the heart muscle with oxygen-rich blood

answer the following questions

1 What are the names of the two atrioventricular valves of the heartA Atrial and ventricularB Tricuspid and mitralC Pulmonary and pulmonicD Aortic and superior vena cava

2 Which chambers of the heart contain trabeculaeA AtriaB VentriclesC Right-sided chambers onlyD Left-sided chambers only

14 The nuts and bolts of implantable device therapy pacemakers

3 Why can trabeculae be important to device cliniciansA They are good places to secure a passive-fixa-

tion leadB They help regulate blood pressureC They prevent atherosclerosisD All of the above

4 Which of the following is not true about the aortaA It is an arteryB It consists of an ascending portion a descend-

ing portion and an archC It carries only oxygenated bloodD It is the largest vein in the body

5 From which artery does blood enter the right atriumA Superior vena cavaB Inferior vena cavaC Coronary sinusD All of the above

6 What is atrial kickA Ventricular pressure on the tricuspid valve

forcing it shutB Electrical impulses originating in the atrium

to cause the heart to beat fasterC Atrial systole that forces the most blood possible

into the already-full and diastolic ventriclesD A type of arrhythmia

7 Why would a person with chronic AF likely have decreased cardiac outputA AF is associated with low blood volumeB AF causes the heart to beat very slowlyC AF may cause the heart to experience fre-

quent pausesD AF reduces or eliminates atrial kickmdashand

atrial kick contributes to cardiac output

8 Which of the following is often a preferred location for affixing a cardiac pacing leadA The right ventricular outflow tract (RVOT)B The left ventricular outflow tract (LVOT)C The excised right atrial appendageD The tricuspid valve

9 What connects the valve leaflets to the papillary musclesA TrabeculaeB ArteriesC Chordae tendineaeD Tiny valves

10 Which coronary artery is nicknamed the widow maker because an occlusion in this vessel will deprive the majority of the heartrsquos muscle mass of oxygen-rich bloodA Left mainB CircumflexC Right coronary arteryD Inferior vena cava

15

The Nuts and Bolts of Implantable Device Therapy Pacemakers First Edition Tom Kenny copy 2015 John Wiley amp Sons Ltd Published 2015 by John Wiley amp Sons Ltd

Chapter 2

Cardiac conduction system

Introduction

The sinoatrial (SA) node is the heartrsquos natural pacemaker In the healthy heart the SA node fires and the electrical stimulus conducts along special pathways through the heart that cause cells in specific areas to depolarize After the stimulus passes the area the cells repolarize Myocardial cell depolarization and repolarization govern the heartrsquos contraction and relaxation sequencemdashin other words the heart rhythm Rhythm disorders occur when this healthy conduction system is in some way disrupted or impaired

the conduction pathways

When the SA node fires and sends out electrical energy the stimulus travels from the SA node in the high right atrium out over the atria via path-ways known as internodal tracts Most of these internodal tracts are located in the right atrium the

main tract over to the left atrium is called Bachmannrsquos bundle It is because of Bachmannrsquos bundle linking the right atrium to the left atrium that the atria can contract simultaneously that is as these two upper chambers contract as one coherent unit After traversing the atria the impulse comes down through conduction tracts to the atrioventricular (AV) node

The AV node is an area of highly specialized tissue located below the atria and above the ventri-cles on the right side of the septum Anatomically the AV node rests in the AV groove The tissue in the healthy AV node has the ability to change the speed of the electrical impulse specifically it slows the stimulus down The slowdown serves two important purposes First slowing the impulse allows the atria time to contract and provide the very valuable atrial kick to ventricular filling Second the slowed impulse protects the ventricles from being forced to beat too quickly that is to beat before they have been adequately filled with blood

Once the impulse crosses the AV node it travels down into the ventricles and accelerates its speed The impulse travels through the bundle of His (pronounced hiss) to the bundle branches (right and left) and then out to the network of Purkinje fibers This allows both ventricles to depolarize and contract simultaneously (Figure 21)

polarization depolarization and repolarization

Cardiac conduction is driven by the ability of myo-cardial cells to be depolarized and repolarized Depolarization occurs at the cellular level and is

2

Learning objectives

bullensp Describe the electrical conduction system of the healthy heart

bullensp List the main anatomical landmarks of normal cardiac conduction

bullensp Name the anatomical location of the heartrsquos natural pacemaker

bullensp Explain depolarization and repolarization in electrical terms and how it relates to the cardiac contraction

bullensp Define the term automaticity and how it relates to cardiac conduction

16 The nuts and bolts of implantable device therapy pacemakers

in turn driven by electrolytes or positively and negatively charged ions (potassium sodium and calcium mainly) which are able to move in and out of the cardiac cells A myocardial cell may be said to be polarized when in its resting state that is there is no electrical activity going on In the rest-ing state the interior of the myocardial cell is somewhat more negatively charged that the outside This state is called the resting state or sometimes phase 0 of the action potential

An electrical stimulus changes the resting state abruptly The electrical energy allows for the movement of ions to the point that the interior of the myocardial cell is now more positive than the outside In other words the charge reverses and the cell is said to be depolarized When this happens the cell contracts when this happens to a large area of the myocardium the heart muscle contracts Cardiac contraction occurs as a result of cellular depolarization

When the electrical stimulus moves past the myocardial cell the ions move in reverse and the interior of the cell assumes its previous negative charge with respect to the outside This process is

called repolarization and it brings the myocar-dial cell back to the resting state When this happens the cell relaxes When a large area of the myocardium repolarizes the heart resumes its previous shape (loses its contraction)

Intrinsic pacemakers

The healthy heartrsquos natural pacemaker is the SA node but the SA node is not the only area of the heart that can generate an electrical pulse The SA node serves as the heartrsquos pacemaker because it generates these pulses faster than other areas In the healthy heart the SA node can fire at around 60ndash100 times a minute corresponding to a healthy heart rate The AV junction can also fire but it is much slower at around 40ndash60 times a minute Even the Purkinje fibers (the ventri-cles) can act as a pacemaker but they fire at only 20ndash40 times a minute Thus in a healthy individual the SA node outpaces these other potential pacemakers The AV junction and ven-tricles are sometimes called backup pacemakers because they take over the heart rate in the event

Sinoatrial (SA) node

Internodal tracts

Right bundle branch

Left bundle branch

Purkinje fibers

Bachmannrsquos bundle

Bundle of His

Atrioventricular (AV) node

Figure 21 The conduction pathways in the healthy heart

Chapter 2 Cardiac conduction system 17

that the SA node is not able to pace the heart While these backup pacemakers may help keep the heart beating in the event of a dysfunctional SA node they are slow and not as reliable as the SA node

The healthy heart is able to pace itself because of a property called automaticity Automaticity is an inherent property of individual cardiac conductive cells that allows them to depolarize spontaneously In other words some cells in the heart (such as the cells of the SA node) are able to generate an electrical stimulus on their own

refractoriness

Refractoriness refers to the temporary inability of a cardiac cell to respond to an electrical stimulus Refractoriness can be the normal appropriate and expected behavior of cardiac cells For example immediately after electrical energy has passed through cardiac cells they go through a phase where they are unresponsive to electrical energy no matter how much voltage is delivered This normal refractoriness is called the physiological refractory period and it is very brief (it is mea-sured in thousandths of a second) Of course the exact duration of a refractory period is based in part on the heart rate the faster the heart rate the shorter the refractory period and vice versa

Refractoriness may also occur when certain cells become dysfunctional that is cardiac cells may lose their ability to conduct properly Furthermore the heartrsquos inherent pacemakers may change their rate for instance the AV junction may speed up and start to deliver impulses at 80 or 100 times a minute When this happensmdashthat is when another pacemaker takes over the role of the SA nodemdashthat pacemaker is called a usurping pacemaker because it usurps the rightful role of the SA node In the event that other pacemakers of the heart start to compete with the SA node the fastest pacemaker always wins

Conductivity

Conductivity is the cardiac cellrsquos ability to transmit an electrical impulse to an adjacent cell Typically cardiac conduction is antegrade that

is it moves forward Electrical impulses actually travel out in all different directions but refracto-riness prevents those cells that were just stimu-lated from depolarization This keeps conduction moving in a single direction through the heart Normal antegrade conduction in a healthy heart goes from SA node to atria to AV node over the bundle of His and down to the ventricles Some individuals have the ability to conduct retro-grade retrograde conduction is abnormal but not uncommon

autonomic nervous system

The heart does not need direct stimulation from the nervous system to generate electricity and to beat but that is not to say that the nervous system does not control the heart rate The autonomic nervous system indirectly affects cardiac conduction by regulating how fast the heart will beat and how hard the heart muscle will squeeze (contractility) The autonomic ner-vous system communicates with the heart through a network of baroreceptors which respond to chemical signals

The autonomic nervous system of the body like certain other physiological systems exists as two systems in balance The autonomic nervous system consists of a sympathetic system and a parasympathetic system They both work together and in a healthy individual one may overdrive the other during specific times The sympathetic ner-vous system is mainly associated with epinephrine (adrenaline) and it has the ability to speed the heart rate and increase cardiac contractility and can increase the speed by which electrical impulses can cross the AV node The sympathetic nervous system takes over when we are stressed frightened (fight-or-flight response) or exerting ourselves

On the other hand the autonomic nervous system also has a parasympathetic system that is associated with acetylcholine This system can decrease the heart rate decrease cardiac contrac-tility and cause impulses crossing the AV node to be slowed The parasympathetic system might take over when we are sleeping or at rest

18 The nuts and bolts of implantable device therapy pacemakers

the nuts and bolts of the cardiac conduction system

bull The heartrsquos natural pacemaker is the sinoatrial (SA) node which generates an electrical impulse spontaneously about 60ndash100 times a minute in a healthy individual

bull The normal conduction path is from the SA node over the atria to the atrioventricular (AV) node and then via the bundle of His down the right and left bundle branches and into the Purkinje fibers

bull The AV node has the ability to slow down the electrical impulse This serves two main purposes it allows for atrial kick and it protects the ventricles from beating too rapidly

bull In the healthy heart the right and left atria contract simultaneously and then the right and left ventricles contract simultaneously

bull Depolarization and repolarization of myocar-dial cells are driven by positively and negatively charged ions which cross the cell membrane A resting cardiac cell is more negative on the inside than the surrounding media but when it depolarizes it becomes more positive on the inside relative to the surrounding media Following depolarization the ions move and the cell returns to being more negative on the inside than the outside in a process known as repolarization

bull Depolarization causes the cardiac cell to contract

bull The main ions involved in depolarization and repolarization are sodium potassium and calcium ions

bull Although the heartrsquos natural pacemaker is the SA node the AV junction and the Purkinje fibers (ventricles) can also assume (usurp) this role if the SA node cannot generate electricity properly The AV junction and Purkinje fibers are not as reliable as a healthy AV node and typically generate electricity more slowly (the AV junction about 40ndash60 times a minute and the Purkinje fibers 20ndash40 times a minute)

bull However sometimes the AV junction or Purkinje fibers may generate electricity very

quickly this is abnormal When they do they may take over the heart When pacemakers of the heart compete the fastest one drives the heart

bull Automaticity is an inherent property of certain cardiac conductive cells to depolarize spontaneously Not all cardiac conduction cells possess automaticity but the SA node AV junction and Purkinje fibers possess automaticity

bull Refractoriness is the natural and expected behavior of cardiac conductive cells that makes them unresponsive to electrical impulses immediately after they have depolarized The natural refractory period is very brief

bull The faster the heart rate the shorter the refractory period the slower the heart rate the longer the refractory period

bull Refractoriness can also occur as a dysfunction bull Conductivity is the cardiac cellrsquos ability to

transmit an electrical impulse to adjacent cells Refractoriness means that cells that just depolarized will not depolarize again This keeps conduction moving forward (antegrade conduction)

bull Antegrade conduction is normal retrograde conduction is abnormal but is not rare

bull The autonomic nervous system regulates the heart rate by controlling how fast the heart beats and how vigorously it contracts

bull The autonomic nervous system consists of a sympathetic and parasympathetic nervous system which work together but at certain times one or the other is dominant The sympathetic nervous system makes the heart beat faster and more vigorously it takes over during times of stress exertion or fight or flight The parasympathetic ner-vous system makes the heart beat slowly and less vigorously it takes over during periods of rest

bull The neurotransmitters associated with these two systems are epinephrine or adrenaline (sympathetic nervous system) and acetylcho-line (parasympathetic nervous system)

Chapter 2 Cardiac conduction system 19

test your knowledge

Please write in the landmarks of the cardiac conduction system Select from the following (each is used only once)

AV node Bachmanrsquos bundle Bundle of His

Internodal tracts Left bundle branch Purkinje fibers

Right bundle branch Ventricles

1 Which of the following is not considered one of the heartrsquos intrinsic pacemakersA AV junctionB SA nodeC Purkinje fibersD Aorta

2 Depolarization and repolarization involve the movement of charged ions across cardiac cell membranes What types of ions are primarily involvedA Calcium potassium and sodiumB Acetylcholine and epinephrineC Magnesium and folateD All of the above

3 Refractoriness refers toA The ability of cardiac conduction cells to

contract

B The inability of cardiac conduction cells to respond to an electrical stimulus

C The unique permeability of the cardiac cellsD Abnormal cardiac conduction

4 When a cardiac cell depolarizes the interior of the cell becomes what with respect to the surrounding mediaA More positiveB More negativeC More acidicD Hotter

5 Fill in the blank In the healthy heart the faster the heart rate the ___ the refractory periodA More vigorousB Less reliableC LongerD Shorter

20 The nuts and bolts of implantable device therapy pacemakers

6 Which specific part of the nervous system governs the fight-or-flight response and is associated with epinephrine (adrenaline) (Pick the most exact answer)A Autonomic nervous systemB Parasympathetic nervous systemC Sympathetic nervous systemD Central nervous system

7 In normal antegrade conduction the electrical impulse travels from the SA node over the atria to the AV node and to what structure before reaching the ventriclesA The intermodal tractsB The Purkinje fibersC The bundle of HisD The ventricular apex

8 Which of the following is true about retrograde cardiac conductionA Everyone has itB It is perfectly normal and can be helpful in

stressful situations

C It is abnormal but not uncommonD It is impossible to diagnose definitively

except in an autopsy

9 What chemical is associated with the parasympathetic nervous systemA AcetylcholineB PotassiumC EpinephrineD Dopamine

10 The stimulation of the myocardial cells of which of the following structures is most closely associated with ventricular depolarizationA SA nodeB AV junctionC Bundle of HisD Purkinje fibers

vi

Preface

Cardiac pacing has changed dramatically in the past decades and yet certain things have not changed at all While technology has advanced cardiac pacing to provide benefits to more and more patients while streamlining device function and downsizing the pulse generators cardiac pacing is still a field that can be difficult to learn and slow to master Those who become cardiac pacing specialists are those who take the time to learn the concepts work sys-tematically through follow-up and listen carefully to their patients It has been more than three decades since I entered the cardiac rhythm management field and two decades since I wrote my first work-book on pacing to assist those trying to learn the fieldmdashand while everyone talks about how much cardiac pacing has changed in the past decades it strikes me how much has stayed the same

After all it is still about a battery a wire in the heart and timing tiny electrical pulses to fire at precisely the right split-second It is still about knowing rate and mode and being able to find a pacing spike on an ECG And it is still about restoring as near-to-normal a cardiac rhythm as possible to our patients

For those embarking on a career in cardiac pacing or clinicians who just need to know more about device-based cardiac rhythm management therapy it is my hope that this book will help

break down some of the complexities of this cardiac pacing Cardiac pacing can seem overwhelmingly complex because it involves the simultaneous inter-play of many different factors Somebody once told me that there are more than a million pos-sible device parameter combinations available in todayrsquos most advanced pacemakers So how does a busy clinician find that one-in-a-million pacing prescription The number of options may have changed but it is still about the basics You find the right mode The right rate You program an AV delay You just work through the programming one simple step at a time For those willing to learn the concepts and approach things systematically cardiac pacing can be mastered

It has always been about the patient and how that patient interacts with the pacemaker Our patients come to the clinicmdashoften nervous about their new ldquobattery-poweredrdquo statusmdashand expect to find in us clinicians both expertise and reas-surance I hope this book conveys my sentiment that the greatest expert is one who not only knows what to do but also knows how to talk warmly frankly and helpfully to each and every patient

Tom KennyNovember 2014

vii

Acknowledgments

Nobody writes a book alone even if there is just one name on the front cover First I must acknowl-edge with deep gratitude my students past and pre-sent over the years From these men and women I have learned a great deal about the kind of person who seeks a career in device-based therapy and how to break down difficult concepts to make them not just understandable but logical I think every time one of my students had an ldquoah-hardquo moment I had one too I hope that my instruction my expla-nations my anecdotes and my method of teaching serve you well in your future clinical work I would also like to thank my mentors that have taught me so much over the years I know much of what I know because of people like Orlando Maytin Michael Chizner Kathy King Barbara Perra and Eliot Ostrow Thank you for sharing your clinical expertise and sparking my interest in cardiology

Second I want to thank my friends and family who supported me in the writing of this book

I owe a great debt to my publishers who not only believed in this project but who went the extra mile to make it a reality

Third I must thank my editor Jo Ann LeQuang who assisted in all of the work that goes into turning a course into a manuscript She worked with me on my very first workbook when I worked in industry and she was a key contributor to this project as well I want to thank Matt OrsquoNeal Bob Matioli and all of the folks at PrepMD where I work today for their support and encouragement with this project

Finally I want to thank the many people who encouraged me to write about cardiac pacing No matter how much I teach this subject lecture on it or write about itmdashI still have a lot to say That shows the depth of this subject and also my passion for it It is my wish that this book not just informs future pacing experts but inspires them as well

1

The Nuts and Bolts of Implantable Device Therapy Pacemakers First Edition Tom Kenny copy 2015 John Wiley amp Sons Ltd Published 2015 by John Wiley amp Sons Ltd

Chapter 1

Cardiovascular anatomy and physiology

Introduction

An encyclopedia could be written on the anatomy and physiology of the human heart and that is not our purpose Device clinicians must understand the cardiovascular system to understand arrhyth-mias and device therapy This chapter will intro-duce the important concepts of cardiac anatomy and physiology necessary for an understanding of cardiac rhythm management To that end this chapter will describe the chambers valves and major vessels of the heart and how these control the flow of blood in the body Although we think of the heartmdashrightlymdashas a pump it also possesses a com-plex electrical system The cells of the human heart are unique in many ways and how they produce conduct and dissipate electrical energy is very important particularly to pacing Our goal here is to describe the anatomy and physiology of the

healthy heart and cardiovascular system in terms of what device clinicians need to know

the healthy heart

The human heart is a double pump (right and left) that sits in the middle of the chest slightly to the left and rotated so that the right side is more anterior than the left An average adult human heart is relatively large about 13 by 9 by 6 cm and weighing about 300 g The heart is protected by the rib cage and sits directly behind one of the bodyrsquos thickest bones the sternum The bottom of the heart rests on the diaphragm muscle The heart is encased in this protected but somewhat crowded areamdashit also contains the lungs (three lobes on the right two on the left) the stomach and the intestines

The bottom tip of the heart (called the apex) taps up against the chest when the heart contracts By placing his hands on the chest a physician can feel the place where the apex of the heart makes contact with the chest this place is called the point of max-imal impulse (PMI) Knowing the precise location of the PMI can be very useful in treating cardiology patients because the PMI of a healthy heart occurs slightly to the left while the PMI of a person with an enlarged heart is going to occur much farther to the left even off to the side A healthy heart is roughly the size of the fist but when hearts enlarge such as occurs with disease progression the enlargement occurs toward the left Thus PMI can

1

Learning objectives

bullensp Point out the key landmarks in the human heart relevant to cardiac rhythm management

bullensp Name the four chambers of the heart the four valves and the major vessels

bullensp Describe the flow of blood through the heartbullensp Define AV synchrony and explain why it is

importantbullensp State the difference between the bodyrsquos arterial

versus venous systems

2 The nuts and bolts of implantable device therapy pacemakers

provide a fast noninvasive way of determining if and to what degree the heart has enlarged

The left ventricle composes most of the mass of the heart being by far the largest of the four pump-ing chambers A healthy heart circulates about 4ndash6 l of blood a minutemdashwhich is the entire blood volume of the body That means the entire circulating volume of blood in the body moves around every minute or once per beat

The heart consists of four chambers two upper chambers called atria (singular atrium) and two lower and larger chambers called ventricles To understand the healthy heart it is useful to think of the heart in terms of right side (right atrium and right ventricle) and left side (left atrium and left ventricle) The right side of the heart circulates deoxygenated blood to the lungs (where it can be oxygenated) The left side of the heart pumps oxy-genated blood out to the rest of the body (see Figure 11)

The heart is a muscle and consists of four dis-tinct layers The endocardium is the innermost layer and composes a lining for the interior of the heart The epicardium is the outer layer of the heart Between the endocardium and epicardium lies the myocardiummdashthe thickest layermdashwhich is muscle The entire heart is encased in a

liquid-filled sac called the pericardium which acts like a shock absorber for the heart The pericardial sac contains about 15ndash20 cc of pericardial fluid in a healthy individual In the event that fluid builds up to abnormally high levels in the pericardial sac (such as might occur when a lead or catheter inside the heart perforates the endocardium myocar-dium and epicardium and goes exterior to the heart) this fluid can place pressure on the heart in a condition known as cardiac tamponade Since the heart is contained in a relatively small space this pressure can compromise the heartrsquos ability to fill with blood and pump efficiently During device implantation perforation is an important concern because it can lead to cardiac tamponade In the event that perforation results in cardiac tamponade a needle is inserted into the pericardial sac (through the chest wall) to drain the blood Lead perforation does not always result in cardiac tamponade but it is a serious concern

Blood flow through the heart

The heart is a pump and it is located amid a net-work of vessels that carry deoxygenated blood into the right side of the heart and reoxygenated blood into the left side of the heart The flow is actually

Right atrium

Left atrium

Left pulmonaryarteries

Left pulmonaryveins

Left ventricle

Right ventricle

Aorta

Aortic valve

Mitral valve

Tricuspidvalve

Pulmonaryvalve

SA node

AV node

Right pulmonaryarteries

Right pulmonaryveins

Figure 11 Cross section of the heart showing the chambers

Chapter 1 Cardiovascular anatomy and physiology 3

fairly simple Deoxygenated blood enters the right side of the heart and is pumped over to the lungs via the pulmonary arteries and is returned back (as oxygen-rich blood) to the left side of the heart by way of the pulmonary veins (PV) While both right and left sides of the heart contract at the same time as a single unit the right side is busy pumping deoxygenated blood to the lungs while the left side is pumping reoxygenated blood out to the rest of the body

Deoxygenated blood enters the right side of the heart via the superior vena cava (SVC) but once it has become oxygenated again blood is pumped back out from the left side of the heart into the aorta The aorta is the largest vessel in the body and it forms a U shape at the top of the heart These portions of the aorta are called the ascending the descending and the arch Coming off the aortic arch are three main arteries the left subclavian artery the left common carotid artery and the brachiocephalic trunk

To better understand the blood flow through the heart it is important to review the structure of the heart The atria or upper chambers of the heart are smaller have thinner walls and are smoother on the inside than the ventricles Within the ventri-cles is a network of fibrous strands known as trabeculae These structural differences become important in lead implantation within the heart it is much easier to affix or lodge a lead in the trabeculae of the ventricles than to try to anchor the lead to a smooth atrial wall Historically atrial leads have almost always been active-fixation screw-in-type leads while ventricular leads were almost always passive-fixation leads (fins or tines that lodge in the trabeculae) Today active-fixation leads are often used in both chambers since they facilitate lead removal (Figure 12)

Overall blood flow to the heart is discussed right and left sides although it is important to recognize that what happens in the heart that is systole (contraction) and diastole (relaxation) are happening on both sides at the same time The right atrium of the heart receives blood from the SVC the inferior vena cava (IVC) and the coronary sinus (CS) The CS is technically a vein and it has an opening or ostium (sometimes just called os) at the base of the right atrium slightly posterior The CS delivers oxygen-depleted blood

to the right atrium from the coronary arteries that encircle the exterior of the heart The CS is of interest in cardiac resynchronization therapy (CRT) because the left ventricular lead is passed through the CS (counter to the flow of blood) in order to be placed into the coronary vessels to pace the left ventricle CRT is used in patients with heart failure whose hearts have remodeled that is enlarged and changed shape (It may be said that with heart failure the heart changes from the shape of a football to the shape of a basketball) The CS may be relocated in this remodeling which can be challenging in implanting a CRT lead because the physician must first locate the os of the CS and then navigate through it in order to implant the left ventricular lead

Anatomically the heart is dominated by the large muscle mass of the left ventricle which makes up about two-thirds of the heart in terms of weight and volume This greater size is typically ascribed to the fact that the left ventricle must pump blood throughout the whole body whereas the right ventricle only has to pump blood to the lungs The left and right ventricles pump blood to different destinations but the left ventricle is larger and more muscular for a reasonmdashpressure It is impor-tant to review the pressures against which the heart must work to understand cardiac blood flow (Figure 13)

Deoxygenated blood in the right side of the heart must travel over the lungs to pick up oxygen This means that blood in the right ventricle travels across the pulmonary valve into the pulmonary artery and then out toward the lungs The pulmonary valve opens automatically when pressure from the contracting right ventricle forces it open This occurs when the pressure in the right ventricle exceeds the pressure in the pulmonary artery Pressure gradients are key concepts in understanding blood flow Valves are like gates that open and close in response to pressure In general the pressure in the PV is fairly low around 12 mmHg Thus the right ventricle does not need to create a lot of force to open the pulmonary valve

Meanwhile as the left ventricle contracts it cre-ates pressure on the aortic valve leading to the aorta In order to open the aortic valve and pump blood out into the aorta the heart must overcome the pressure in the aortic valve Pressure in the

4 The nuts and bolts of implantable device therapy pacemakers

Right atrium

Left atrium

Left ventricle

Right ventricle

Figure 12 Note that the atria are smooth walled while the ventricles contain a spongelike fibrous network of trabeculae

Left pulmonaryarteries

Left pulmonaryveins

Aorta

Aortic valve

Pulmonaryvalve

Right pulmonaryarteries

Superiorvena cava

Inferiorvena cava

Right pulmonaryveins

Figure 13 The blood flow within the heart takes oxygen-depleted blood from the body into the right atrium where it flows to the right ventricle and is pumped out over the lungs the reoxygenated blood from the lungs is pumped into the left atrium where it flows to the left ventricle and is pumped out via the aorta to the body

Chapter 1 Cardiovascular anatomy and physiology 5

aorta is high around 120 mmHg or ten times higher than the pressure in the PV The left ventricle must therefore work much harder to pump blood than the right ventricle This requires the left ventricle to be larger and more muscular than the right ven-tricle (see Figure 14)

On the right side of the heart blood travels from the right atrium into the right ventricle via the tricuspid valve The tricuspid valve gets its name from its characteristic shape involving three leaves or cusps Attached to these cusps are cords that anchor into the base of the ventricle known as chordae tendineae they look almost like little parachutes The strands of the chordae tendineae attach to tiny papillary muscles These chords attach to the valve leaves at one end and a papil-lary muscle at the other end On the right side of the heart the tricuspid valve is associated with three papillary muscles The purpose of these chords and muscles is to assure that the valve is

effectively closed and opened at the proper times (Figure 15)

The heart can rightly be thought of as a pump but it must be remembered that the heart is also a muscle and all muscles need a steady supply of oxy-gen-rich blood The heart muscle is supplied with blood through a network of coronary arteries that surround the outside of the heart Blockage in a coronary artery results in ischemia which can lead to death of cardiac muscle including the chordae tendineae and papillary muscles While the patient may survive such an ischemic event the damage to the heart may lead to an incompetent valve that is a valve that is no longer able to function effectively

In tracing the blood flow from the right ventricle to the pulmonary artery it should be clear that the blood has to go from down in the right ventricle to up through the pulmonary valve and into the pulmonary artery The blood is able to make this journey because of the pumping pressure of the heart The route the blood takes as it exits the right ventricle and journeys up toward the pulmonary valve is known as the right ventricular outflow tract (RVOT) On the other side of the heart there is also a corresponding left ventricular outflow tract (LVOT) of approximately the same size Cardiac leads are sometimes fixated in the RVOT (see Figure 16)

Blood pumped out of the right ventricle crosses the pulmonary valve and enters the pulmonary artery which splits into two branches right and left The right pulmonary artery takes blood to the right lung while the left pulmonary artery takes blood to the left lung In this respect the pulmonary artery is unique in the body in that it is an artery but it carries deoxygenated blood Blood travels through the lung to the alveoli where it gains oxygen and loses carbon dioxide Once it is reoxygenated the blood gathers into the PV (which are also unique being the only veins to carry oxygenated blood) There are four PV in total the two right-sided PV take oxygen-ated blood from the right lung while the two left-sided PV take oxygenated blood from the left lung and they all bring this reoxygenated blood to the left atrium

The left atrium is smooth walled like the right atrium and although the left atrium is much

Figure 14 The left ventricular is far more muscular than the right ventricle because it must overcome 10 times the pressure of the right ventricle in order to pump blood out via the aortic valve and into the aorta

6 The nuts and bolts of implantable device therapy pacemakers

Right atrium

Left atrium

Left ventricle

Right ventricle

Figure 15 The leaflets of the valves are attached by chordae tendineae at one end and papillary muscles on the other which assure the effective opening and closing of the valves

Right-ventricularoutflow tract (RVOT)

Figure 16 There are two outflow tracts in the heart one associated with the right ventricle and the other the left ventricle These outflow tracts are roughly the same size The illustration shows the right ventricular outflow tract (RVOT) a preferred location for right ventricular lead fixation The left ventricular outflow tract (LVOT) cannot be seen in this illustration as it is posterior

Chapter 1 Cardiovascular anatomy and physiology 7

smaller than the left ventricle it is larger than the right atrium The blood will travel from the left atrium into the left ventricle by way of the mitral valve The mitral valve gets its name from the miter a bishoprsquos hat The mitral valve has two cusps and papillary muscles connected by chordae tendineae Once blood is in the left ventricle a contraction will force it toward the aorta by way of the LVOT In persons with hypertrophic obstructive cardiomy-opathy (HOCM) the septum or wall in the heart can become thick and enlarged effectively narrow-ing the LVOT In the healthy heart the blood flows up the LVOT across the aortic valve and into the aorta From the aorta blood will be directed upward toward the head downward toward the legs and feet and some blood will be redirected back to the coronary arteries of the heart

The blood flow from the left ventricle to the aorta and back to the coronary arteries is unique in the body in that most of that flow occurs during diastole rather than systole Think of it this way as blood is pumped by the left ventricle during systole upward and downward to the brain and the rest of the body some of that blood flows back during diastole and drains into the coronary arteries Although it may seem counterintuitive it is impor-tant to remember that coronary artery perfusion occurs primarily during diastole

Volume valves and pressure

While it is tempting to think of the heart as simply a pump with an electrical system it is more accu-rate to say that the heart moves blood because of variations in volume and pressure

The heart has four valves two are atrioventricular (AV) valves because they connect the atrium to the ventricle and two are semilunar valves connecting the heart with pulmonary arteries or the aorta The right AV valve is the tricuspid while the left AV valve is the mitral valve The flow or movement of blood is guided and controlled by the heartrsquos muscular action in the form of systole (contraction) or diastole (relaxation)

When blood pours into the right atrium it dis-tends and expands the right atrium When this volume of blood creates sufficient pressure it forces the tricuspid valve open Think of it as volume creating pressure and pressure opening

valves When the tricuspid valve opens the blood dumps into the right ventricle During this period of diastole blood pours into the right ventricle creating volume and expanding it As blood pours in and ventricular diastole nears the end the atria contract while the AV valves are still open in something nicknamed atrial kick Atrial systole occurs at the end of ventricular diastole Atrial kick forces the maximum amount of blood from the atria into the ventricles It is estimated that atrial kick delivers 20ndash30 of cardiac output

The atrial contribution to ventricular filling (atrial kick) is of enormous clinical significance Patients with certain atrial arrhythmias such as atrial fibrillation (AF) lose atrial kick Even milder forms of atrial tachyarrhythmias may compromise atrial kick Moreover patients who do not have AV synchrony (one atrial beat corresponding to one ventricular beat) will lose atrial kick The loss of atrial kick can reduce cardiac output by 20ndash30

Once the atria have contracted and the ventricles are filled with blood the AV valves snap closed The ventricles now start to contract and push open the semilunar valves For example as the right ven-tricle contracts (systole) it opens the PV (The PV is sometimes called the pulmonic valve itrsquos the same thing) The whole process works on volume pressure and contractionrelaxation

Of the four valves in the heart the tricuspid valve is of most interest to device specialists because at least one lead will be placed into the right ventricle This means that pacing and defi-brillation leads typically cross the tricuspid valve These leads may interfere with the proper closing of the tricuspid valve particularly if the lead is large diameter or several leads are passed over the tricuspid valve A single lead through the tricuspid valve is likely to cause minimal to no dysfunction However it is not unusual to see patients with three or more leads in the right ventricle and these leads may hamper the tricuspid valve in its closing leading to tricuspid regurgitation or backflow of blood Patients with multiple leads (such as chil-dren who grow up with pacemakers) may need to have some of them removed to avoid tricuspid regurgitation

Right ventricular leads are typically fixated at the right ventricular apex or in the RVOT and may be

8 The nuts and bolts of implantable device therapy pacemakers

lodged in the trabeculae Trabeculae are a fibrous network that could be described as similar to the pores of a sponge (see Figure 17)

On the left side of the heart the blood flow pattern is similar Blood flows into the left atrium over the open mitral valve and into the left atrium At the end of ventricular diastole as the ventricle is distended and stretched atrial kick delivers more blood into left ventricle The volume of blood cre-ates pressure The mitral valve closes As the left ventricle contracts the growing pressure over-comes the resistance on the aortic valve and it opens allowing blood to flow out into the aorta and beyond

While any valve can become diseased or dys-functional valvular disease more commonly affects the mitral and the aortic valves (left-sided valves) than the right-sided valves (tricuspid and pul-monary) These left valves are more likely to suffer damage or disease because of the high-pressure environment in which they function exposing them to more potential damage Other damage can occur when plaque builds up in the coronary arteries and the aorta The aorta can become

diseased typically in the case of an aortic aneurysm or dissection Carotid arteries in the neck are also frequently a site for atherosclerotic disease

the right atrial appendage

The right atrial appendage (RAA) is an area near the right atrium which is a preferred site for attach-ing active-fixation atrial leads (see Figure 18) The RAA serves no obvious purpose and may be sacri-ficed in certain heart surgeries such as a coronary artery bypass graft (CABG) procedure CABG or bypass surgery involves stopping the heart and running the blood through a machine to reoxygen-ate it during the course of the procedure This is done by attaching a large-diameter hose to the RAA when surgery concludes and this hose is removed it can damage the RAA to the point that it is completely or partially surgically removed In patients who have had a CABG procedure the RAA may not be available for right atrial lead placement For such patients the right atrial lead is often placed against the lateral wall of the right atrium It is sometimes possible to fixate a right

Figure 17 Trabeculae form a fibrous network within the ventricles Passive-fixation ventricular leads are held in place by lodging their tines or fins in the trabeculae

Chapter 1 Cardiovascular anatomy and physiology 9

atrial lead in the remnants of the RAA if a portion of it is preserved

arteries and veins

Any discussion about the heart necessarily involves the vasculature which is why we commonly refer to our circulatory system as the cardiovascular

system As most of us remember from anatomy classes arteries carry blood away from the heart while veins transport blood back to the heart (The exceptions are the PV and arteries described earlier) Arteries carry oxygen-rich blood while veins carry deoxygenated blood What is of importance to device clinicians is that arteries are high-pressure vessels while veins are low-pressure vessels Arteries tend to be more muscular in structure and muscular contractions of the arteries help to move blood outward into the body Veins tend to be less muscular than arteries but unlike arteries they contain a system of tiny interior valves The purpose of these little valves is to maintain a unidirectional flow of blood (see Figure 19)

Some of the most important veins in the body are the SVC and its counterpart the IVC The SVC takes blood from the upper part of the body and delivers it into the right atrium with the IVC doing the same for blood from the lower part of the body The coronary veins networked around the outside of the heart carry blood from the heart tissue and deliver it back to the right atrium they are closely linked to the coronary arteries which carry reoxygenated blood from the left ventricle back to the heart muscle The CS is the site where

Right atrial appendage

Figure 18 The right atrial appendage serves no obvious hemodynamic purpose but is often the site of fixation of atrial leads

Artery Vein

Figure 19 Veins and arteries have different purposes different anatomical structure and different pressures Overall veins are low-pressure systems that move blood by a series of little valves that keep blood flowing in the same direction Arteries are high-pressure systems and are more muscular so that they force blood forward by squeezing

10 The nuts and bolts of implantable device therapy pacemakers

the great middle and small cardiac veins all drain the CS takes this blood and delivers it back to the right atrium

The PV carry reoxygenated blood from the lungs and deliver it to the left atrium so that it can be pumped back out to the body by the left ventricle

The most important artery in the body is by far the aorta The aorta is connected to the left ven-tricle and it is the first and main conduit that takes reoxygenated blood from the left ventricle and sends it out to the body As this reoxygenated blood makes its way out into the body its first two stops are the head (brain) and the heart muscle Inadequate oxygenated blood to the brain can very quickly provoke symptoms This is the reason that depressed cardiac output is associated with symptoms like dizziness light-headedness feeling woozy and fatigue Severe symptoms might include syncope

The coronary arteries arise from the root of the aorta just above the cusps of the aortic valve Coronary arteries receive most blood during dias-tole so blood is more or less pouring back into these vessels The coronary artery system is a networkmdashalmost a meshmdashof middle-sized to very small vessels that cover the exterior surface of the heart (see Figure 110)

Coronary arteries get their name from the Latin word for crown (such as in our word coronation) because this network of vessels sits like a crown atop the heart and encircles it The heart gets blood from outside in as oxygen-rich blood in these coro-nary arteries is delivered to the exterior of the heart muscle Coronary arteries are not within the heart they are epicardial structures The two main coro-nary arteries are the right coronary artery (RCA) and the left coronary artery (LCA) The RCA and LCA come off the aorta and are located in the AV groove an exterior structure below the atria and above the ventricles The RCA runs from the aorta in the right AV groove while the LCA comes off the aorta and quickly branches to the left main The left main is a very short artery (about the size of a thumbnail) which branches quickly into the left anterior descending artery (LAD) and the circum-flex artery The LAD travels down the front of the

left side of the heart The circumflex artery can be found in the left AV groove

The RCA bifurcates to the posterior descending artery The right atrium and right ventricle get their supply of oxygen-rich blood from the RCA Much of the cardiac conduction system is right sided that is it commences with the sinoatrial (SA) node in the high right atrium and travels down to the AV node on the right side of the septum These important electrical structures get their oxygen-rich blood supply from the RCA The electrical system travels down the ventricles to the inferior wall of the left ventricle (located at the bottom of the heart but belonging to the left ventricle) the inferior wall is also fed oxygen-rich blood by the RCA An occlusion of the RCA such as might occur in a myocardial infarction (MI) may result in death of the heart muscle it feeds which includes the right atrium the right ventricle and the inferior wall of the left ventricle and it might affect the conduction system including the SA node and the AV node An inferior wall MI is typically the result of a clogged RCA

RCA

LCA

Aorta

Figure 110 The coronary arteries of the heart branch out to surround the entire heart muscle The two most prominent coronary arteries are the right coronary artery (RCA) and the left coronary artery (LCA)

Chapter 1 Cardiovascular anatomy and physiology 11

For an MI the more distal the occlusion (that is further from the aorta) the less potential damage the heart attack will have In other words the worst occlusions are proximal Whether or not a patient survives an MI comes down to the numbers The loss of over 40 of the left ventricular muscle mass results in death However that 40 is cumulative For example a person can survive an MI that costs him 30 of left ventricular muscle mass but if that heart attack is followed by another that kills 15 of the left ventricular muscle mass the second (milder) heart attack will prove fatal This is the reason many people survive a first heart attack but die of a second or third attack even though their fatal MI may be relatively mild

The LCA branches quickly to the left main which branches almost at once to the LAD The LAD runs on the outside of the heart along the front of the heart along the septum The left main supplies the left atrium and the majority of the left ventricle in other words the major pumping por-tion of the heart An occlusion in the left main can be particularly disastrous because it is so far upstream and affects the heartrsquos left ventricular muscle mass No wonder they call the left main the widow maker

The coronary arteries do not run in straight lines they twist and turn and sometimes make very sharp bends People with coronary artery disease (CAD) typically have plaque buildup in the areas where the coronary arteries bend at sharp angles These are areas where the blood flow creates a lot of turbulence and plaque and other substances can collect and build up It is typically to find blockage and occlusions at these places

The LAD supplies the anterior wall of the left ventricle and the septum with oxygen-rich blood Since part of the heartrsquos conduction system runs through the septum (the right and left bundle branches) an LAD occlusion can damage the heartrsquos ability to conduct electricity properly

The circumflex artery runs along the AV groove laterally and thus supplies oxygen-rich blood to the lateral wall of the left ventricle along with the left atrium The circumflex runs along the AV groove to the left heading toward the back of the heart where it meets with the RCA which

travels along the AV groove to the left toward the back of the heart Both the circumflex and the RCA are responsible for providing oxygenated blood to the posterior wall of the ventricle But which is the more important provider That depends on crux and whether the heart is right or left dominant

The crux refers to an imaginary line drawn exactly down the middle of the ventricular poste-rior wall without any regard to anatomical landmarks If the RCA crosses the crux then the heart is said to be right dominant and the RCA is the greater supplier of oxygenated blood to the ven-tricular posterior wall On the other hand if the circumflex crosses the crux then the heart is said to be left dominant and the circumflex is the more significant provider Roughly 60 of the population is right dominant

While this information about the vasculature is important it is even more important for clini-cians to recognize that there is tremendous interpatient variability in terms of venous anatomy While we can describe major structures and typical venous formations anomalies are very common These anomalies may be minor variations or they can be very pronounced such as missing veins and arteries Clinicians who deal with the cardiovascular system must be prepared for large and small differences in vascular anatomy The cardiovascular system is incredibly resilient and can adapt to some anomalies by building up collateral circulation For example a person with a missing or incompetent vessel may over time build up a network of smaller collateral vessels that compensates by doing the same job It is not unusual to see vessels grow larger or branch out to compensate Sometimes when an artery is blocked collateral circulation will build up and compensate by delivering oxygen-rich blood to the heart muscle

Every artery in the body has a matching or corresponding vein usually located in close proximity This applies to the coronary system as well Note that there are great differences in coro-nary venous anatomy among patients which becomes important when implanting left ventric-ular leads for CRT devices (see Figure 111)

12 The nuts and bolts of implantable device therapy pacemakers

the nuts and bolts of cardiovascular anatomy and physiology

bull The heart consists of four chambers which may be considered the upper chambers (right atrium and left atrium) and lower chambers (right ven-tricle and left ventricle) or may be thought of as the right heart (right atrium and right ventricle) and left heart (left atrium and left ventricle)

bull Deoxygenated blood enters the right atrium flows to the right ventricle and is pumped out via the pulmonary valve to the lungs The blood is reoxygenated in the lungs and then enters the left atrium and flows to the left atrium and out of the aortic valve into the aorta and the rest of the body

bull The heart has two outflow tracts a right ven-tricular outflow tract (RVOT) and a left ven-tricular outflow tract (LVOT)

bull The largest chamber of the heart is the left ven-tricle making up about 23 of the cardiac mass The left ventricle is muscular and massive because it must pump against approximately 10 times the pressure against which the right ven-tricle pumps (120 vs 12 mmHg)

bull The two cardiac valves most susceptible to dysfunction and disease are the mitral and aortic valve because they function in a high-pressure environment

bull The chordae tendineae connect the leaflets of the valves to the papillary muscles and look like little parachutes These help the valves open and close properly

bull In general arteries transport blood away from the heart are muscular and contract to help move the blood on its way and have thicker walls than veins Veins have thinner less muscular walls and carry oxygen-depleted blood back to the heart using tiny interior valves to keep the blood flow moving forward Arteries transport oxygenated blood and can be considered high-pressure systems while veins transported deoxygenated blood in a low-pressure system

bull Cardiac contraction is systole cardiac relaxation is diastole Atrial systole occurs at the very end of ventricular diastole in something nicknamed atrial kick In a healthy individual about 20ndash30 of cardiac output comes from atrial kick

bull The heart is a muscle that is fed with oxygenated blood through a network of coronary arteries Coronary arteries get oxygen-rich blood from the aorta and they perfuse the heart muscle during diastole as blood drains back

bull Although arteries and veins generally take blood away from or to the heart respectively

Rightventricle

Leftventricle

Leftatrium

Aorta

Rightatrium

Rightcoronaryartery

Leftcoronaryartery

Circumflexartery

Superiorvena cava

Inferiorvena cava

Arterial system

Rightventricle

Leftventricle

Greatcardiac vein

Smallcardiac vein

Middlecardiac vein

Leftatrium

Aorta

Rightatrium

Superiorvena cava

Inferiorvena cava

Venous system

Coronarysinus

Figure 111 The coronary artery system is matched by a coronary venous system

Chapter 1 Cardiovascular anatomy and physiology 13

test your knowledge

Fill in these anatomical landmarks of the heart They include the four chambers the four valves and the bodyrsquos main artery

the pulmonary vessels are the exception The pulmonary artery transports deoxygenated blood while the pulmonary veins carry oxygen-ated blood This is because of their special loca-tion and the fact that their work is taking blood from the right side to the left side of the heart

bull The main coronary arteries are the right coro-nary artery and the left coronary artery The left coronary artery branches quickly to a short vessel known as the left main (the widow maker) and the circumflex artery

bull A myocardial infarction can kill the heart muscle The clogged arteries determine the loc ation of the affected muscle A right coro-nary artery occlusion will affect the right atrium and ventricle and the inferior wall of the left ventricle while a left coronary artery occlusion will affect the left atrium and most of the left ventricle By far the most dangerous occlusion is in the left main because it feeds the majority of the heart muscle with oxygen-rich blood

answer the following questions

1 What are the names of the two atrioventricular valves of the heartA Atrial and ventricularB Tricuspid and mitralC Pulmonary and pulmonicD Aortic and superior vena cava

2 Which chambers of the heart contain trabeculaeA AtriaB VentriclesC Right-sided chambers onlyD Left-sided chambers only

14 The nuts and bolts of implantable device therapy pacemakers

3 Why can trabeculae be important to device cliniciansA They are good places to secure a passive-fixa-

tion leadB They help regulate blood pressureC They prevent atherosclerosisD All of the above

4 Which of the following is not true about the aortaA It is an arteryB It consists of an ascending portion a descend-

ing portion and an archC It carries only oxygenated bloodD It is the largest vein in the body

5 From which artery does blood enter the right atriumA Superior vena cavaB Inferior vena cavaC Coronary sinusD All of the above

6 What is atrial kickA Ventricular pressure on the tricuspid valve

forcing it shutB Electrical impulses originating in the atrium

to cause the heart to beat fasterC Atrial systole that forces the most blood possible

into the already-full and diastolic ventriclesD A type of arrhythmia

7 Why would a person with chronic AF likely have decreased cardiac outputA AF is associated with low blood volumeB AF causes the heart to beat very slowlyC AF may cause the heart to experience fre-

quent pausesD AF reduces or eliminates atrial kickmdashand

atrial kick contributes to cardiac output

8 Which of the following is often a preferred location for affixing a cardiac pacing leadA The right ventricular outflow tract (RVOT)B The left ventricular outflow tract (LVOT)C The excised right atrial appendageD The tricuspid valve

9 What connects the valve leaflets to the papillary musclesA TrabeculaeB ArteriesC Chordae tendineaeD Tiny valves

10 Which coronary artery is nicknamed the widow maker because an occlusion in this vessel will deprive the majority of the heartrsquos muscle mass of oxygen-rich bloodA Left mainB CircumflexC Right coronary arteryD Inferior vena cava

15

The Nuts and Bolts of Implantable Device Therapy Pacemakers First Edition Tom Kenny copy 2015 John Wiley amp Sons Ltd Published 2015 by John Wiley amp Sons Ltd

Chapter 2

Cardiac conduction system

Introduction

The sinoatrial (SA) node is the heartrsquos natural pacemaker In the healthy heart the SA node fires and the electrical stimulus conducts along special pathways through the heart that cause cells in specific areas to depolarize After the stimulus passes the area the cells repolarize Myocardial cell depolarization and repolarization govern the heartrsquos contraction and relaxation sequencemdashin other words the heart rhythm Rhythm disorders occur when this healthy conduction system is in some way disrupted or impaired

the conduction pathways

When the SA node fires and sends out electrical energy the stimulus travels from the SA node in the high right atrium out over the atria via path-ways known as internodal tracts Most of these internodal tracts are located in the right atrium the

main tract over to the left atrium is called Bachmannrsquos bundle It is because of Bachmannrsquos bundle linking the right atrium to the left atrium that the atria can contract simultaneously that is as these two upper chambers contract as one coherent unit After traversing the atria the impulse comes down through conduction tracts to the atrioventricular (AV) node

The AV node is an area of highly specialized tissue located below the atria and above the ventri-cles on the right side of the septum Anatomically the AV node rests in the AV groove The tissue in the healthy AV node has the ability to change the speed of the electrical impulse specifically it slows the stimulus down The slowdown serves two important purposes First slowing the impulse allows the atria time to contract and provide the very valuable atrial kick to ventricular filling Second the slowed impulse protects the ventricles from being forced to beat too quickly that is to beat before they have been adequately filled with blood

Once the impulse crosses the AV node it travels down into the ventricles and accelerates its speed The impulse travels through the bundle of His (pronounced hiss) to the bundle branches (right and left) and then out to the network of Purkinje fibers This allows both ventricles to depolarize and contract simultaneously (Figure 21)

polarization depolarization and repolarization

Cardiac conduction is driven by the ability of myo-cardial cells to be depolarized and repolarized Depolarization occurs at the cellular level and is

2

Learning objectives

bullensp Describe the electrical conduction system of the healthy heart

bullensp List the main anatomical landmarks of normal cardiac conduction

bullensp Name the anatomical location of the heartrsquos natural pacemaker

bullensp Explain depolarization and repolarization in electrical terms and how it relates to the cardiac contraction

bullensp Define the term automaticity and how it relates to cardiac conduction

16 The nuts and bolts of implantable device therapy pacemakers

in turn driven by electrolytes or positively and negatively charged ions (potassium sodium and calcium mainly) which are able to move in and out of the cardiac cells A myocardial cell may be said to be polarized when in its resting state that is there is no electrical activity going on In the rest-ing state the interior of the myocardial cell is somewhat more negatively charged that the outside This state is called the resting state or sometimes phase 0 of the action potential

An electrical stimulus changes the resting state abruptly The electrical energy allows for the movement of ions to the point that the interior of the myocardial cell is now more positive than the outside In other words the charge reverses and the cell is said to be depolarized When this happens the cell contracts when this happens to a large area of the myocardium the heart muscle contracts Cardiac contraction occurs as a result of cellular depolarization

When the electrical stimulus moves past the myocardial cell the ions move in reverse and the interior of the cell assumes its previous negative charge with respect to the outside This process is

called repolarization and it brings the myocar-dial cell back to the resting state When this happens the cell relaxes When a large area of the myocardium repolarizes the heart resumes its previous shape (loses its contraction)

Intrinsic pacemakers

The healthy heartrsquos natural pacemaker is the SA node but the SA node is not the only area of the heart that can generate an electrical pulse The SA node serves as the heartrsquos pacemaker because it generates these pulses faster than other areas In the healthy heart the SA node can fire at around 60ndash100 times a minute corresponding to a healthy heart rate The AV junction can also fire but it is much slower at around 40ndash60 times a minute Even the Purkinje fibers (the ventri-cles) can act as a pacemaker but they fire at only 20ndash40 times a minute Thus in a healthy individual the SA node outpaces these other potential pacemakers The AV junction and ven-tricles are sometimes called backup pacemakers because they take over the heart rate in the event

Sinoatrial (SA) node

Internodal tracts

Right bundle branch

Left bundle branch

Purkinje fibers

Bachmannrsquos bundle

Bundle of His

Atrioventricular (AV) node

Figure 21 The conduction pathways in the healthy heart

Chapter 2 Cardiac conduction system 17

that the SA node is not able to pace the heart While these backup pacemakers may help keep the heart beating in the event of a dysfunctional SA node they are slow and not as reliable as the SA node

The healthy heart is able to pace itself because of a property called automaticity Automaticity is an inherent property of individual cardiac conductive cells that allows them to depolarize spontaneously In other words some cells in the heart (such as the cells of the SA node) are able to generate an electrical stimulus on their own

refractoriness

Refractoriness refers to the temporary inability of a cardiac cell to respond to an electrical stimulus Refractoriness can be the normal appropriate and expected behavior of cardiac cells For example immediately after electrical energy has passed through cardiac cells they go through a phase where they are unresponsive to electrical energy no matter how much voltage is delivered This normal refractoriness is called the physiological refractory period and it is very brief (it is mea-sured in thousandths of a second) Of course the exact duration of a refractory period is based in part on the heart rate the faster the heart rate the shorter the refractory period and vice versa

Refractoriness may also occur when certain cells become dysfunctional that is cardiac cells may lose their ability to conduct properly Furthermore the heartrsquos inherent pacemakers may change their rate for instance the AV junction may speed up and start to deliver impulses at 80 or 100 times a minute When this happensmdashthat is when another pacemaker takes over the role of the SA nodemdashthat pacemaker is called a usurping pacemaker because it usurps the rightful role of the SA node In the event that other pacemakers of the heart start to compete with the SA node the fastest pacemaker always wins

Conductivity

Conductivity is the cardiac cellrsquos ability to transmit an electrical impulse to an adjacent cell Typically cardiac conduction is antegrade that

is it moves forward Electrical impulses actually travel out in all different directions but refracto-riness prevents those cells that were just stimu-lated from depolarization This keeps conduction moving in a single direction through the heart Normal antegrade conduction in a healthy heart goes from SA node to atria to AV node over the bundle of His and down to the ventricles Some individuals have the ability to conduct retro-grade retrograde conduction is abnormal but not uncommon

autonomic nervous system

The heart does not need direct stimulation from the nervous system to generate electricity and to beat but that is not to say that the nervous system does not control the heart rate The autonomic nervous system indirectly affects cardiac conduction by regulating how fast the heart will beat and how hard the heart muscle will squeeze (contractility) The autonomic ner-vous system communicates with the heart through a network of baroreceptors which respond to chemical signals

The autonomic nervous system of the body like certain other physiological systems exists as two systems in balance The autonomic nervous system consists of a sympathetic system and a parasympathetic system They both work together and in a healthy individual one may overdrive the other during specific times The sympathetic ner-vous system is mainly associated with epinephrine (adrenaline) and it has the ability to speed the heart rate and increase cardiac contractility and can increase the speed by which electrical impulses can cross the AV node The sympathetic nervous system takes over when we are stressed frightened (fight-or-flight response) or exerting ourselves

On the other hand the autonomic nervous system also has a parasympathetic system that is associated with acetylcholine This system can decrease the heart rate decrease cardiac contrac-tility and cause impulses crossing the AV node to be slowed The parasympathetic system might take over when we are sleeping or at rest

18 The nuts and bolts of implantable device therapy pacemakers

the nuts and bolts of the cardiac conduction system

bull The heartrsquos natural pacemaker is the sinoatrial (SA) node which generates an electrical impulse spontaneously about 60ndash100 times a minute in a healthy individual

bull The normal conduction path is from the SA node over the atria to the atrioventricular (AV) node and then via the bundle of His down the right and left bundle branches and into the Purkinje fibers

bull The AV node has the ability to slow down the electrical impulse This serves two main purposes it allows for atrial kick and it protects the ventricles from beating too rapidly

bull In the healthy heart the right and left atria contract simultaneously and then the right and left ventricles contract simultaneously

bull Depolarization and repolarization of myocar-dial cells are driven by positively and negatively charged ions which cross the cell membrane A resting cardiac cell is more negative on the inside than the surrounding media but when it depolarizes it becomes more positive on the inside relative to the surrounding media Following depolarization the ions move and the cell returns to being more negative on the inside than the outside in a process known as repolarization

bull Depolarization causes the cardiac cell to contract

bull The main ions involved in depolarization and repolarization are sodium potassium and calcium ions

bull Although the heartrsquos natural pacemaker is the SA node the AV junction and the Purkinje fibers (ventricles) can also assume (usurp) this role if the SA node cannot generate electricity properly The AV junction and Purkinje fibers are not as reliable as a healthy AV node and typically generate electricity more slowly (the AV junction about 40ndash60 times a minute and the Purkinje fibers 20ndash40 times a minute)

bull However sometimes the AV junction or Purkinje fibers may generate electricity very

quickly this is abnormal When they do they may take over the heart When pacemakers of the heart compete the fastest one drives the heart

bull Automaticity is an inherent property of certain cardiac conductive cells to depolarize spontaneously Not all cardiac conduction cells possess automaticity but the SA node AV junction and Purkinje fibers possess automaticity

bull Refractoriness is the natural and expected behavior of cardiac conductive cells that makes them unresponsive to electrical impulses immediately after they have depolarized The natural refractory period is very brief

bull The faster the heart rate the shorter the refractory period the slower the heart rate the longer the refractory period

bull Refractoriness can also occur as a dysfunction bull Conductivity is the cardiac cellrsquos ability to

transmit an electrical impulse to adjacent cells Refractoriness means that cells that just depolarized will not depolarize again This keeps conduction moving forward (antegrade conduction)

bull Antegrade conduction is normal retrograde conduction is abnormal but is not rare

bull The autonomic nervous system regulates the heart rate by controlling how fast the heart beats and how vigorously it contracts

bull The autonomic nervous system consists of a sympathetic and parasympathetic nervous system which work together but at certain times one or the other is dominant The sympathetic nervous system makes the heart beat faster and more vigorously it takes over during times of stress exertion or fight or flight The parasympathetic ner-vous system makes the heart beat slowly and less vigorously it takes over during periods of rest

bull The neurotransmitters associated with these two systems are epinephrine or adrenaline (sympathetic nervous system) and acetylcho-line (parasympathetic nervous system)

Chapter 2 Cardiac conduction system 19

test your knowledge

Please write in the landmarks of the cardiac conduction system Select from the following (each is used only once)

AV node Bachmanrsquos bundle Bundle of His

Internodal tracts Left bundle branch Purkinje fibers

Right bundle branch Ventricles

1 Which of the following is not considered one of the heartrsquos intrinsic pacemakersA AV junctionB SA nodeC Purkinje fibersD Aorta

2 Depolarization and repolarization involve the movement of charged ions across cardiac cell membranes What types of ions are primarily involvedA Calcium potassium and sodiumB Acetylcholine and epinephrineC Magnesium and folateD All of the above

3 Refractoriness refers toA The ability of cardiac conduction cells to

contract

B The inability of cardiac conduction cells to respond to an electrical stimulus

C The unique permeability of the cardiac cellsD Abnormal cardiac conduction

4 When a cardiac cell depolarizes the interior of the cell becomes what with respect to the surrounding mediaA More positiveB More negativeC More acidicD Hotter

5 Fill in the blank In the healthy heart the faster the heart rate the ___ the refractory periodA More vigorousB Less reliableC LongerD Shorter

20 The nuts and bolts of implantable device therapy pacemakers

6 Which specific part of the nervous system governs the fight-or-flight response and is associated with epinephrine (adrenaline) (Pick the most exact answer)A Autonomic nervous systemB Parasympathetic nervous systemC Sympathetic nervous systemD Central nervous system

7 In normal antegrade conduction the electrical impulse travels from the SA node over the atria to the AV node and to what structure before reaching the ventriclesA The intermodal tractsB The Purkinje fibersC The bundle of HisD The ventricular apex

8 Which of the following is true about retrograde cardiac conductionA Everyone has itB It is perfectly normal and can be helpful in

stressful situations

C It is abnormal but not uncommonD It is impossible to diagnose definitively

except in an autopsy

9 What chemical is associated with the parasympathetic nervous systemA AcetylcholineB PotassiumC EpinephrineD Dopamine

10 The stimulation of the myocardial cells of which of the following structures is most closely associated with ventricular depolarizationA SA nodeB AV junctionC Bundle of HisD Purkinje fibers

vii

Acknowledgments

Nobody writes a book alone even if there is just one name on the front cover First I must acknowl-edge with deep gratitude my students past and pre-sent over the years From these men and women I have learned a great deal about the kind of person who seeks a career in device-based therapy and how to break down difficult concepts to make them not just understandable but logical I think every time one of my students had an ldquoah-hardquo moment I had one too I hope that my instruction my expla-nations my anecdotes and my method of teaching serve you well in your future clinical work I would also like to thank my mentors that have taught me so much over the years I know much of what I know because of people like Orlando Maytin Michael Chizner Kathy King Barbara Perra and Eliot Ostrow Thank you for sharing your clinical expertise and sparking my interest in cardiology

Second I want to thank my friends and family who supported me in the writing of this book

I owe a great debt to my publishers who not only believed in this project but who went the extra mile to make it a reality

Third I must thank my editor Jo Ann LeQuang who assisted in all of the work that goes into turning a course into a manuscript She worked with me on my very first workbook when I worked in industry and she was a key contributor to this project as well I want to thank Matt OrsquoNeal Bob Matioli and all of the folks at PrepMD where I work today for their support and encouragement with this project

Finally I want to thank the many people who encouraged me to write about cardiac pacing No matter how much I teach this subject lecture on it or write about itmdashI still have a lot to say That shows the depth of this subject and also my passion for it It is my wish that this book not just informs future pacing experts but inspires them as well

1

The Nuts and Bolts of Implantable Device Therapy Pacemakers First Edition Tom Kenny copy 2015 John Wiley amp Sons Ltd Published 2015 by John Wiley amp Sons Ltd

Chapter 1

Cardiovascular anatomy and physiology

Introduction

An encyclopedia could be written on the anatomy and physiology of the human heart and that is not our purpose Device clinicians must understand the cardiovascular system to understand arrhyth-mias and device therapy This chapter will intro-duce the important concepts of cardiac anatomy and physiology necessary for an understanding of cardiac rhythm management To that end this chapter will describe the chambers valves and major vessels of the heart and how these control the flow of blood in the body Although we think of the heartmdashrightlymdashas a pump it also possesses a com-plex electrical system The cells of the human heart are unique in many ways and how they produce conduct and dissipate electrical energy is very important particularly to pacing Our goal here is to describe the anatomy and physiology of the

healthy heart and cardiovascular system in terms of what device clinicians need to know

the healthy heart

The human heart is a double pump (right and left) that sits in the middle of the chest slightly to the left and rotated so that the right side is more anterior than the left An average adult human heart is relatively large about 13 by 9 by 6 cm and weighing about 300 g The heart is protected by the rib cage and sits directly behind one of the bodyrsquos thickest bones the sternum The bottom of the heart rests on the diaphragm muscle The heart is encased in this protected but somewhat crowded areamdashit also contains the lungs (three lobes on the right two on the left) the stomach and the intestines

The bottom tip of the heart (called the apex) taps up against the chest when the heart contracts By placing his hands on the chest a physician can feel the place where the apex of the heart makes contact with the chest this place is called the point of max-imal impulse (PMI) Knowing the precise location of the PMI can be very useful in treating cardiology patients because the PMI of a healthy heart occurs slightly to the left while the PMI of a person with an enlarged heart is going to occur much farther to the left even off to the side A healthy heart is roughly the size of the fist but when hearts enlarge such as occurs with disease progression the enlargement occurs toward the left Thus PMI can

1

Learning objectives

bullensp Point out the key landmarks in the human heart relevant to cardiac rhythm management

bullensp Name the four chambers of the heart the four valves and the major vessels

bullensp Describe the flow of blood through the heartbullensp Define AV synchrony and explain why it is

importantbullensp State the difference between the bodyrsquos arterial

versus venous systems

2 The nuts and bolts of implantable device therapy pacemakers

provide a fast noninvasive way of determining if and to what degree the heart has enlarged

The left ventricle composes most of the mass of the heart being by far the largest of the four pump-ing chambers A healthy heart circulates about 4ndash6 l of blood a minutemdashwhich is the entire blood volume of the body That means the entire circulating volume of blood in the body moves around every minute or once per beat

The heart consists of four chambers two upper chambers called atria (singular atrium) and two lower and larger chambers called ventricles To understand the healthy heart it is useful to think of the heart in terms of right side (right atrium and right ventricle) and left side (left atrium and left ventricle) The right side of the heart circulates deoxygenated blood to the lungs (where it can be oxygenated) The left side of the heart pumps oxy-genated blood out to the rest of the body (see Figure 11)

The heart is a muscle and consists of four dis-tinct layers The endocardium is the innermost layer and composes a lining for the interior of the heart The epicardium is the outer layer of the heart Between the endocardium and epicardium lies the myocardiummdashthe thickest layermdashwhich is muscle The entire heart is encased in a

liquid-filled sac called the pericardium which acts like a shock absorber for the heart The pericardial sac contains about 15ndash20 cc of pericardial fluid in a healthy individual In the event that fluid builds up to abnormally high levels in the pericardial sac (such as might occur when a lead or catheter inside the heart perforates the endocardium myocar-dium and epicardium and goes exterior to the heart) this fluid can place pressure on the heart in a condition known as cardiac tamponade Since the heart is contained in a relatively small space this pressure can compromise the heartrsquos ability to fill with blood and pump efficiently During device implantation perforation is an important concern because it can lead to cardiac tamponade In the event that perforation results in cardiac tamponade a needle is inserted into the pericardial sac (through the chest wall) to drain the blood Lead perforation does not always result in cardiac tamponade but it is a serious concern

Blood flow through the heart

The heart is a pump and it is located amid a net-work of vessels that carry deoxygenated blood into the right side of the heart and reoxygenated blood into the left side of the heart The flow is actually

Right atrium

Left atrium

Left pulmonaryarteries

Left pulmonaryveins

Left ventricle

Right ventricle

Aorta

Aortic valve

Mitral valve

Tricuspidvalve

Pulmonaryvalve

SA node

AV node

Right pulmonaryarteries

Right pulmonaryveins

Figure 11 Cross section of the heart showing the chambers

Chapter 1 Cardiovascular anatomy and physiology 3

fairly simple Deoxygenated blood enters the right side of the heart and is pumped over to the lungs via the pulmonary arteries and is returned back (as oxygen-rich blood) to the left side of the heart by way of the pulmonary veins (PV) While both right and left sides of the heart contract at the same time as a single unit the right side is busy pumping deoxygenated blood to the lungs while the left side is pumping reoxygenated blood out to the rest of the body

Deoxygenated blood enters the right side of the heart via the superior vena cava (SVC) but once it has become oxygenated again blood is pumped back out from the left side of the heart into the aorta The aorta is the largest vessel in the body and it forms a U shape at the top of the heart These portions of the aorta are called the ascending the descending and the arch Coming off the aortic arch are three main arteries the left subclavian artery the left common carotid artery and the brachiocephalic trunk

To better understand the blood flow through the heart it is important to review the structure of the heart The atria or upper chambers of the heart are smaller have thinner walls and are smoother on the inside than the ventricles Within the ventri-cles is a network of fibrous strands known as trabeculae These structural differences become important in lead implantation within the heart it is much easier to affix or lodge a lead in the trabeculae of the ventricles than to try to anchor the lead to a smooth atrial wall Historically atrial leads have almost always been active-fixation screw-in-type leads while ventricular leads were almost always passive-fixation leads (fins or tines that lodge in the trabeculae) Today active-fixation leads are often used in both chambers since they facilitate lead removal (Figure 12)

Overall blood flow to the heart is discussed right and left sides although it is important to recognize that what happens in the heart that is systole (contraction) and diastole (relaxation) are happening on both sides at the same time The right atrium of the heart receives blood from the SVC the inferior vena cava (IVC) and the coronary sinus (CS) The CS is technically a vein and it has an opening or ostium (sometimes just called os) at the base of the right atrium slightly posterior The CS delivers oxygen-depleted blood

to the right atrium from the coronary arteries that encircle the exterior of the heart The CS is of interest in cardiac resynchronization therapy (CRT) because the left ventricular lead is passed through the CS (counter to the flow of blood) in order to be placed into the coronary vessels to pace the left ventricle CRT is used in patients with heart failure whose hearts have remodeled that is enlarged and changed shape (It may be said that with heart failure the heart changes from the shape of a football to the shape of a basketball) The CS may be relocated in this remodeling which can be challenging in implanting a CRT lead because the physician must first locate the os of the CS and then navigate through it in order to implant the left ventricular lead

Anatomically the heart is dominated by the large muscle mass of the left ventricle which makes up about two-thirds of the heart in terms of weight and volume This greater size is typically ascribed to the fact that the left ventricle must pump blood throughout the whole body whereas the right ventricle only has to pump blood to the lungs The left and right ventricles pump blood to different destinations but the left ventricle is larger and more muscular for a reasonmdashpressure It is impor-tant to review the pressures against which the heart must work to understand cardiac blood flow (Figure 13)

Deoxygenated blood in the right side of the heart must travel over the lungs to pick up oxygen This means that blood in the right ventricle travels across the pulmonary valve into the pulmonary artery and then out toward the lungs The pulmonary valve opens automatically when pressure from the contracting right ventricle forces it open This occurs when the pressure in the right ventricle exceeds the pressure in the pulmonary artery Pressure gradients are key concepts in understanding blood flow Valves are like gates that open and close in response to pressure In general the pressure in the PV is fairly low around 12 mmHg Thus the right ventricle does not need to create a lot of force to open the pulmonary valve

Meanwhile as the left ventricle contracts it cre-ates pressure on the aortic valve leading to the aorta In order to open the aortic valve and pump blood out into the aorta the heart must overcome the pressure in the aortic valve Pressure in the

4 The nuts and bolts of implantable device therapy pacemakers

Right atrium

Left atrium

Left ventricle

Right ventricle

Figure 12 Note that the atria are smooth walled while the ventricles contain a spongelike fibrous network of trabeculae

Left pulmonaryarteries

Left pulmonaryveins

Aorta

Aortic valve

Pulmonaryvalve

Right pulmonaryarteries

Superiorvena cava

Inferiorvena cava

Right pulmonaryveins

Figure 13 The blood flow within the heart takes oxygen-depleted blood from the body into the right atrium where it flows to the right ventricle and is pumped out over the lungs the reoxygenated blood from the lungs is pumped into the left atrium where it flows to the left ventricle and is pumped out via the aorta to the body

Chapter 1 Cardiovascular anatomy and physiology 5

aorta is high around 120 mmHg or ten times higher than the pressure in the PV The left ventricle must therefore work much harder to pump blood than the right ventricle This requires the left ventricle to be larger and more muscular than the right ven-tricle (see Figure 14)

On the right side of the heart blood travels from the right atrium into the right ventricle via the tricuspid valve The tricuspid valve gets its name from its characteristic shape involving three leaves or cusps Attached to these cusps are cords that anchor into the base of the ventricle known as chordae tendineae they look almost like little parachutes The strands of the chordae tendineae attach to tiny papillary muscles These chords attach to the valve leaves at one end and a papil-lary muscle at the other end On the right side of the heart the tricuspid valve is associated with three papillary muscles The purpose of these chords and muscles is to assure that the valve is

effectively closed and opened at the proper times (Figure 15)

The heart can rightly be thought of as a pump but it must be remembered that the heart is also a muscle and all muscles need a steady supply of oxy-gen-rich blood The heart muscle is supplied with blood through a network of coronary arteries that surround the outside of the heart Blockage in a coronary artery results in ischemia which can lead to death of cardiac muscle including the chordae tendineae and papillary muscles While the patient may survive such an ischemic event the damage to the heart may lead to an incompetent valve that is a valve that is no longer able to function effectively

In tracing the blood flow from the right ventricle to the pulmonary artery it should be clear that the blood has to go from down in the right ventricle to up through the pulmonary valve and into the pulmonary artery The blood is able to make this journey because of the pumping pressure of the heart The route the blood takes as it exits the right ventricle and journeys up toward the pulmonary valve is known as the right ventricular outflow tract (RVOT) On the other side of the heart there is also a corresponding left ventricular outflow tract (LVOT) of approximately the same size Cardiac leads are sometimes fixated in the RVOT (see Figure 16)

Blood pumped out of the right ventricle crosses the pulmonary valve and enters the pulmonary artery which splits into two branches right and left The right pulmonary artery takes blood to the right lung while the left pulmonary artery takes blood to the left lung In this respect the pulmonary artery is unique in the body in that it is an artery but it carries deoxygenated blood Blood travels through the lung to the alveoli where it gains oxygen and loses carbon dioxide Once it is reoxygenated the blood gathers into the PV (which are also unique being the only veins to carry oxygenated blood) There are four PV in total the two right-sided PV take oxygen-ated blood from the right lung while the two left-sided PV take oxygenated blood from the left lung and they all bring this reoxygenated blood to the left atrium

The left atrium is smooth walled like the right atrium and although the left atrium is much

Figure 14 The left ventricular is far more muscular than the right ventricle because it must overcome 10 times the pressure of the right ventricle in order to pump blood out via the aortic valve and into the aorta

6 The nuts and bolts of implantable device therapy pacemakers

Right atrium

Left atrium

Left ventricle

Right ventricle

Figure 15 The leaflets of the valves are attached by chordae tendineae at one end and papillary muscles on the other which assure the effective opening and closing of the valves

Right-ventricularoutflow tract (RVOT)

Figure 16 There are two outflow tracts in the heart one associated with the right ventricle and the other the left ventricle These outflow tracts are roughly the same size The illustration shows the right ventricular outflow tract (RVOT) a preferred location for right ventricular lead fixation The left ventricular outflow tract (LVOT) cannot be seen in this illustration as it is posterior

Chapter 1 Cardiovascular anatomy and physiology 7

smaller than the left ventricle it is larger than the right atrium The blood will travel from the left atrium into the left ventricle by way of the mitral valve The mitral valve gets its name from the miter a bishoprsquos hat The mitral valve has two cusps and papillary muscles connected by chordae tendineae Once blood is in the left ventricle a contraction will force it toward the aorta by way of the LVOT In persons with hypertrophic obstructive cardiomy-opathy (HOCM) the septum or wall in the heart can become thick and enlarged effectively narrow-ing the LVOT In the healthy heart the blood flows up the LVOT across the aortic valve and into the aorta From the aorta blood will be directed upward toward the head downward toward the legs and feet and some blood will be redirected back to the coronary arteries of the heart

The blood flow from the left ventricle to the aorta and back to the coronary arteries is unique in the body in that most of that flow occurs during diastole rather than systole Think of it this way as blood is pumped by the left ventricle during systole upward and downward to the brain and the rest of the body some of that blood flows back during diastole and drains into the coronary arteries Although it may seem counterintuitive it is impor-tant to remember that coronary artery perfusion occurs primarily during diastole

Volume valves and pressure

While it is tempting to think of the heart as simply a pump with an electrical system it is more accu-rate to say that the heart moves blood because of variations in volume and pressure

The heart has four valves two are atrioventricular (AV) valves because they connect the atrium to the ventricle and two are semilunar valves connecting the heart with pulmonary arteries or the aorta The right AV valve is the tricuspid while the left AV valve is the mitral valve The flow or movement of blood is guided and controlled by the heartrsquos muscular action in the form of systole (contraction) or diastole (relaxation)

When blood pours into the right atrium it dis-tends and expands the right atrium When this volume of blood creates sufficient pressure it forces the tricuspid valve open Think of it as volume creating pressure and pressure opening

valves When the tricuspid valve opens the blood dumps into the right ventricle During this period of diastole blood pours into the right ventricle creating volume and expanding it As blood pours in and ventricular diastole nears the end the atria contract while the AV valves are still open in something nicknamed atrial kick Atrial systole occurs at the end of ventricular diastole Atrial kick forces the maximum amount of blood from the atria into the ventricles It is estimated that atrial kick delivers 20ndash30 of cardiac output

The atrial contribution to ventricular filling (atrial kick) is of enormous clinical significance Patients with certain atrial arrhythmias such as atrial fibrillation (AF) lose atrial kick Even milder forms of atrial tachyarrhythmias may compromise atrial kick Moreover patients who do not have AV synchrony (one atrial beat corresponding to one ventricular beat) will lose atrial kick The loss of atrial kick can reduce cardiac output by 20ndash30

Once the atria have contracted and the ventricles are filled with blood the AV valves snap closed The ventricles now start to contract and push open the semilunar valves For example as the right ven-tricle contracts (systole) it opens the PV (The PV is sometimes called the pulmonic valve itrsquos the same thing) The whole process works on volume pressure and contractionrelaxation

Of the four valves in the heart the tricuspid valve is of most interest to device specialists because at least one lead will be placed into the right ventricle This means that pacing and defi-brillation leads typically cross the tricuspid valve These leads may interfere with the proper closing of the tricuspid valve particularly if the lead is large diameter or several leads are passed over the tricuspid valve A single lead through the tricuspid valve is likely to cause minimal to no dysfunction However it is not unusual to see patients with three or more leads in the right ventricle and these leads may hamper the tricuspid valve in its closing leading to tricuspid regurgitation or backflow of blood Patients with multiple leads (such as chil-dren who grow up with pacemakers) may need to have some of them removed to avoid tricuspid regurgitation

Right ventricular leads are typically fixated at the right ventricular apex or in the RVOT and may be

8 The nuts and bolts of implantable device therapy pacemakers

lodged in the trabeculae Trabeculae are a fibrous network that could be described as similar to the pores of a sponge (see Figure 17)

On the left side of the heart the blood flow pattern is similar Blood flows into the left atrium over the open mitral valve and into the left atrium At the end of ventricular diastole as the ventricle is distended and stretched atrial kick delivers more blood into left ventricle The volume of blood cre-ates pressure The mitral valve closes As the left ventricle contracts the growing pressure over-comes the resistance on the aortic valve and it opens allowing blood to flow out into the aorta and beyond

While any valve can become diseased or dys-functional valvular disease more commonly affects the mitral and the aortic valves (left-sided valves) than the right-sided valves (tricuspid and pul-monary) These left valves are more likely to suffer damage or disease because of the high-pressure environment in which they function exposing them to more potential damage Other damage can occur when plaque builds up in the coronary arteries and the aorta The aorta can become

diseased typically in the case of an aortic aneurysm or dissection Carotid arteries in the neck are also frequently a site for atherosclerotic disease

the right atrial appendage

The right atrial appendage (RAA) is an area near the right atrium which is a preferred site for attach-ing active-fixation atrial leads (see Figure 18) The RAA serves no obvious purpose and may be sacri-ficed in certain heart surgeries such as a coronary artery bypass graft (CABG) procedure CABG or bypass surgery involves stopping the heart and running the blood through a machine to reoxygen-ate it during the course of the procedure This is done by attaching a large-diameter hose to the RAA when surgery concludes and this hose is removed it can damage the RAA to the point that it is completely or partially surgically removed In patients who have had a CABG procedure the RAA may not be available for right atrial lead placement For such patients the right atrial lead is often placed against the lateral wall of the right atrium It is sometimes possible to fixate a right

Figure 17 Trabeculae form a fibrous network within the ventricles Passive-fixation ventricular leads are held in place by lodging their tines or fins in the trabeculae

Chapter 1 Cardiovascular anatomy and physiology 9

atrial lead in the remnants of the RAA if a portion of it is preserved

arteries and veins

Any discussion about the heart necessarily involves the vasculature which is why we commonly refer to our circulatory system as the cardiovascular

system As most of us remember from anatomy classes arteries carry blood away from the heart while veins transport blood back to the heart (The exceptions are the PV and arteries described earlier) Arteries carry oxygen-rich blood while veins carry deoxygenated blood What is of importance to device clinicians is that arteries are high-pressure vessels while veins are low-pressure vessels Arteries tend to be more muscular in structure and muscular contractions of the arteries help to move blood outward into the body Veins tend to be less muscular than arteries but unlike arteries they contain a system of tiny interior valves The purpose of these little valves is to maintain a unidirectional flow of blood (see Figure 19)

Some of the most important veins in the body are the SVC and its counterpart the IVC The SVC takes blood from the upper part of the body and delivers it into the right atrium with the IVC doing the same for blood from the lower part of the body The coronary veins networked around the outside of the heart carry blood from the heart tissue and deliver it back to the right atrium they are closely linked to the coronary arteries which carry reoxygenated blood from the left ventricle back to the heart muscle The CS is the site where

Right atrial appendage

Figure 18 The right atrial appendage serves no obvious hemodynamic purpose but is often the site of fixation of atrial leads

Artery Vein

Figure 19 Veins and arteries have different purposes different anatomical structure and different pressures Overall veins are low-pressure systems that move blood by a series of little valves that keep blood flowing in the same direction Arteries are high-pressure systems and are more muscular so that they force blood forward by squeezing

10 The nuts and bolts of implantable device therapy pacemakers

the great middle and small cardiac veins all drain the CS takes this blood and delivers it back to the right atrium

The PV carry reoxygenated blood from the lungs and deliver it to the left atrium so that it can be pumped back out to the body by the left ventricle

The most important artery in the body is by far the aorta The aorta is connected to the left ven-tricle and it is the first and main conduit that takes reoxygenated blood from the left ventricle and sends it out to the body As this reoxygenated blood makes its way out into the body its first two stops are the head (brain) and the heart muscle Inadequate oxygenated blood to the brain can very quickly provoke symptoms This is the reason that depressed cardiac output is associated with symptoms like dizziness light-headedness feeling woozy and fatigue Severe symptoms might include syncope

The coronary arteries arise from the root of the aorta just above the cusps of the aortic valve Coronary arteries receive most blood during dias-tole so blood is more or less pouring back into these vessels The coronary artery system is a networkmdashalmost a meshmdashof middle-sized to very small vessels that cover the exterior surface of the heart (see Figure 110)

Coronary arteries get their name from the Latin word for crown (such as in our word coronation) because this network of vessels sits like a crown atop the heart and encircles it The heart gets blood from outside in as oxygen-rich blood in these coro-nary arteries is delivered to the exterior of the heart muscle Coronary arteries are not within the heart they are epicardial structures The two main coro-nary arteries are the right coronary artery (RCA) and the left coronary artery (LCA) The RCA and LCA come off the aorta and are located in the AV groove an exterior structure below the atria and above the ventricles The RCA runs from the aorta in the right AV groove while the LCA comes off the aorta and quickly branches to the left main The left main is a very short artery (about the size of a thumbnail) which branches quickly into the left anterior descending artery (LAD) and the circum-flex artery The LAD travels down the front of the

left side of the heart The circumflex artery can be found in the left AV groove

The RCA bifurcates to the posterior descending artery The right atrium and right ventricle get their supply of oxygen-rich blood from the RCA Much of the cardiac conduction system is right sided that is it commences with the sinoatrial (SA) node in the high right atrium and travels down to the AV node on the right side of the septum These important electrical structures get their oxygen-rich blood supply from the RCA The electrical system travels down the ventricles to the inferior wall of the left ventricle (located at the bottom of the heart but belonging to the left ventricle) the inferior wall is also fed oxygen-rich blood by the RCA An occlusion of the RCA such as might occur in a myocardial infarction (MI) may result in death of the heart muscle it feeds which includes the right atrium the right ventricle and the inferior wall of the left ventricle and it might affect the conduction system including the SA node and the AV node An inferior wall MI is typically the result of a clogged RCA

RCA

LCA

Aorta

Figure 110 The coronary arteries of the heart branch out to surround the entire heart muscle The two most prominent coronary arteries are the right coronary artery (RCA) and the left coronary artery (LCA)

Chapter 1 Cardiovascular anatomy and physiology 11

For an MI the more distal the occlusion (that is further from the aorta) the less potential damage the heart attack will have In other words the worst occlusions are proximal Whether or not a patient survives an MI comes down to the numbers The loss of over 40 of the left ventricular muscle mass results in death However that 40 is cumulative For example a person can survive an MI that costs him 30 of left ventricular muscle mass but if that heart attack is followed by another that kills 15 of the left ventricular muscle mass the second (milder) heart attack will prove fatal This is the reason many people survive a first heart attack but die of a second or third attack even though their fatal MI may be relatively mild

The LCA branches quickly to the left main which branches almost at once to the LAD The LAD runs on the outside of the heart along the front of the heart along the septum The left main supplies the left atrium and the majority of the left ventricle in other words the major pumping por-tion of the heart An occlusion in the left main can be particularly disastrous because it is so far upstream and affects the heartrsquos left ventricular muscle mass No wonder they call the left main the widow maker

The coronary arteries do not run in straight lines they twist and turn and sometimes make very sharp bends People with coronary artery disease (CAD) typically have plaque buildup in the areas where the coronary arteries bend at sharp angles These are areas where the blood flow creates a lot of turbulence and plaque and other substances can collect and build up It is typically to find blockage and occlusions at these places

The LAD supplies the anterior wall of the left ventricle and the septum with oxygen-rich blood Since part of the heartrsquos conduction system runs through the septum (the right and left bundle branches) an LAD occlusion can damage the heartrsquos ability to conduct electricity properly

The circumflex artery runs along the AV groove laterally and thus supplies oxygen-rich blood to the lateral wall of the left ventricle along with the left atrium The circumflex runs along the AV groove to the left heading toward the back of the heart where it meets with the RCA which

travels along the AV groove to the left toward the back of the heart Both the circumflex and the RCA are responsible for providing oxygenated blood to the posterior wall of the ventricle But which is the more important provider That depends on crux and whether the heart is right or left dominant

The crux refers to an imaginary line drawn exactly down the middle of the ventricular poste-rior wall without any regard to anatomical landmarks If the RCA crosses the crux then the heart is said to be right dominant and the RCA is the greater supplier of oxygenated blood to the ven-tricular posterior wall On the other hand if the circumflex crosses the crux then the heart is said to be left dominant and the circumflex is the more significant provider Roughly 60 of the population is right dominant

While this information about the vasculature is important it is even more important for clini-cians to recognize that there is tremendous interpatient variability in terms of venous anatomy While we can describe major structures and typical venous formations anomalies are very common These anomalies may be minor variations or they can be very pronounced such as missing veins and arteries Clinicians who deal with the cardiovascular system must be prepared for large and small differences in vascular anatomy The cardiovascular system is incredibly resilient and can adapt to some anomalies by building up collateral circulation For example a person with a missing or incompetent vessel may over time build up a network of smaller collateral vessels that compensates by doing the same job It is not unusual to see vessels grow larger or branch out to compensate Sometimes when an artery is blocked collateral circulation will build up and compensate by delivering oxygen-rich blood to the heart muscle

Every artery in the body has a matching or corresponding vein usually located in close proximity This applies to the coronary system as well Note that there are great differences in coro-nary venous anatomy among patients which becomes important when implanting left ventric-ular leads for CRT devices (see Figure 111)

12 The nuts and bolts of implantable device therapy pacemakers

the nuts and bolts of cardiovascular anatomy and physiology

bull The heart consists of four chambers which may be considered the upper chambers (right atrium and left atrium) and lower chambers (right ven-tricle and left ventricle) or may be thought of as the right heart (right atrium and right ventricle) and left heart (left atrium and left ventricle)

bull Deoxygenated blood enters the right atrium flows to the right ventricle and is pumped out via the pulmonary valve to the lungs The blood is reoxygenated in the lungs and then enters the left atrium and flows to the left atrium and out of the aortic valve into the aorta and the rest of the body

bull The heart has two outflow tracts a right ven-tricular outflow tract (RVOT) and a left ven-tricular outflow tract (LVOT)

bull The largest chamber of the heart is the left ven-tricle making up about 23 of the cardiac mass The left ventricle is muscular and massive because it must pump against approximately 10 times the pressure against which the right ven-tricle pumps (120 vs 12 mmHg)

bull The two cardiac valves most susceptible to dysfunction and disease are the mitral and aortic valve because they function in a high-pressure environment

bull The chordae tendineae connect the leaflets of the valves to the papillary muscles and look like little parachutes These help the valves open and close properly

bull In general arteries transport blood away from the heart are muscular and contract to help move the blood on its way and have thicker walls than veins Veins have thinner less muscular walls and carry oxygen-depleted blood back to the heart using tiny interior valves to keep the blood flow moving forward Arteries transport oxygenated blood and can be considered high-pressure systems while veins transported deoxygenated blood in a low-pressure system

bull Cardiac contraction is systole cardiac relaxation is diastole Atrial systole occurs at the very end of ventricular diastole in something nicknamed atrial kick In a healthy individual about 20ndash30 of cardiac output comes from atrial kick

bull The heart is a muscle that is fed with oxygenated blood through a network of coronary arteries Coronary arteries get oxygen-rich blood from the aorta and they perfuse the heart muscle during diastole as blood drains back

bull Although arteries and veins generally take blood away from or to the heart respectively

Rightventricle

Leftventricle

Leftatrium

Aorta

Rightatrium

Rightcoronaryartery

Leftcoronaryartery

Circumflexartery

Superiorvena cava

Inferiorvena cava

Arterial system

Rightventricle

Leftventricle

Greatcardiac vein

Smallcardiac vein

Middlecardiac vein

Leftatrium

Aorta

Rightatrium

Superiorvena cava

Inferiorvena cava

Venous system

Coronarysinus

Figure 111 The coronary artery system is matched by a coronary venous system

Chapter 1 Cardiovascular anatomy and physiology 13

test your knowledge

Fill in these anatomical landmarks of the heart They include the four chambers the four valves and the bodyrsquos main artery

the pulmonary vessels are the exception The pulmonary artery transports deoxygenated blood while the pulmonary veins carry oxygen-ated blood This is because of their special loca-tion and the fact that their work is taking blood from the right side to the left side of the heart

bull The main coronary arteries are the right coro-nary artery and the left coronary artery The left coronary artery branches quickly to a short vessel known as the left main (the widow maker) and the circumflex artery

bull A myocardial infarction can kill the heart muscle The clogged arteries determine the loc ation of the affected muscle A right coro-nary artery occlusion will affect the right atrium and ventricle and the inferior wall of the left ventricle while a left coronary artery occlusion will affect the left atrium and most of the left ventricle By far the most dangerous occlusion is in the left main because it feeds the majority of the heart muscle with oxygen-rich blood

answer the following questions

1 What are the names of the two atrioventricular valves of the heartA Atrial and ventricularB Tricuspid and mitralC Pulmonary and pulmonicD Aortic and superior vena cava

2 Which chambers of the heart contain trabeculaeA AtriaB VentriclesC Right-sided chambers onlyD Left-sided chambers only

14 The nuts and bolts of implantable device therapy pacemakers

3 Why can trabeculae be important to device cliniciansA They are good places to secure a passive-fixa-

tion leadB They help regulate blood pressureC They prevent atherosclerosisD All of the above

4 Which of the following is not true about the aortaA It is an arteryB It consists of an ascending portion a descend-

ing portion and an archC It carries only oxygenated bloodD It is the largest vein in the body

5 From which artery does blood enter the right atriumA Superior vena cavaB Inferior vena cavaC Coronary sinusD All of the above

6 What is atrial kickA Ventricular pressure on the tricuspid valve

forcing it shutB Electrical impulses originating in the atrium

to cause the heart to beat fasterC Atrial systole that forces the most blood possible

into the already-full and diastolic ventriclesD A type of arrhythmia

7 Why would a person with chronic AF likely have decreased cardiac outputA AF is associated with low blood volumeB AF causes the heart to beat very slowlyC AF may cause the heart to experience fre-

quent pausesD AF reduces or eliminates atrial kickmdashand

atrial kick contributes to cardiac output

8 Which of the following is often a preferred location for affixing a cardiac pacing leadA The right ventricular outflow tract (RVOT)B The left ventricular outflow tract (LVOT)C The excised right atrial appendageD The tricuspid valve

9 What connects the valve leaflets to the papillary musclesA TrabeculaeB ArteriesC Chordae tendineaeD Tiny valves

10 Which coronary artery is nicknamed the widow maker because an occlusion in this vessel will deprive the majority of the heartrsquos muscle mass of oxygen-rich bloodA Left mainB CircumflexC Right coronary arteryD Inferior vena cava

15

The Nuts and Bolts of Implantable Device Therapy Pacemakers First Edition Tom Kenny copy 2015 John Wiley amp Sons Ltd Published 2015 by John Wiley amp Sons Ltd

Chapter 2

Cardiac conduction system

Introduction

The sinoatrial (SA) node is the heartrsquos natural pacemaker In the healthy heart the SA node fires and the electrical stimulus conducts along special pathways through the heart that cause cells in specific areas to depolarize After the stimulus passes the area the cells repolarize Myocardial cell depolarization and repolarization govern the heartrsquos contraction and relaxation sequencemdashin other words the heart rhythm Rhythm disorders occur when this healthy conduction system is in some way disrupted or impaired

the conduction pathways

When the SA node fires and sends out electrical energy the stimulus travels from the SA node in the high right atrium out over the atria via path-ways known as internodal tracts Most of these internodal tracts are located in the right atrium the

main tract over to the left atrium is called Bachmannrsquos bundle It is because of Bachmannrsquos bundle linking the right atrium to the left atrium that the atria can contract simultaneously that is as these two upper chambers contract as one coherent unit After traversing the atria the impulse comes down through conduction tracts to the atrioventricular (AV) node

The AV node is an area of highly specialized tissue located below the atria and above the ventri-cles on the right side of the septum Anatomically the AV node rests in the AV groove The tissue in the healthy AV node has the ability to change the speed of the electrical impulse specifically it slows the stimulus down The slowdown serves two important purposes First slowing the impulse allows the atria time to contract and provide the very valuable atrial kick to ventricular filling Second the slowed impulse protects the ventricles from being forced to beat too quickly that is to beat before they have been adequately filled with blood

Once the impulse crosses the AV node it travels down into the ventricles and accelerates its speed The impulse travels through the bundle of His (pronounced hiss) to the bundle branches (right and left) and then out to the network of Purkinje fibers This allows both ventricles to depolarize and contract simultaneously (Figure 21)

polarization depolarization and repolarization

Cardiac conduction is driven by the ability of myo-cardial cells to be depolarized and repolarized Depolarization occurs at the cellular level and is

2

Learning objectives

bullensp Describe the electrical conduction system of the healthy heart

bullensp List the main anatomical landmarks of normal cardiac conduction

bullensp Name the anatomical location of the heartrsquos natural pacemaker

bullensp Explain depolarization and repolarization in electrical terms and how it relates to the cardiac contraction

bullensp Define the term automaticity and how it relates to cardiac conduction

16 The nuts and bolts of implantable device therapy pacemakers

in turn driven by electrolytes or positively and negatively charged ions (potassium sodium and calcium mainly) which are able to move in and out of the cardiac cells A myocardial cell may be said to be polarized when in its resting state that is there is no electrical activity going on In the rest-ing state the interior of the myocardial cell is somewhat more negatively charged that the outside This state is called the resting state or sometimes phase 0 of the action potential

An electrical stimulus changes the resting state abruptly The electrical energy allows for the movement of ions to the point that the interior of the myocardial cell is now more positive than the outside In other words the charge reverses and the cell is said to be depolarized When this happens the cell contracts when this happens to a large area of the myocardium the heart muscle contracts Cardiac contraction occurs as a result of cellular depolarization

When the electrical stimulus moves past the myocardial cell the ions move in reverse and the interior of the cell assumes its previous negative charge with respect to the outside This process is

called repolarization and it brings the myocar-dial cell back to the resting state When this happens the cell relaxes When a large area of the myocardium repolarizes the heart resumes its previous shape (loses its contraction)

Intrinsic pacemakers

The healthy heartrsquos natural pacemaker is the SA node but the SA node is not the only area of the heart that can generate an electrical pulse The SA node serves as the heartrsquos pacemaker because it generates these pulses faster than other areas In the healthy heart the SA node can fire at around 60ndash100 times a minute corresponding to a healthy heart rate The AV junction can also fire but it is much slower at around 40ndash60 times a minute Even the Purkinje fibers (the ventri-cles) can act as a pacemaker but they fire at only 20ndash40 times a minute Thus in a healthy individual the SA node outpaces these other potential pacemakers The AV junction and ven-tricles are sometimes called backup pacemakers because they take over the heart rate in the event

Sinoatrial (SA) node

Internodal tracts

Right bundle branch

Left bundle branch

Purkinje fibers

Bachmannrsquos bundle

Bundle of His

Atrioventricular (AV) node

Figure 21 The conduction pathways in the healthy heart

Chapter 2 Cardiac conduction system 17

that the SA node is not able to pace the heart While these backup pacemakers may help keep the heart beating in the event of a dysfunctional SA node they are slow and not as reliable as the SA node

The healthy heart is able to pace itself because of a property called automaticity Automaticity is an inherent property of individual cardiac conductive cells that allows them to depolarize spontaneously In other words some cells in the heart (such as the cells of the SA node) are able to generate an electrical stimulus on their own

refractoriness

Refractoriness refers to the temporary inability of a cardiac cell to respond to an electrical stimulus Refractoriness can be the normal appropriate and expected behavior of cardiac cells For example immediately after electrical energy has passed through cardiac cells they go through a phase where they are unresponsive to electrical energy no matter how much voltage is delivered This normal refractoriness is called the physiological refractory period and it is very brief (it is mea-sured in thousandths of a second) Of course the exact duration of a refractory period is based in part on the heart rate the faster the heart rate the shorter the refractory period and vice versa

Refractoriness may also occur when certain cells become dysfunctional that is cardiac cells may lose their ability to conduct properly Furthermore the heartrsquos inherent pacemakers may change their rate for instance the AV junction may speed up and start to deliver impulses at 80 or 100 times a minute When this happensmdashthat is when another pacemaker takes over the role of the SA nodemdashthat pacemaker is called a usurping pacemaker because it usurps the rightful role of the SA node In the event that other pacemakers of the heart start to compete with the SA node the fastest pacemaker always wins

Conductivity

Conductivity is the cardiac cellrsquos ability to transmit an electrical impulse to an adjacent cell Typically cardiac conduction is antegrade that

is it moves forward Electrical impulses actually travel out in all different directions but refracto-riness prevents those cells that were just stimu-lated from depolarization This keeps conduction moving in a single direction through the heart Normal antegrade conduction in a healthy heart goes from SA node to atria to AV node over the bundle of His and down to the ventricles Some individuals have the ability to conduct retro-grade retrograde conduction is abnormal but not uncommon

autonomic nervous system

The heart does not need direct stimulation from the nervous system to generate electricity and to beat but that is not to say that the nervous system does not control the heart rate The autonomic nervous system indirectly affects cardiac conduction by regulating how fast the heart will beat and how hard the heart muscle will squeeze (contractility) The autonomic ner-vous system communicates with the heart through a network of baroreceptors which respond to chemical signals

The autonomic nervous system of the body like certain other physiological systems exists as two systems in balance The autonomic nervous system consists of a sympathetic system and a parasympathetic system They both work together and in a healthy individual one may overdrive the other during specific times The sympathetic ner-vous system is mainly associated with epinephrine (adrenaline) and it has the ability to speed the heart rate and increase cardiac contractility and can increase the speed by which electrical impulses can cross the AV node The sympathetic nervous system takes over when we are stressed frightened (fight-or-flight response) or exerting ourselves

On the other hand the autonomic nervous system also has a parasympathetic system that is associated with acetylcholine This system can decrease the heart rate decrease cardiac contrac-tility and cause impulses crossing the AV node to be slowed The parasympathetic system might take over when we are sleeping or at rest

18 The nuts and bolts of implantable device therapy pacemakers

the nuts and bolts of the cardiac conduction system

bull The heartrsquos natural pacemaker is the sinoatrial (SA) node which generates an electrical impulse spontaneously about 60ndash100 times a minute in a healthy individual

bull The normal conduction path is from the SA node over the atria to the atrioventricular (AV) node and then via the bundle of His down the right and left bundle branches and into the Purkinje fibers

bull The AV node has the ability to slow down the electrical impulse This serves two main purposes it allows for atrial kick and it protects the ventricles from beating too rapidly

bull In the healthy heart the right and left atria contract simultaneously and then the right and left ventricles contract simultaneously

bull Depolarization and repolarization of myocar-dial cells are driven by positively and negatively charged ions which cross the cell membrane A resting cardiac cell is more negative on the inside than the surrounding media but when it depolarizes it becomes more positive on the inside relative to the surrounding media Following depolarization the ions move and the cell returns to being more negative on the inside than the outside in a process known as repolarization

bull Depolarization causes the cardiac cell to contract

bull The main ions involved in depolarization and repolarization are sodium potassium and calcium ions

bull Although the heartrsquos natural pacemaker is the SA node the AV junction and the Purkinje fibers (ventricles) can also assume (usurp) this role if the SA node cannot generate electricity properly The AV junction and Purkinje fibers are not as reliable as a healthy AV node and typically generate electricity more slowly (the AV junction about 40ndash60 times a minute and the Purkinje fibers 20ndash40 times a minute)

bull However sometimes the AV junction or Purkinje fibers may generate electricity very

quickly this is abnormal When they do they may take over the heart When pacemakers of the heart compete the fastest one drives the heart

bull Automaticity is an inherent property of certain cardiac conductive cells to depolarize spontaneously Not all cardiac conduction cells possess automaticity but the SA node AV junction and Purkinje fibers possess automaticity

bull Refractoriness is the natural and expected behavior of cardiac conductive cells that makes them unresponsive to electrical impulses immediately after they have depolarized The natural refractory period is very brief

bull The faster the heart rate the shorter the refractory period the slower the heart rate the longer the refractory period

bull Refractoriness can also occur as a dysfunction bull Conductivity is the cardiac cellrsquos ability to

transmit an electrical impulse to adjacent cells Refractoriness means that cells that just depolarized will not depolarize again This keeps conduction moving forward (antegrade conduction)

bull Antegrade conduction is normal retrograde conduction is abnormal but is not rare

bull The autonomic nervous system regulates the heart rate by controlling how fast the heart beats and how vigorously it contracts

bull The autonomic nervous system consists of a sympathetic and parasympathetic nervous system which work together but at certain times one or the other is dominant The sympathetic nervous system makes the heart beat faster and more vigorously it takes over during times of stress exertion or fight or flight The parasympathetic ner-vous system makes the heart beat slowly and less vigorously it takes over during periods of rest

bull The neurotransmitters associated with these two systems are epinephrine or adrenaline (sympathetic nervous system) and acetylcho-line (parasympathetic nervous system)

Chapter 2 Cardiac conduction system 19

test your knowledge

Please write in the landmarks of the cardiac conduction system Select from the following (each is used only once)

AV node Bachmanrsquos bundle Bundle of His

Internodal tracts Left bundle branch Purkinje fibers

Right bundle branch Ventricles

1 Which of the following is not considered one of the heartrsquos intrinsic pacemakersA AV junctionB SA nodeC Purkinje fibersD Aorta

2 Depolarization and repolarization involve the movement of charged ions across cardiac cell membranes What types of ions are primarily involvedA Calcium potassium and sodiumB Acetylcholine and epinephrineC Magnesium and folateD All of the above

3 Refractoriness refers toA The ability of cardiac conduction cells to

contract

B The inability of cardiac conduction cells to respond to an electrical stimulus

C The unique permeability of the cardiac cellsD Abnormal cardiac conduction

4 When a cardiac cell depolarizes the interior of the cell becomes what with respect to the surrounding mediaA More positiveB More negativeC More acidicD Hotter

5 Fill in the blank In the healthy heart the faster the heart rate the ___ the refractory periodA More vigorousB Less reliableC LongerD Shorter

20 The nuts and bolts of implantable device therapy pacemakers

6 Which specific part of the nervous system governs the fight-or-flight response and is associated with epinephrine (adrenaline) (Pick the most exact answer)A Autonomic nervous systemB Parasympathetic nervous systemC Sympathetic nervous systemD Central nervous system

7 In normal antegrade conduction the electrical impulse travels from the SA node over the atria to the AV node and to what structure before reaching the ventriclesA The intermodal tractsB The Purkinje fibersC The bundle of HisD The ventricular apex

8 Which of the following is true about retrograde cardiac conductionA Everyone has itB It is perfectly normal and can be helpful in

stressful situations

C It is abnormal but not uncommonD It is impossible to diagnose definitively

except in an autopsy

9 What chemical is associated with the parasympathetic nervous systemA AcetylcholineB PotassiumC EpinephrineD Dopamine

10 The stimulation of the myocardial cells of which of the following structures is most closely associated with ventricular depolarizationA SA nodeB AV junctionC Bundle of HisD Purkinje fibers

1

The Nuts and Bolts of Implantable Device Therapy Pacemakers First Edition Tom Kenny copy 2015 John Wiley amp Sons Ltd Published 2015 by John Wiley amp Sons Ltd

Chapter 1

Cardiovascular anatomy and physiology

Introduction

An encyclopedia could be written on the anatomy and physiology of the human heart and that is not our purpose Device clinicians must understand the cardiovascular system to understand arrhyth-mias and device therapy This chapter will intro-duce the important concepts of cardiac anatomy and physiology necessary for an understanding of cardiac rhythm management To that end this chapter will describe the chambers valves and major vessels of the heart and how these control the flow of blood in the body Although we think of the heartmdashrightlymdashas a pump it also possesses a com-plex electrical system The cells of the human heart are unique in many ways and how they produce conduct and dissipate electrical energy is very important particularly to pacing Our goal here is to describe the anatomy and physiology of the

healthy heart and cardiovascular system in terms of what device clinicians need to know

the healthy heart

The human heart is a double pump (right and left) that sits in the middle of the chest slightly to the left and rotated so that the right side is more anterior than the left An average adult human heart is relatively large about 13 by 9 by 6 cm and weighing about 300 g The heart is protected by the rib cage and sits directly behind one of the bodyrsquos thickest bones the sternum The bottom of the heart rests on the diaphragm muscle The heart is encased in this protected but somewhat crowded areamdashit also contains the lungs (three lobes on the right two on the left) the stomach and the intestines

The bottom tip of the heart (called the apex) taps up against the chest when the heart contracts By placing his hands on the chest a physician can feel the place where the apex of the heart makes contact with the chest this place is called the point of max-imal impulse (PMI) Knowing the precise location of the PMI can be very useful in treating cardiology patients because the PMI of a healthy heart occurs slightly to the left while the PMI of a person with an enlarged heart is going to occur much farther to the left even off to the side A healthy heart is roughly the size of the fist but when hearts enlarge such as occurs with disease progression the enlargement occurs toward the left Thus PMI can

1

Learning objectives

bullensp Point out the key landmarks in the human heart relevant to cardiac rhythm management

bullensp Name the four chambers of the heart the four valves and the major vessels

bullensp Describe the flow of blood through the heartbullensp Define AV synchrony and explain why it is

importantbullensp State the difference between the bodyrsquos arterial

versus venous systems

2 The nuts and bolts of implantable device therapy pacemakers

provide a fast noninvasive way of determining if and to what degree the heart has enlarged

The left ventricle composes most of the mass of the heart being by far the largest of the four pump-ing chambers A healthy heart circulates about 4ndash6 l of blood a minutemdashwhich is the entire blood volume of the body That means the entire circulating volume of blood in the body moves around every minute or once per beat

The heart consists of four chambers two upper chambers called atria (singular atrium) and two lower and larger chambers called ventricles To understand the healthy heart it is useful to think of the heart in terms of right side (right atrium and right ventricle) and left side (left atrium and left ventricle) The right side of the heart circulates deoxygenated blood to the lungs (where it can be oxygenated) The left side of the heart pumps oxy-genated blood out to the rest of the body (see Figure 11)

The heart is a muscle and consists of four dis-tinct layers The endocardium is the innermost layer and composes a lining for the interior of the heart The epicardium is the outer layer of the heart Between the endocardium and epicardium lies the myocardiummdashthe thickest layermdashwhich is muscle The entire heart is encased in a

liquid-filled sac called the pericardium which acts like a shock absorber for the heart The pericardial sac contains about 15ndash20 cc of pericardial fluid in a healthy individual In the event that fluid builds up to abnormally high levels in the pericardial sac (such as might occur when a lead or catheter inside the heart perforates the endocardium myocar-dium and epicardium and goes exterior to the heart) this fluid can place pressure on the heart in a condition known as cardiac tamponade Since the heart is contained in a relatively small space this pressure can compromise the heartrsquos ability to fill with blood and pump efficiently During device implantation perforation is an important concern because it can lead to cardiac tamponade In the event that perforation results in cardiac tamponade a needle is inserted into the pericardial sac (through the chest wall) to drain the blood Lead perforation does not always result in cardiac tamponade but it is a serious concern

Blood flow through the heart

The heart is a pump and it is located amid a net-work of vessels that carry deoxygenated blood into the right side of the heart and reoxygenated blood into the left side of the heart The flow is actually

Right atrium

Left atrium

Left pulmonaryarteries

Left pulmonaryveins

Left ventricle

Right ventricle

Aorta

Aortic valve

Mitral valve

Tricuspidvalve

Pulmonaryvalve

SA node

AV node

Right pulmonaryarteries

Right pulmonaryveins

Figure 11 Cross section of the heart showing the chambers

Chapter 1 Cardiovascular anatomy and physiology 3

fairly simple Deoxygenated blood enters the right side of the heart and is pumped over to the lungs via the pulmonary arteries and is returned back (as oxygen-rich blood) to the left side of the heart by way of the pulmonary veins (PV) While both right and left sides of the heart contract at the same time as a single unit the right side is busy pumping deoxygenated blood to the lungs while the left side is pumping reoxygenated blood out to the rest of the body

Deoxygenated blood enters the right side of the heart via the superior vena cava (SVC) but once it has become oxygenated again blood is pumped back out from the left side of the heart into the aorta The aorta is the largest vessel in the body and it forms a U shape at the top of the heart These portions of the aorta are called the ascending the descending and the arch Coming off the aortic arch are three main arteries the left subclavian artery the left common carotid artery and the brachiocephalic trunk

To better understand the blood flow through the heart it is important to review the structure of the heart The atria or upper chambers of the heart are smaller have thinner walls and are smoother on the inside than the ventricles Within the ventri-cles is a network of fibrous strands known as trabeculae These structural differences become important in lead implantation within the heart it is much easier to affix or lodge a lead in the trabeculae of the ventricles than to try to anchor the lead to a smooth atrial wall Historically atrial leads have almost always been active-fixation screw-in-type leads while ventricular leads were almost always passive-fixation leads (fins or tines that lodge in the trabeculae) Today active-fixation leads are often used in both chambers since they facilitate lead removal (Figure 12)

Overall blood flow to the heart is discussed right and left sides although it is important to recognize that what happens in the heart that is systole (contraction) and diastole (relaxation) are happening on both sides at the same time The right atrium of the heart receives blood from the SVC the inferior vena cava (IVC) and the coronary sinus (CS) The CS is technically a vein and it has an opening or ostium (sometimes just called os) at the base of the right atrium slightly posterior The CS delivers oxygen-depleted blood

to the right atrium from the coronary arteries that encircle the exterior of the heart The CS is of interest in cardiac resynchronization therapy (CRT) because the left ventricular lead is passed through the CS (counter to the flow of blood) in order to be placed into the coronary vessels to pace the left ventricle CRT is used in patients with heart failure whose hearts have remodeled that is enlarged and changed shape (It may be said that with heart failure the heart changes from the shape of a football to the shape of a basketball) The CS may be relocated in this remodeling which can be challenging in implanting a CRT lead because the physician must first locate the os of the CS and then navigate through it in order to implant the left ventricular lead

Anatomically the heart is dominated by the large muscle mass of the left ventricle which makes up about two-thirds of the heart in terms of weight and volume This greater size is typically ascribed to the fact that the left ventricle must pump blood throughout the whole body whereas the right ventricle only has to pump blood to the lungs The left and right ventricles pump blood to different destinations but the left ventricle is larger and more muscular for a reasonmdashpressure It is impor-tant to review the pressures against which the heart must work to understand cardiac blood flow (Figure 13)

Deoxygenated blood in the right side of the heart must travel over the lungs to pick up oxygen This means that blood in the right ventricle travels across the pulmonary valve into the pulmonary artery and then out toward the lungs The pulmonary valve opens automatically when pressure from the contracting right ventricle forces it open This occurs when the pressure in the right ventricle exceeds the pressure in the pulmonary artery Pressure gradients are key concepts in understanding blood flow Valves are like gates that open and close in response to pressure In general the pressure in the PV is fairly low around 12 mmHg Thus the right ventricle does not need to create a lot of force to open the pulmonary valve

Meanwhile as the left ventricle contracts it cre-ates pressure on the aortic valve leading to the aorta In order to open the aortic valve and pump blood out into the aorta the heart must overcome the pressure in the aortic valve Pressure in the

4 The nuts and bolts of implantable device therapy pacemakers

Right atrium

Left atrium

Left ventricle

Right ventricle

Figure 12 Note that the atria are smooth walled while the ventricles contain a spongelike fibrous network of trabeculae

Left pulmonaryarteries

Left pulmonaryveins

Aorta

Aortic valve

Pulmonaryvalve

Right pulmonaryarteries

Superiorvena cava

Inferiorvena cava

Right pulmonaryveins

Figure 13 The blood flow within the heart takes oxygen-depleted blood from the body into the right atrium where it flows to the right ventricle and is pumped out over the lungs the reoxygenated blood from the lungs is pumped into the left atrium where it flows to the left ventricle and is pumped out via the aorta to the body

Chapter 1 Cardiovascular anatomy and physiology 5

aorta is high around 120 mmHg or ten times higher than the pressure in the PV The left ventricle must therefore work much harder to pump blood than the right ventricle This requires the left ventricle to be larger and more muscular than the right ven-tricle (see Figure 14)

On the right side of the heart blood travels from the right atrium into the right ventricle via the tricuspid valve The tricuspid valve gets its name from its characteristic shape involving three leaves or cusps Attached to these cusps are cords that anchor into the base of the ventricle known as chordae tendineae they look almost like little parachutes The strands of the chordae tendineae attach to tiny papillary muscles These chords attach to the valve leaves at one end and a papil-lary muscle at the other end On the right side of the heart the tricuspid valve is associated with three papillary muscles The purpose of these chords and muscles is to assure that the valve is

effectively closed and opened at the proper times (Figure 15)

The heart can rightly be thought of as a pump but it must be remembered that the heart is also a muscle and all muscles need a steady supply of oxy-gen-rich blood The heart muscle is supplied with blood through a network of coronary arteries that surround the outside of the heart Blockage in a coronary artery results in ischemia which can lead to death of cardiac muscle including the chordae tendineae and papillary muscles While the patient may survive such an ischemic event the damage to the heart may lead to an incompetent valve that is a valve that is no longer able to function effectively

In tracing the blood flow from the right ventricle to the pulmonary artery it should be clear that the blood has to go from down in the right ventricle to up through the pulmonary valve and into the pulmonary artery The blood is able to make this journey because of the pumping pressure of the heart The route the blood takes as it exits the right ventricle and journeys up toward the pulmonary valve is known as the right ventricular outflow tract (RVOT) On the other side of the heart there is also a corresponding left ventricular outflow tract (LVOT) of approximately the same size Cardiac leads are sometimes fixated in the RVOT (see Figure 16)

Blood pumped out of the right ventricle crosses the pulmonary valve and enters the pulmonary artery which splits into two branches right and left The right pulmonary artery takes blood to the right lung while the left pulmonary artery takes blood to the left lung In this respect the pulmonary artery is unique in the body in that it is an artery but it carries deoxygenated blood Blood travels through the lung to the alveoli where it gains oxygen and loses carbon dioxide Once it is reoxygenated the blood gathers into the PV (which are also unique being the only veins to carry oxygenated blood) There are four PV in total the two right-sided PV take oxygen-ated blood from the right lung while the two left-sided PV take oxygenated blood from the left lung and they all bring this reoxygenated blood to the left atrium

The left atrium is smooth walled like the right atrium and although the left atrium is much

Figure 14 The left ventricular is far more muscular than the right ventricle because it must overcome 10 times the pressure of the right ventricle in order to pump blood out via the aortic valve and into the aorta

6 The nuts and bolts of implantable device therapy pacemakers

Right atrium

Left atrium

Left ventricle

Right ventricle

Figure 15 The leaflets of the valves are attached by chordae tendineae at one end and papillary muscles on the other which assure the effective opening and closing of the valves

Right-ventricularoutflow tract (RVOT)

Figure 16 There are two outflow tracts in the heart one associated with the right ventricle and the other the left ventricle These outflow tracts are roughly the same size The illustration shows the right ventricular outflow tract (RVOT) a preferred location for right ventricular lead fixation The left ventricular outflow tract (LVOT) cannot be seen in this illustration as it is posterior

Chapter 1 Cardiovascular anatomy and physiology 7

smaller than the left ventricle it is larger than the right atrium The blood will travel from the left atrium into the left ventricle by way of the mitral valve The mitral valve gets its name from the miter a bishoprsquos hat The mitral valve has two cusps and papillary muscles connected by chordae tendineae Once blood is in the left ventricle a contraction will force it toward the aorta by way of the LVOT In persons with hypertrophic obstructive cardiomy-opathy (HOCM) the septum or wall in the heart can become thick and enlarged effectively narrow-ing the LVOT In the healthy heart the blood flows up the LVOT across the aortic valve and into the aorta From the aorta blood will be directed upward toward the head downward toward the legs and feet and some blood will be redirected back to the coronary arteries of the heart

The blood flow from the left ventricle to the aorta and back to the coronary arteries is unique in the body in that most of that flow occurs during diastole rather than systole Think of it this way as blood is pumped by the left ventricle during systole upward and downward to the brain and the rest of the body some of that blood flows back during diastole and drains into the coronary arteries Although it may seem counterintuitive it is impor-tant to remember that coronary artery perfusion occurs primarily during diastole

Volume valves and pressure

While it is tempting to think of the heart as simply a pump with an electrical system it is more accu-rate to say that the heart moves blood because of variations in volume and pressure

The heart has four valves two are atrioventricular (AV) valves because they connect the atrium to the ventricle and two are semilunar valves connecting the heart with pulmonary arteries or the aorta The right AV valve is the tricuspid while the left AV valve is the mitral valve The flow or movement of blood is guided and controlled by the heartrsquos muscular action in the form of systole (contraction) or diastole (relaxation)

When blood pours into the right atrium it dis-tends and expands the right atrium When this volume of blood creates sufficient pressure it forces the tricuspid valve open Think of it as volume creating pressure and pressure opening

valves When the tricuspid valve opens the blood dumps into the right ventricle During this period of diastole blood pours into the right ventricle creating volume and expanding it As blood pours in and ventricular diastole nears the end the atria contract while the AV valves are still open in something nicknamed atrial kick Atrial systole occurs at the end of ventricular diastole Atrial kick forces the maximum amount of blood from the atria into the ventricles It is estimated that atrial kick delivers 20ndash30 of cardiac output

The atrial contribution to ventricular filling (atrial kick) is of enormous clinical significance Patients with certain atrial arrhythmias such as atrial fibrillation (AF) lose atrial kick Even milder forms of atrial tachyarrhythmias may compromise atrial kick Moreover patients who do not have AV synchrony (one atrial beat corresponding to one ventricular beat) will lose atrial kick The loss of atrial kick can reduce cardiac output by 20ndash30

Once the atria have contracted and the ventricles are filled with blood the AV valves snap closed The ventricles now start to contract and push open the semilunar valves For example as the right ven-tricle contracts (systole) it opens the PV (The PV is sometimes called the pulmonic valve itrsquos the same thing) The whole process works on volume pressure and contractionrelaxation

Of the four valves in the heart the tricuspid valve is of most interest to device specialists because at least one lead will be placed into the right ventricle This means that pacing and defi-brillation leads typically cross the tricuspid valve These leads may interfere with the proper closing of the tricuspid valve particularly if the lead is large diameter or several leads are passed over the tricuspid valve A single lead through the tricuspid valve is likely to cause minimal to no dysfunction However it is not unusual to see patients with three or more leads in the right ventricle and these leads may hamper the tricuspid valve in its closing leading to tricuspid regurgitation or backflow of blood Patients with multiple leads (such as chil-dren who grow up with pacemakers) may need to have some of them removed to avoid tricuspid regurgitation

Right ventricular leads are typically fixated at the right ventricular apex or in the RVOT and may be

8 The nuts and bolts of implantable device therapy pacemakers

lodged in the trabeculae Trabeculae are a fibrous network that could be described as similar to the pores of a sponge (see Figure 17)

On the left side of the heart the blood flow pattern is similar Blood flows into the left atrium over the open mitral valve and into the left atrium At the end of ventricular diastole as the ventricle is distended and stretched atrial kick delivers more blood into left ventricle The volume of blood cre-ates pressure The mitral valve closes As the left ventricle contracts the growing pressure over-comes the resistance on the aortic valve and it opens allowing blood to flow out into the aorta and beyond

While any valve can become diseased or dys-functional valvular disease more commonly affects the mitral and the aortic valves (left-sided valves) than the right-sided valves (tricuspid and pul-monary) These left valves are more likely to suffer damage or disease because of the high-pressure environment in which they function exposing them to more potential damage Other damage can occur when plaque builds up in the coronary arteries and the aorta The aorta can become

diseased typically in the case of an aortic aneurysm or dissection Carotid arteries in the neck are also frequently a site for atherosclerotic disease

the right atrial appendage

The right atrial appendage (RAA) is an area near the right atrium which is a preferred site for attach-ing active-fixation atrial leads (see Figure 18) The RAA serves no obvious purpose and may be sacri-ficed in certain heart surgeries such as a coronary artery bypass graft (CABG) procedure CABG or bypass surgery involves stopping the heart and running the blood through a machine to reoxygen-ate it during the course of the procedure This is done by attaching a large-diameter hose to the RAA when surgery concludes and this hose is removed it can damage the RAA to the point that it is completely or partially surgically removed In patients who have had a CABG procedure the RAA may not be available for right atrial lead placement For such patients the right atrial lead is often placed against the lateral wall of the right atrium It is sometimes possible to fixate a right

Figure 17 Trabeculae form a fibrous network within the ventricles Passive-fixation ventricular leads are held in place by lodging their tines or fins in the trabeculae

Chapter 1 Cardiovascular anatomy and physiology 9

atrial lead in the remnants of the RAA if a portion of it is preserved

arteries and veins

Any discussion about the heart necessarily involves the vasculature which is why we commonly refer to our circulatory system as the cardiovascular

system As most of us remember from anatomy classes arteries carry blood away from the heart while veins transport blood back to the heart (The exceptions are the PV and arteries described earlier) Arteries carry oxygen-rich blood while veins carry deoxygenated blood What is of importance to device clinicians is that arteries are high-pressure vessels while veins are low-pressure vessels Arteries tend to be more muscular in structure and muscular contractions of the arteries help to move blood outward into the body Veins tend to be less muscular than arteries but unlike arteries they contain a system of tiny interior valves The purpose of these little valves is to maintain a unidirectional flow of blood (see Figure 19)

Some of the most important veins in the body are the SVC and its counterpart the IVC The SVC takes blood from the upper part of the body and delivers it into the right atrium with the IVC doing the same for blood from the lower part of the body The coronary veins networked around the outside of the heart carry blood from the heart tissue and deliver it back to the right atrium they are closely linked to the coronary arteries which carry reoxygenated blood from the left ventricle back to the heart muscle The CS is the site where

Right atrial appendage

Figure 18 The right atrial appendage serves no obvious hemodynamic purpose but is often the site of fixation of atrial leads

Artery Vein

Figure 19 Veins and arteries have different purposes different anatomical structure and different pressures Overall veins are low-pressure systems that move blood by a series of little valves that keep blood flowing in the same direction Arteries are high-pressure systems and are more muscular so that they force blood forward by squeezing

10 The nuts and bolts of implantable device therapy pacemakers

the great middle and small cardiac veins all drain the CS takes this blood and delivers it back to the right atrium

The PV carry reoxygenated blood from the lungs and deliver it to the left atrium so that it can be pumped back out to the body by the left ventricle

The most important artery in the body is by far the aorta The aorta is connected to the left ven-tricle and it is the first and main conduit that takes reoxygenated blood from the left ventricle and sends it out to the body As this reoxygenated blood makes its way out into the body its first two stops are the head (brain) and the heart muscle Inadequate oxygenated blood to the brain can very quickly provoke symptoms This is the reason that depressed cardiac output is associated with symptoms like dizziness light-headedness feeling woozy and fatigue Severe symptoms might include syncope

The coronary arteries arise from the root of the aorta just above the cusps of the aortic valve Coronary arteries receive most blood during dias-tole so blood is more or less pouring back into these vessels The coronary artery system is a networkmdashalmost a meshmdashof middle-sized to very small vessels that cover the exterior surface of the heart (see Figure 110)

Coronary arteries get their name from the Latin word for crown (such as in our word coronation) because this network of vessels sits like a crown atop the heart and encircles it The heart gets blood from outside in as oxygen-rich blood in these coro-nary arteries is delivered to the exterior of the heart muscle Coronary arteries are not within the heart they are epicardial structures The two main coro-nary arteries are the right coronary artery (RCA) and the left coronary artery (LCA) The RCA and LCA come off the aorta and are located in the AV groove an exterior structure below the atria and above the ventricles The RCA runs from the aorta in the right AV groove while the LCA comes off the aorta and quickly branches to the left main The left main is a very short artery (about the size of a thumbnail) which branches quickly into the left anterior descending artery (LAD) and the circum-flex artery The LAD travels down the front of the

left side of the heart The circumflex artery can be found in the left AV groove

The RCA bifurcates to the posterior descending artery The right atrium and right ventricle get their supply of oxygen-rich blood from the RCA Much of the cardiac conduction system is right sided that is it commences with the sinoatrial (SA) node in the high right atrium and travels down to the AV node on the right side of the septum These important electrical structures get their oxygen-rich blood supply from the RCA The electrical system travels down the ventricles to the inferior wall of the left ventricle (located at the bottom of the heart but belonging to the left ventricle) the inferior wall is also fed oxygen-rich blood by the RCA An occlusion of the RCA such as might occur in a myocardial infarction (MI) may result in death of the heart muscle it feeds which includes the right atrium the right ventricle and the inferior wall of the left ventricle and it might affect the conduction system including the SA node and the AV node An inferior wall MI is typically the result of a clogged RCA

RCA

LCA

Aorta

Figure 110 The coronary arteries of the heart branch out to surround the entire heart muscle The two most prominent coronary arteries are the right coronary artery (RCA) and the left coronary artery (LCA)

Chapter 1 Cardiovascular anatomy and physiology 11

For an MI the more distal the occlusion (that is further from the aorta) the less potential damage the heart attack will have In other words the worst occlusions are proximal Whether or not a patient survives an MI comes down to the numbers The loss of over 40 of the left ventricular muscle mass results in death However that 40 is cumulative For example a person can survive an MI that costs him 30 of left ventricular muscle mass but if that heart attack is followed by another that kills 15 of the left ventricular muscle mass the second (milder) heart attack will prove fatal This is the reason many people survive a first heart attack but die of a second or third attack even though their fatal MI may be relatively mild

The LCA branches quickly to the left main which branches almost at once to the LAD The LAD runs on the outside of the heart along the front of the heart along the septum The left main supplies the left atrium and the majority of the left ventricle in other words the major pumping por-tion of the heart An occlusion in the left main can be particularly disastrous because it is so far upstream and affects the heartrsquos left ventricular muscle mass No wonder they call the left main the widow maker

The coronary arteries do not run in straight lines they twist and turn and sometimes make very sharp bends People with coronary artery disease (CAD) typically have plaque buildup in the areas where the coronary arteries bend at sharp angles These are areas where the blood flow creates a lot of turbulence and plaque and other substances can collect and build up It is typically to find blockage and occlusions at these places

The LAD supplies the anterior wall of the left ventricle and the septum with oxygen-rich blood Since part of the heartrsquos conduction system runs through the septum (the right and left bundle branches) an LAD occlusion can damage the heartrsquos ability to conduct electricity properly

The circumflex artery runs along the AV groove laterally and thus supplies oxygen-rich blood to the lateral wall of the left ventricle along with the left atrium The circumflex runs along the AV groove to the left heading toward the back of the heart where it meets with the RCA which

travels along the AV groove to the left toward the back of the heart Both the circumflex and the RCA are responsible for providing oxygenated blood to the posterior wall of the ventricle But which is the more important provider That depends on crux and whether the heart is right or left dominant

The crux refers to an imaginary line drawn exactly down the middle of the ventricular poste-rior wall without any regard to anatomical landmarks If the RCA crosses the crux then the heart is said to be right dominant and the RCA is the greater supplier of oxygenated blood to the ven-tricular posterior wall On the other hand if the circumflex crosses the crux then the heart is said to be left dominant and the circumflex is the more significant provider Roughly 60 of the population is right dominant

While this information about the vasculature is important it is even more important for clini-cians to recognize that there is tremendous interpatient variability in terms of venous anatomy While we can describe major structures and typical venous formations anomalies are very common These anomalies may be minor variations or they can be very pronounced such as missing veins and arteries Clinicians who deal with the cardiovascular system must be prepared for large and small differences in vascular anatomy The cardiovascular system is incredibly resilient and can adapt to some anomalies by building up collateral circulation For example a person with a missing or incompetent vessel may over time build up a network of smaller collateral vessels that compensates by doing the same job It is not unusual to see vessels grow larger or branch out to compensate Sometimes when an artery is blocked collateral circulation will build up and compensate by delivering oxygen-rich blood to the heart muscle

Every artery in the body has a matching or corresponding vein usually located in close proximity This applies to the coronary system as well Note that there are great differences in coro-nary venous anatomy among patients which becomes important when implanting left ventric-ular leads for CRT devices (see Figure 111)

12 The nuts and bolts of implantable device therapy pacemakers

the nuts and bolts of cardiovascular anatomy and physiology

bull The heart consists of four chambers which may be considered the upper chambers (right atrium and left atrium) and lower chambers (right ven-tricle and left ventricle) or may be thought of as the right heart (right atrium and right ventricle) and left heart (left atrium and left ventricle)

bull Deoxygenated blood enters the right atrium flows to the right ventricle and is pumped out via the pulmonary valve to the lungs The blood is reoxygenated in the lungs and then enters the left atrium and flows to the left atrium and out of the aortic valve into the aorta and the rest of the body

bull The heart has two outflow tracts a right ven-tricular outflow tract (RVOT) and a left ven-tricular outflow tract (LVOT)

bull The largest chamber of the heart is the left ven-tricle making up about 23 of the cardiac mass The left ventricle is muscular and massive because it must pump against approximately 10 times the pressure against which the right ven-tricle pumps (120 vs 12 mmHg)

bull The two cardiac valves most susceptible to dysfunction and disease are the mitral and aortic valve because they function in a high-pressure environment

bull The chordae tendineae connect the leaflets of the valves to the papillary muscles and look like little parachutes These help the valves open and close properly

bull In general arteries transport blood away from the heart are muscular and contract to help move the blood on its way and have thicker walls than veins Veins have thinner less muscular walls and carry oxygen-depleted blood back to the heart using tiny interior valves to keep the blood flow moving forward Arteries transport oxygenated blood and can be considered high-pressure systems while veins transported deoxygenated blood in a low-pressure system

bull Cardiac contraction is systole cardiac relaxation is diastole Atrial systole occurs at the very end of ventricular diastole in something nicknamed atrial kick In a healthy individual about 20ndash30 of cardiac output comes from atrial kick

bull The heart is a muscle that is fed with oxygenated blood through a network of coronary arteries Coronary arteries get oxygen-rich blood from the aorta and they perfuse the heart muscle during diastole as blood drains back

bull Although arteries and veins generally take blood away from or to the heart respectively

Rightventricle

Leftventricle

Leftatrium

Aorta

Rightatrium

Rightcoronaryartery

Leftcoronaryartery

Circumflexartery

Superiorvena cava

Inferiorvena cava

Arterial system

Rightventricle

Leftventricle

Greatcardiac vein

Smallcardiac vein

Middlecardiac vein

Leftatrium

Aorta

Rightatrium

Superiorvena cava

Inferiorvena cava

Venous system

Coronarysinus

Figure 111 The coronary artery system is matched by a coronary venous system

Chapter 1 Cardiovascular anatomy and physiology 13

test your knowledge

Fill in these anatomical landmarks of the heart They include the four chambers the four valves and the bodyrsquos main artery

the pulmonary vessels are the exception The pulmonary artery transports deoxygenated blood while the pulmonary veins carry oxygen-ated blood This is because of their special loca-tion and the fact that their work is taking blood from the right side to the left side of the heart

bull The main coronary arteries are the right coro-nary artery and the left coronary artery The left coronary artery branches quickly to a short vessel known as the left main (the widow maker) and the circumflex artery

bull A myocardial infarction can kill the heart muscle The clogged arteries determine the loc ation of the affected muscle A right coro-nary artery occlusion will affect the right atrium and ventricle and the inferior wall of the left ventricle while a left coronary artery occlusion will affect the left atrium and most of the left ventricle By far the most dangerous occlusion is in the left main because it feeds the majority of the heart muscle with oxygen-rich blood

answer the following questions

1 What are the names of the two atrioventricular valves of the heartA Atrial and ventricularB Tricuspid and mitralC Pulmonary and pulmonicD Aortic and superior vena cava

2 Which chambers of the heart contain trabeculaeA AtriaB VentriclesC Right-sided chambers onlyD Left-sided chambers only

14 The nuts and bolts of implantable device therapy pacemakers

3 Why can trabeculae be important to device cliniciansA They are good places to secure a passive-fixa-

tion leadB They help regulate blood pressureC They prevent atherosclerosisD All of the above

4 Which of the following is not true about the aortaA It is an arteryB It consists of an ascending portion a descend-

ing portion and an archC It carries only oxygenated bloodD It is the largest vein in the body

5 From which artery does blood enter the right atriumA Superior vena cavaB Inferior vena cavaC Coronary sinusD All of the above

6 What is atrial kickA Ventricular pressure on the tricuspid valve

forcing it shutB Electrical impulses originating in the atrium

to cause the heart to beat fasterC Atrial systole that forces the most blood possible

into the already-full and diastolic ventriclesD A type of arrhythmia

7 Why would a person with chronic AF likely have decreased cardiac outputA AF is associated with low blood volumeB AF causes the heart to beat very slowlyC AF may cause the heart to experience fre-

quent pausesD AF reduces or eliminates atrial kickmdashand

atrial kick contributes to cardiac output

8 Which of the following is often a preferred location for affixing a cardiac pacing leadA The right ventricular outflow tract (RVOT)B The left ventricular outflow tract (LVOT)C The excised right atrial appendageD The tricuspid valve

9 What connects the valve leaflets to the papillary musclesA TrabeculaeB ArteriesC Chordae tendineaeD Tiny valves

10 Which coronary artery is nicknamed the widow maker because an occlusion in this vessel will deprive the majority of the heartrsquos muscle mass of oxygen-rich bloodA Left mainB CircumflexC Right coronary arteryD Inferior vena cava

15

The Nuts and Bolts of Implantable Device Therapy Pacemakers First Edition Tom Kenny copy 2015 John Wiley amp Sons Ltd Published 2015 by John Wiley amp Sons Ltd

Chapter 2

Cardiac conduction system

Introduction

The sinoatrial (SA) node is the heartrsquos natural pacemaker In the healthy heart the SA node fires and the electrical stimulus conducts along special pathways through the heart that cause cells in specific areas to depolarize After the stimulus passes the area the cells repolarize Myocardial cell depolarization and repolarization govern the heartrsquos contraction and relaxation sequencemdashin other words the heart rhythm Rhythm disorders occur when this healthy conduction system is in some way disrupted or impaired

the conduction pathways

When the SA node fires and sends out electrical energy the stimulus travels from the SA node in the high right atrium out over the atria via path-ways known as internodal tracts Most of these internodal tracts are located in the right atrium the

main tract over to the left atrium is called Bachmannrsquos bundle It is because of Bachmannrsquos bundle linking the right atrium to the left atrium that the atria can contract simultaneously that is as these two upper chambers contract as one coherent unit After traversing the atria the impulse comes down through conduction tracts to the atrioventricular (AV) node

The AV node is an area of highly specialized tissue located below the atria and above the ventri-cles on the right side of the septum Anatomically the AV node rests in the AV groove The tissue in the healthy AV node has the ability to change the speed of the electrical impulse specifically it slows the stimulus down The slowdown serves two important purposes First slowing the impulse allows the atria time to contract and provide the very valuable atrial kick to ventricular filling Second the slowed impulse protects the ventricles from being forced to beat too quickly that is to beat before they have been adequately filled with blood

Once the impulse crosses the AV node it travels down into the ventricles and accelerates its speed The impulse travels through the bundle of His (pronounced hiss) to the bundle branches (right and left) and then out to the network of Purkinje fibers This allows both ventricles to depolarize and contract simultaneously (Figure 21)

polarization depolarization and repolarization

Cardiac conduction is driven by the ability of myo-cardial cells to be depolarized and repolarized Depolarization occurs at the cellular level and is

2

Learning objectives

bullensp Describe the electrical conduction system of the healthy heart

bullensp List the main anatomical landmarks of normal cardiac conduction

bullensp Name the anatomical location of the heartrsquos natural pacemaker

bullensp Explain depolarization and repolarization in electrical terms and how it relates to the cardiac contraction

bullensp Define the term automaticity and how it relates to cardiac conduction

16 The nuts and bolts of implantable device therapy pacemakers

in turn driven by electrolytes or positively and negatively charged ions (potassium sodium and calcium mainly) which are able to move in and out of the cardiac cells A myocardial cell may be said to be polarized when in its resting state that is there is no electrical activity going on In the rest-ing state the interior of the myocardial cell is somewhat more negatively charged that the outside This state is called the resting state or sometimes phase 0 of the action potential

An electrical stimulus changes the resting state abruptly The electrical energy allows for the movement of ions to the point that the interior of the myocardial cell is now more positive than the outside In other words the charge reverses and the cell is said to be depolarized When this happens the cell contracts when this happens to a large area of the myocardium the heart muscle contracts Cardiac contraction occurs as a result of cellular depolarization

When the electrical stimulus moves past the myocardial cell the ions move in reverse and the interior of the cell assumes its previous negative charge with respect to the outside This process is

called repolarization and it brings the myocar-dial cell back to the resting state When this happens the cell relaxes When a large area of the myocardium repolarizes the heart resumes its previous shape (loses its contraction)

Intrinsic pacemakers

The healthy heartrsquos natural pacemaker is the SA node but the SA node is not the only area of the heart that can generate an electrical pulse The SA node serves as the heartrsquos pacemaker because it generates these pulses faster than other areas In the healthy heart the SA node can fire at around 60ndash100 times a minute corresponding to a healthy heart rate The AV junction can also fire but it is much slower at around 40ndash60 times a minute Even the Purkinje fibers (the ventri-cles) can act as a pacemaker but they fire at only 20ndash40 times a minute Thus in a healthy individual the SA node outpaces these other potential pacemakers The AV junction and ven-tricles are sometimes called backup pacemakers because they take over the heart rate in the event

Sinoatrial (SA) node

Internodal tracts

Right bundle branch

Left bundle branch

Purkinje fibers

Bachmannrsquos bundle

Bundle of His

Atrioventricular (AV) node

Figure 21 The conduction pathways in the healthy heart

Chapter 2 Cardiac conduction system 17

that the SA node is not able to pace the heart While these backup pacemakers may help keep the heart beating in the event of a dysfunctional SA node they are slow and not as reliable as the SA node

The healthy heart is able to pace itself because of a property called automaticity Automaticity is an inherent property of individual cardiac conductive cells that allows them to depolarize spontaneously In other words some cells in the heart (such as the cells of the SA node) are able to generate an electrical stimulus on their own

refractoriness

Refractoriness refers to the temporary inability of a cardiac cell to respond to an electrical stimulus Refractoriness can be the normal appropriate and expected behavior of cardiac cells For example immediately after electrical energy has passed through cardiac cells they go through a phase where they are unresponsive to electrical energy no matter how much voltage is delivered This normal refractoriness is called the physiological refractory period and it is very brief (it is mea-sured in thousandths of a second) Of course the exact duration of a refractory period is based in part on the heart rate the faster the heart rate the shorter the refractory period and vice versa

Refractoriness may also occur when certain cells become dysfunctional that is cardiac cells may lose their ability to conduct properly Furthermore the heartrsquos inherent pacemakers may change their rate for instance the AV junction may speed up and start to deliver impulses at 80 or 100 times a minute When this happensmdashthat is when another pacemaker takes over the role of the SA nodemdashthat pacemaker is called a usurping pacemaker because it usurps the rightful role of the SA node In the event that other pacemakers of the heart start to compete with the SA node the fastest pacemaker always wins

Conductivity

Conductivity is the cardiac cellrsquos ability to transmit an electrical impulse to an adjacent cell Typically cardiac conduction is antegrade that

is it moves forward Electrical impulses actually travel out in all different directions but refracto-riness prevents those cells that were just stimu-lated from depolarization This keeps conduction moving in a single direction through the heart Normal antegrade conduction in a healthy heart goes from SA node to atria to AV node over the bundle of His and down to the ventricles Some individuals have the ability to conduct retro-grade retrograde conduction is abnormal but not uncommon

autonomic nervous system

The heart does not need direct stimulation from the nervous system to generate electricity and to beat but that is not to say that the nervous system does not control the heart rate The autonomic nervous system indirectly affects cardiac conduction by regulating how fast the heart will beat and how hard the heart muscle will squeeze (contractility) The autonomic ner-vous system communicates with the heart through a network of baroreceptors which respond to chemical signals

The autonomic nervous system of the body like certain other physiological systems exists as two systems in balance The autonomic nervous system consists of a sympathetic system and a parasympathetic system They both work together and in a healthy individual one may overdrive the other during specific times The sympathetic ner-vous system is mainly associated with epinephrine (adrenaline) and it has the ability to speed the heart rate and increase cardiac contractility and can increase the speed by which electrical impulses can cross the AV node The sympathetic nervous system takes over when we are stressed frightened (fight-or-flight response) or exerting ourselves

On the other hand the autonomic nervous system also has a parasympathetic system that is associated with acetylcholine This system can decrease the heart rate decrease cardiac contrac-tility and cause impulses crossing the AV node to be slowed The parasympathetic system might take over when we are sleeping or at rest

18 The nuts and bolts of implantable device therapy pacemakers

the nuts and bolts of the cardiac conduction system

bull The heartrsquos natural pacemaker is the sinoatrial (SA) node which generates an electrical impulse spontaneously about 60ndash100 times a minute in a healthy individual

bull The normal conduction path is from the SA node over the atria to the atrioventricular (AV) node and then via the bundle of His down the right and left bundle branches and into the Purkinje fibers

bull The AV node has the ability to slow down the electrical impulse This serves two main purposes it allows for atrial kick and it protects the ventricles from beating too rapidly

bull In the healthy heart the right and left atria contract simultaneously and then the right and left ventricles contract simultaneously

bull Depolarization and repolarization of myocar-dial cells are driven by positively and negatively charged ions which cross the cell membrane A resting cardiac cell is more negative on the inside than the surrounding media but when it depolarizes it becomes more positive on the inside relative to the surrounding media Following depolarization the ions move and the cell returns to being more negative on the inside than the outside in a process known as repolarization

bull Depolarization causes the cardiac cell to contract

bull The main ions involved in depolarization and repolarization are sodium potassium and calcium ions

bull Although the heartrsquos natural pacemaker is the SA node the AV junction and the Purkinje fibers (ventricles) can also assume (usurp) this role if the SA node cannot generate electricity properly The AV junction and Purkinje fibers are not as reliable as a healthy AV node and typically generate electricity more slowly (the AV junction about 40ndash60 times a minute and the Purkinje fibers 20ndash40 times a minute)

bull However sometimes the AV junction or Purkinje fibers may generate electricity very

quickly this is abnormal When they do they may take over the heart When pacemakers of the heart compete the fastest one drives the heart

bull Automaticity is an inherent property of certain cardiac conductive cells to depolarize spontaneously Not all cardiac conduction cells possess automaticity but the SA node AV junction and Purkinje fibers possess automaticity

bull Refractoriness is the natural and expected behavior of cardiac conductive cells that makes them unresponsive to electrical impulses immediately after they have depolarized The natural refractory period is very brief

bull The faster the heart rate the shorter the refractory period the slower the heart rate the longer the refractory period

bull Refractoriness can also occur as a dysfunction bull Conductivity is the cardiac cellrsquos ability to

transmit an electrical impulse to adjacent cells Refractoriness means that cells that just depolarized will not depolarize again This keeps conduction moving forward (antegrade conduction)

bull Antegrade conduction is normal retrograde conduction is abnormal but is not rare

bull The autonomic nervous system regulates the heart rate by controlling how fast the heart beats and how vigorously it contracts

bull The autonomic nervous system consists of a sympathetic and parasympathetic nervous system which work together but at certain times one or the other is dominant The sympathetic nervous system makes the heart beat faster and more vigorously it takes over during times of stress exertion or fight or flight The parasympathetic ner-vous system makes the heart beat slowly and less vigorously it takes over during periods of rest

bull The neurotransmitters associated with these two systems are epinephrine or adrenaline (sympathetic nervous system) and acetylcho-line (parasympathetic nervous system)

Chapter 2 Cardiac conduction system 19

test your knowledge

Please write in the landmarks of the cardiac conduction system Select from the following (each is used only once)

AV node Bachmanrsquos bundle Bundle of His

Internodal tracts Left bundle branch Purkinje fibers

Right bundle branch Ventricles

1 Which of the following is not considered one of the heartrsquos intrinsic pacemakersA AV junctionB SA nodeC Purkinje fibersD Aorta

2 Depolarization and repolarization involve the movement of charged ions across cardiac cell membranes What types of ions are primarily involvedA Calcium potassium and sodiumB Acetylcholine and epinephrineC Magnesium and folateD All of the above

3 Refractoriness refers toA The ability of cardiac conduction cells to

contract

B The inability of cardiac conduction cells to respond to an electrical stimulus

C The unique permeability of the cardiac cellsD Abnormal cardiac conduction

4 When a cardiac cell depolarizes the interior of the cell becomes what with respect to the surrounding mediaA More positiveB More negativeC More acidicD Hotter

5 Fill in the blank In the healthy heart the faster the heart rate the ___ the refractory periodA More vigorousB Less reliableC LongerD Shorter

20 The nuts and bolts of implantable device therapy pacemakers

6 Which specific part of the nervous system governs the fight-or-flight response and is associated with epinephrine (adrenaline) (Pick the most exact answer)A Autonomic nervous systemB Parasympathetic nervous systemC Sympathetic nervous systemD Central nervous system

7 In normal antegrade conduction the electrical impulse travels from the SA node over the atria to the AV node and to what structure before reaching the ventriclesA The intermodal tractsB The Purkinje fibersC The bundle of HisD The ventricular apex

8 Which of the following is true about retrograde cardiac conductionA Everyone has itB It is perfectly normal and can be helpful in

stressful situations

C It is abnormal but not uncommonD It is impossible to diagnose definitively

except in an autopsy

9 What chemical is associated with the parasympathetic nervous systemA AcetylcholineB PotassiumC EpinephrineD Dopamine

10 The stimulation of the myocardial cells of which of the following structures is most closely associated with ventricular depolarizationA SA nodeB AV junctionC Bundle of HisD Purkinje fibers

2 The nuts and bolts of implantable device therapy pacemakers

provide a fast noninvasive way of determining if and to what degree the heart has enlarged

The left ventricle composes most of the mass of the heart being by far the largest of the four pump-ing chambers A healthy heart circulates about 4ndash6 l of blood a minutemdashwhich is the entire blood volume of the body That means the entire circulating volume of blood in the body moves around every minute or once per beat

The heart consists of four chambers two upper chambers called atria (singular atrium) and two lower and larger chambers called ventricles To understand the healthy heart it is useful to think of the heart in terms of right side (right atrium and right ventricle) and left side (left atrium and left ventricle) The right side of the heart circulates deoxygenated blood to the lungs (where it can be oxygenated) The left side of the heart pumps oxy-genated blood out to the rest of the body (see Figure 11)

The heart is a muscle and consists of four dis-tinct layers The endocardium is the innermost layer and composes a lining for the interior of the heart The epicardium is the outer layer of the heart Between the endocardium and epicardium lies the myocardiummdashthe thickest layermdashwhich is muscle The entire heart is encased in a

liquid-filled sac called the pericardium which acts like a shock absorber for the heart The pericardial sac contains about 15ndash20 cc of pericardial fluid in a healthy individual In the event that fluid builds up to abnormally high levels in the pericardial sac (such as might occur when a lead or catheter inside the heart perforates the endocardium myocar-dium and epicardium and goes exterior to the heart) this fluid can place pressure on the heart in a condition known as cardiac tamponade Since the heart is contained in a relatively small space this pressure can compromise the heartrsquos ability to fill with blood and pump efficiently During device implantation perforation is an important concern because it can lead to cardiac tamponade In the event that perforation results in cardiac tamponade a needle is inserted into the pericardial sac (through the chest wall) to drain the blood Lead perforation does not always result in cardiac tamponade but it is a serious concern

Blood flow through the heart

The heart is a pump and it is located amid a net-work of vessels that carry deoxygenated blood into the right side of the heart and reoxygenated blood into the left side of the heart The flow is actually

Right atrium

Left atrium

Left pulmonaryarteries

Left pulmonaryveins

Left ventricle

Right ventricle

Aorta

Aortic valve

Mitral valve

Tricuspidvalve

Pulmonaryvalve

SA node

AV node

Right pulmonaryarteries

Right pulmonaryveins

Figure 11 Cross section of the heart showing the chambers

Chapter 1 Cardiovascular anatomy and physiology 3

fairly simple Deoxygenated blood enters the right side of the heart and is pumped over to the lungs via the pulmonary arteries and is returned back (as oxygen-rich blood) to the left side of the heart by way of the pulmonary veins (PV) While both right and left sides of the heart contract at the same time as a single unit the right side is busy pumping deoxygenated blood to the lungs while the left side is pumping reoxygenated blood out to the rest of the body

Deoxygenated blood enters the right side of the heart via the superior vena cava (SVC) but once it has become oxygenated again blood is pumped back out from the left side of the heart into the aorta The aorta is the largest vessel in the body and it forms a U shape at the top of the heart These portions of the aorta are called the ascending the descending and the arch Coming off the aortic arch are three main arteries the left subclavian artery the left common carotid artery and the brachiocephalic trunk

To better understand the blood flow through the heart it is important to review the structure of the heart The atria or upper chambers of the heart are smaller have thinner walls and are smoother on the inside than the ventricles Within the ventri-cles is a network of fibrous strands known as trabeculae These structural differences become important in lead implantation within the heart it is much easier to affix or lodge a lead in the trabeculae of the ventricles than to try to anchor the lead to a smooth atrial wall Historically atrial leads have almost always been active-fixation screw-in-type leads while ventricular leads were almost always passive-fixation leads (fins or tines that lodge in the trabeculae) Today active-fixation leads are often used in both chambers since they facilitate lead removal (Figure 12)

Overall blood flow to the heart is discussed right and left sides although it is important to recognize that what happens in the heart that is systole (contraction) and diastole (relaxation) are happening on both sides at the same time The right atrium of the heart receives blood from the SVC the inferior vena cava (IVC) and the coronary sinus (CS) The CS is technically a vein and it has an opening or ostium (sometimes just called os) at the base of the right atrium slightly posterior The CS delivers oxygen-depleted blood

to the right atrium from the coronary arteries that encircle the exterior of the heart The CS is of interest in cardiac resynchronization therapy (CRT) because the left ventricular lead is passed through the CS (counter to the flow of blood) in order to be placed into the coronary vessels to pace the left ventricle CRT is used in patients with heart failure whose hearts have remodeled that is enlarged and changed shape (It may be said that with heart failure the heart changes from the shape of a football to the shape of a basketball) The CS may be relocated in this remodeling which can be challenging in implanting a CRT lead because the physician must first locate the os of the CS and then navigate through it in order to implant the left ventricular lead

Anatomically the heart is dominated by the large muscle mass of the left ventricle which makes up about two-thirds of the heart in terms of weight and volume This greater size is typically ascribed to the fact that the left ventricle must pump blood throughout the whole body whereas the right ventricle only has to pump blood to the lungs The left and right ventricles pump blood to different destinations but the left ventricle is larger and more muscular for a reasonmdashpressure It is impor-tant to review the pressures against which the heart must work to understand cardiac blood flow (Figure 13)

Deoxygenated blood in the right side of the heart must travel over the lungs to pick up oxygen This means that blood in the right ventricle travels across the pulmonary valve into the pulmonary artery and then out toward the lungs The pulmonary valve opens automatically when pressure from the contracting right ventricle forces it open This occurs when the pressure in the right ventricle exceeds the pressure in the pulmonary artery Pressure gradients are key concepts in understanding blood flow Valves are like gates that open and close in response to pressure In general the pressure in the PV is fairly low around 12 mmHg Thus the right ventricle does not need to create a lot of force to open the pulmonary valve

Meanwhile as the left ventricle contracts it cre-ates pressure on the aortic valve leading to the aorta In order to open the aortic valve and pump blood out into the aorta the heart must overcome the pressure in the aortic valve Pressure in the

4 The nuts and bolts of implantable device therapy pacemakers

Right atrium

Left atrium

Left ventricle

Right ventricle

Figure 12 Note that the atria are smooth walled while the ventricles contain a spongelike fibrous network of trabeculae

Left pulmonaryarteries

Left pulmonaryveins

Aorta

Aortic valve

Pulmonaryvalve

Right pulmonaryarteries

Superiorvena cava

Inferiorvena cava

Right pulmonaryveins

Figure 13 The blood flow within the heart takes oxygen-depleted blood from the body into the right atrium where it flows to the right ventricle and is pumped out over the lungs the reoxygenated blood from the lungs is pumped into the left atrium where it flows to the left ventricle and is pumped out via the aorta to the body

Chapter 1 Cardiovascular anatomy and physiology 5

aorta is high around 120 mmHg or ten times higher than the pressure in the PV The left ventricle must therefore work much harder to pump blood than the right ventricle This requires the left ventricle to be larger and more muscular than the right ven-tricle (see Figure 14)

On the right side of the heart blood travels from the right atrium into the right ventricle via the tricuspid valve The tricuspid valve gets its name from its characteristic shape involving three leaves or cusps Attached to these cusps are cords that anchor into the base of the ventricle known as chordae tendineae they look almost like little parachutes The strands of the chordae tendineae attach to tiny papillary muscles These chords attach to the valve leaves at one end and a papil-lary muscle at the other end On the right side of the heart the tricuspid valve is associated with three papillary muscles The purpose of these chords and muscles is to assure that the valve is

effectively closed and opened at the proper times (Figure 15)

The heart can rightly be thought of as a pump but it must be remembered that the heart is also a muscle and all muscles need a steady supply of oxy-gen-rich blood The heart muscle is supplied with blood through a network of coronary arteries that surround the outside of the heart Blockage in a coronary artery results in ischemia which can lead to death of cardiac muscle including the chordae tendineae and papillary muscles While the patient may survive such an ischemic event the damage to the heart may lead to an incompetent valve that is a valve that is no longer able to function effectively

In tracing the blood flow from the right ventricle to the pulmonary artery it should be clear that the blood has to go from down in the right ventricle to up through the pulmonary valve and into the pulmonary artery The blood is able to make this journey because of the pumping pressure of the heart The route the blood takes as it exits the right ventricle and journeys up toward the pulmonary valve is known as the right ventricular outflow tract (RVOT) On the other side of the heart there is also a corresponding left ventricular outflow tract (LVOT) of approximately the same size Cardiac leads are sometimes fixated in the RVOT (see Figure 16)

Blood pumped out of the right ventricle crosses the pulmonary valve and enters the pulmonary artery which splits into two branches right and left The right pulmonary artery takes blood to the right lung while the left pulmonary artery takes blood to the left lung In this respect the pulmonary artery is unique in the body in that it is an artery but it carries deoxygenated blood Blood travels through the lung to the alveoli where it gains oxygen and loses carbon dioxide Once it is reoxygenated the blood gathers into the PV (which are also unique being the only veins to carry oxygenated blood) There are four PV in total the two right-sided PV take oxygen-ated blood from the right lung while the two left-sided PV take oxygenated blood from the left lung and they all bring this reoxygenated blood to the left atrium

The left atrium is smooth walled like the right atrium and although the left atrium is much

Figure 14 The left ventricular is far more muscular than the right ventricle because it must overcome 10 times the pressure of the right ventricle in order to pump blood out via the aortic valve and into the aorta

6 The nuts and bolts of implantable device therapy pacemakers

Right atrium

Left atrium

Left ventricle

Right ventricle

Figure 15 The leaflets of the valves are attached by chordae tendineae at one end and papillary muscles on the other which assure the effective opening and closing of the valves

Right-ventricularoutflow tract (RVOT)

Figure 16 There are two outflow tracts in the heart one associated with the right ventricle and the other the left ventricle These outflow tracts are roughly the same size The illustration shows the right ventricular outflow tract (RVOT) a preferred location for right ventricular lead fixation The left ventricular outflow tract (LVOT) cannot be seen in this illustration as it is posterior

Chapter 1 Cardiovascular anatomy and physiology 7

smaller than the left ventricle it is larger than the right atrium The blood will travel from the left atrium into the left ventricle by way of the mitral valve The mitral valve gets its name from the miter a bishoprsquos hat The mitral valve has two cusps and papillary muscles connected by chordae tendineae Once blood is in the left ventricle a contraction will force it toward the aorta by way of the LVOT In persons with hypertrophic obstructive cardiomy-opathy (HOCM) the septum or wall in the heart can become thick and enlarged effectively narrow-ing the LVOT In the healthy heart the blood flows up the LVOT across the aortic valve and into the aorta From the aorta blood will be directed upward toward the head downward toward the legs and feet and some blood will be redirected back to the coronary arteries of the heart

The blood flow from the left ventricle to the aorta and back to the coronary arteries is unique in the body in that most of that flow occurs during diastole rather than systole Think of it this way as blood is pumped by the left ventricle during systole upward and downward to the brain and the rest of the body some of that blood flows back during diastole and drains into the coronary arteries Although it may seem counterintuitive it is impor-tant to remember that coronary artery perfusion occurs primarily during diastole

Volume valves and pressure

While it is tempting to think of the heart as simply a pump with an electrical system it is more accu-rate to say that the heart moves blood because of variations in volume and pressure

The heart has four valves two are atrioventricular (AV) valves because they connect the atrium to the ventricle and two are semilunar valves connecting the heart with pulmonary arteries or the aorta The right AV valve is the tricuspid while the left AV valve is the mitral valve The flow or movement of blood is guided and controlled by the heartrsquos muscular action in the form of systole (contraction) or diastole (relaxation)

When blood pours into the right atrium it dis-tends and expands the right atrium When this volume of blood creates sufficient pressure it forces the tricuspid valve open Think of it as volume creating pressure and pressure opening

valves When the tricuspid valve opens the blood dumps into the right ventricle During this period of diastole blood pours into the right ventricle creating volume and expanding it As blood pours in and ventricular diastole nears the end the atria contract while the AV valves are still open in something nicknamed atrial kick Atrial systole occurs at the end of ventricular diastole Atrial kick forces the maximum amount of blood from the atria into the ventricles It is estimated that atrial kick delivers 20ndash30 of cardiac output

The atrial contribution to ventricular filling (atrial kick) is of enormous clinical significance Patients with certain atrial arrhythmias such as atrial fibrillation (AF) lose atrial kick Even milder forms of atrial tachyarrhythmias may compromise atrial kick Moreover patients who do not have AV synchrony (one atrial beat corresponding to one ventricular beat) will lose atrial kick The loss of atrial kick can reduce cardiac output by 20ndash30

Once the atria have contracted and the ventricles are filled with blood the AV valves snap closed The ventricles now start to contract and push open the semilunar valves For example as the right ven-tricle contracts (systole) it opens the PV (The PV is sometimes called the pulmonic valve itrsquos the same thing) The whole process works on volume pressure and contractionrelaxation

Of the four valves in the heart the tricuspid valve is of most interest to device specialists because at least one lead will be placed into the right ventricle This means that pacing and defi-brillation leads typically cross the tricuspid valve These leads may interfere with the proper closing of the tricuspid valve particularly if the lead is large diameter or several leads are passed over the tricuspid valve A single lead through the tricuspid valve is likely to cause minimal to no dysfunction However it is not unusual to see patients with three or more leads in the right ventricle and these leads may hamper the tricuspid valve in its closing leading to tricuspid regurgitation or backflow of blood Patients with multiple leads (such as chil-dren who grow up with pacemakers) may need to have some of them removed to avoid tricuspid regurgitation

Right ventricular leads are typically fixated at the right ventricular apex or in the RVOT and may be

8 The nuts and bolts of implantable device therapy pacemakers

lodged in the trabeculae Trabeculae are a fibrous network that could be described as similar to the pores of a sponge (see Figure 17)

On the left side of the heart the blood flow pattern is similar Blood flows into the left atrium over the open mitral valve and into the left atrium At the end of ventricular diastole as the ventricle is distended and stretched atrial kick delivers more blood into left ventricle The volume of blood cre-ates pressure The mitral valve closes As the left ventricle contracts the growing pressure over-comes the resistance on the aortic valve and it opens allowing blood to flow out into the aorta and beyond

While any valve can become diseased or dys-functional valvular disease more commonly affects the mitral and the aortic valves (left-sided valves) than the right-sided valves (tricuspid and pul-monary) These left valves are more likely to suffer damage or disease because of the high-pressure environment in which they function exposing them to more potential damage Other damage can occur when plaque builds up in the coronary arteries and the aorta The aorta can become

diseased typically in the case of an aortic aneurysm or dissection Carotid arteries in the neck are also frequently a site for atherosclerotic disease

the right atrial appendage

The right atrial appendage (RAA) is an area near the right atrium which is a preferred site for attach-ing active-fixation atrial leads (see Figure 18) The RAA serves no obvious purpose and may be sacri-ficed in certain heart surgeries such as a coronary artery bypass graft (CABG) procedure CABG or bypass surgery involves stopping the heart and running the blood through a machine to reoxygen-ate it during the course of the procedure This is done by attaching a large-diameter hose to the RAA when surgery concludes and this hose is removed it can damage the RAA to the point that it is completely or partially surgically removed In patients who have had a CABG procedure the RAA may not be available for right atrial lead placement For such patients the right atrial lead is often placed against the lateral wall of the right atrium It is sometimes possible to fixate a right

Figure 17 Trabeculae form a fibrous network within the ventricles Passive-fixation ventricular leads are held in place by lodging their tines or fins in the trabeculae

Chapter 1 Cardiovascular anatomy and physiology 9

atrial lead in the remnants of the RAA if a portion of it is preserved

arteries and veins

Any discussion about the heart necessarily involves the vasculature which is why we commonly refer to our circulatory system as the cardiovascular

system As most of us remember from anatomy classes arteries carry blood away from the heart while veins transport blood back to the heart (The exceptions are the PV and arteries described earlier) Arteries carry oxygen-rich blood while veins carry deoxygenated blood What is of importance to device clinicians is that arteries are high-pressure vessels while veins are low-pressure vessels Arteries tend to be more muscular in structure and muscular contractions of the arteries help to move blood outward into the body Veins tend to be less muscular than arteries but unlike arteries they contain a system of tiny interior valves The purpose of these little valves is to maintain a unidirectional flow of blood (see Figure 19)

Some of the most important veins in the body are the SVC and its counterpart the IVC The SVC takes blood from the upper part of the body and delivers it into the right atrium with the IVC doing the same for blood from the lower part of the body The coronary veins networked around the outside of the heart carry blood from the heart tissue and deliver it back to the right atrium they are closely linked to the coronary arteries which carry reoxygenated blood from the left ventricle back to the heart muscle The CS is the site where

Right atrial appendage

Figure 18 The right atrial appendage serves no obvious hemodynamic purpose but is often the site of fixation of atrial leads

Artery Vein

Figure 19 Veins and arteries have different purposes different anatomical structure and different pressures Overall veins are low-pressure systems that move blood by a series of little valves that keep blood flowing in the same direction Arteries are high-pressure systems and are more muscular so that they force blood forward by squeezing

10 The nuts and bolts of implantable device therapy pacemakers

the great middle and small cardiac veins all drain the CS takes this blood and delivers it back to the right atrium

The PV carry reoxygenated blood from the lungs and deliver it to the left atrium so that it can be pumped back out to the body by the left ventricle

The most important artery in the body is by far the aorta The aorta is connected to the left ven-tricle and it is the first and main conduit that takes reoxygenated blood from the left ventricle and sends it out to the body As this reoxygenated blood makes its way out into the body its first two stops are the head (brain) and the heart muscle Inadequate oxygenated blood to the brain can very quickly provoke symptoms This is the reason that depressed cardiac output is associated with symptoms like dizziness light-headedness feeling woozy and fatigue Severe symptoms might include syncope

The coronary arteries arise from the root of the aorta just above the cusps of the aortic valve Coronary arteries receive most blood during dias-tole so blood is more or less pouring back into these vessels The coronary artery system is a networkmdashalmost a meshmdashof middle-sized to very small vessels that cover the exterior surface of the heart (see Figure 110)

Coronary arteries get their name from the Latin word for crown (such as in our word coronation) because this network of vessels sits like a crown atop the heart and encircles it The heart gets blood from outside in as oxygen-rich blood in these coro-nary arteries is delivered to the exterior of the heart muscle Coronary arteries are not within the heart they are epicardial structures The two main coro-nary arteries are the right coronary artery (RCA) and the left coronary artery (LCA) The RCA and LCA come off the aorta and are located in the AV groove an exterior structure below the atria and above the ventricles The RCA runs from the aorta in the right AV groove while the LCA comes off the aorta and quickly branches to the left main The left main is a very short artery (about the size of a thumbnail) which branches quickly into the left anterior descending artery (LAD) and the circum-flex artery The LAD travels down the front of the

left side of the heart The circumflex artery can be found in the left AV groove

The RCA bifurcates to the posterior descending artery The right atrium and right ventricle get their supply of oxygen-rich blood from the RCA Much of the cardiac conduction system is right sided that is it commences with the sinoatrial (SA) node in the high right atrium and travels down to the AV node on the right side of the septum These important electrical structures get their oxygen-rich blood supply from the RCA The electrical system travels down the ventricles to the inferior wall of the left ventricle (located at the bottom of the heart but belonging to the left ventricle) the inferior wall is also fed oxygen-rich blood by the RCA An occlusion of the RCA such as might occur in a myocardial infarction (MI) may result in death of the heart muscle it feeds which includes the right atrium the right ventricle and the inferior wall of the left ventricle and it might affect the conduction system including the SA node and the AV node An inferior wall MI is typically the result of a clogged RCA

RCA

LCA

Aorta

Figure 110 The coronary arteries of the heart branch out to surround the entire heart muscle The two most prominent coronary arteries are the right coronary artery (RCA) and the left coronary artery (LCA)

Chapter 1 Cardiovascular anatomy and physiology 11

For an MI the more distal the occlusion (that is further from the aorta) the less potential damage the heart attack will have In other words the worst occlusions are proximal Whether or not a patient survives an MI comes down to the numbers The loss of over 40 of the left ventricular muscle mass results in death However that 40 is cumulative For example a person can survive an MI that costs him 30 of left ventricular muscle mass but if that heart attack is followed by another that kills 15 of the left ventricular muscle mass the second (milder) heart attack will prove fatal This is the reason many people survive a first heart attack but die of a second or third attack even though their fatal MI may be relatively mild

The LCA branches quickly to the left main which branches almost at once to the LAD The LAD runs on the outside of the heart along the front of the heart along the septum The left main supplies the left atrium and the majority of the left ventricle in other words the major pumping por-tion of the heart An occlusion in the left main can be particularly disastrous because it is so far upstream and affects the heartrsquos left ventricular muscle mass No wonder they call the left main the widow maker

The coronary arteries do not run in straight lines they twist and turn and sometimes make very sharp bends People with coronary artery disease (CAD) typically have plaque buildup in the areas where the coronary arteries bend at sharp angles These are areas where the blood flow creates a lot of turbulence and plaque and other substances can collect and build up It is typically to find blockage and occlusions at these places

The LAD supplies the anterior wall of the left ventricle and the septum with oxygen-rich blood Since part of the heartrsquos conduction system runs through the septum (the right and left bundle branches) an LAD occlusion can damage the heartrsquos ability to conduct electricity properly

The circumflex artery runs along the AV groove laterally and thus supplies oxygen-rich blood to the lateral wall of the left ventricle along with the left atrium The circumflex runs along the AV groove to the left heading toward the back of the heart where it meets with the RCA which

travels along the AV groove to the left toward the back of the heart Both the circumflex and the RCA are responsible for providing oxygenated blood to the posterior wall of the ventricle But which is the more important provider That depends on crux and whether the heart is right or left dominant

The crux refers to an imaginary line drawn exactly down the middle of the ventricular poste-rior wall without any regard to anatomical landmarks If the RCA crosses the crux then the heart is said to be right dominant and the RCA is the greater supplier of oxygenated blood to the ven-tricular posterior wall On the other hand if the circumflex crosses the crux then the heart is said to be left dominant and the circumflex is the more significant provider Roughly 60 of the population is right dominant

While this information about the vasculature is important it is even more important for clini-cians to recognize that there is tremendous interpatient variability in terms of venous anatomy While we can describe major structures and typical venous formations anomalies are very common These anomalies may be minor variations or they can be very pronounced such as missing veins and arteries Clinicians who deal with the cardiovascular system must be prepared for large and small differences in vascular anatomy The cardiovascular system is incredibly resilient and can adapt to some anomalies by building up collateral circulation For example a person with a missing or incompetent vessel may over time build up a network of smaller collateral vessels that compensates by doing the same job It is not unusual to see vessels grow larger or branch out to compensate Sometimes when an artery is blocked collateral circulation will build up and compensate by delivering oxygen-rich blood to the heart muscle

Every artery in the body has a matching or corresponding vein usually located in close proximity This applies to the coronary system as well Note that there are great differences in coro-nary venous anatomy among patients which becomes important when implanting left ventric-ular leads for CRT devices (see Figure 111)

12 The nuts and bolts of implantable device therapy pacemakers

the nuts and bolts of cardiovascular anatomy and physiology

bull The heart consists of four chambers which may be considered the upper chambers (right atrium and left atrium) and lower chambers (right ven-tricle and left ventricle) or may be thought of as the right heart (right atrium and right ventricle) and left heart (left atrium and left ventricle)

bull Deoxygenated blood enters the right atrium flows to the right ventricle and is pumped out via the pulmonary valve to the lungs The blood is reoxygenated in the lungs and then enters the left atrium and flows to the left atrium and out of the aortic valve into the aorta and the rest of the body

bull The heart has two outflow tracts a right ven-tricular outflow tract (RVOT) and a left ven-tricular outflow tract (LVOT)

bull The largest chamber of the heart is the left ven-tricle making up about 23 of the cardiac mass The left ventricle is muscular and massive because it must pump against approximately 10 times the pressure against which the right ven-tricle pumps (120 vs 12 mmHg)

bull The two cardiac valves most susceptible to dysfunction and disease are the mitral and aortic valve because they function in a high-pressure environment

bull The chordae tendineae connect the leaflets of the valves to the papillary muscles and look like little parachutes These help the valves open and close properly

bull In general arteries transport blood away from the heart are muscular and contract to help move the blood on its way and have thicker walls than veins Veins have thinner less muscular walls and carry oxygen-depleted blood back to the heart using tiny interior valves to keep the blood flow moving forward Arteries transport oxygenated blood and can be considered high-pressure systems while veins transported deoxygenated blood in a low-pressure system

bull Cardiac contraction is systole cardiac relaxation is diastole Atrial systole occurs at the very end of ventricular diastole in something nicknamed atrial kick In a healthy individual about 20ndash30 of cardiac output comes from atrial kick

bull The heart is a muscle that is fed with oxygenated blood through a network of coronary arteries Coronary arteries get oxygen-rich blood from the aorta and they perfuse the heart muscle during diastole as blood drains back

bull Although arteries and veins generally take blood away from or to the heart respectively

Rightventricle

Leftventricle

Leftatrium

Aorta

Rightatrium

Rightcoronaryartery

Leftcoronaryartery

Circumflexartery

Superiorvena cava

Inferiorvena cava

Arterial system

Rightventricle

Leftventricle

Greatcardiac vein

Smallcardiac vein

Middlecardiac vein

Leftatrium

Aorta

Rightatrium

Superiorvena cava

Inferiorvena cava

Venous system

Coronarysinus

Figure 111 The coronary artery system is matched by a coronary venous system

Chapter 1 Cardiovascular anatomy and physiology 13

test your knowledge

Fill in these anatomical landmarks of the heart They include the four chambers the four valves and the bodyrsquos main artery

the pulmonary vessels are the exception The pulmonary artery transports deoxygenated blood while the pulmonary veins carry oxygen-ated blood This is because of their special loca-tion and the fact that their work is taking blood from the right side to the left side of the heart

bull The main coronary arteries are the right coro-nary artery and the left coronary artery The left coronary artery branches quickly to a short vessel known as the left main (the widow maker) and the circumflex artery

bull A myocardial infarction can kill the heart muscle The clogged arteries determine the loc ation of the affected muscle A right coro-nary artery occlusion will affect the right atrium and ventricle and the inferior wall of the left ventricle while a left coronary artery occlusion will affect the left atrium and most of the left ventricle By far the most dangerous occlusion is in the left main because it feeds the majority of the heart muscle with oxygen-rich blood

answer the following questions

1 What are the names of the two atrioventricular valves of the heartA Atrial and ventricularB Tricuspid and mitralC Pulmonary and pulmonicD Aortic and superior vena cava

2 Which chambers of the heart contain trabeculaeA AtriaB VentriclesC Right-sided chambers onlyD Left-sided chambers only

14 The nuts and bolts of implantable device therapy pacemakers

3 Why can trabeculae be important to device cliniciansA They are good places to secure a passive-fixa-

tion leadB They help regulate blood pressureC They prevent atherosclerosisD All of the above

4 Which of the following is not true about the aortaA It is an arteryB It consists of an ascending portion a descend-

ing portion and an archC It carries only oxygenated bloodD It is the largest vein in the body

5 From which artery does blood enter the right atriumA Superior vena cavaB Inferior vena cavaC Coronary sinusD All of the above

6 What is atrial kickA Ventricular pressure on the tricuspid valve

forcing it shutB Electrical impulses originating in the atrium

to cause the heart to beat fasterC Atrial systole that forces the most blood possible

into the already-full and diastolic ventriclesD A type of arrhythmia

7 Why would a person with chronic AF likely have decreased cardiac outputA AF is associated with low blood volumeB AF causes the heart to beat very slowlyC AF may cause the heart to experience fre-

quent pausesD AF reduces or eliminates atrial kickmdashand

atrial kick contributes to cardiac output

8 Which of the following is often a preferred location for affixing a cardiac pacing leadA The right ventricular outflow tract (RVOT)B The left ventricular outflow tract (LVOT)C The excised right atrial appendageD The tricuspid valve

9 What connects the valve leaflets to the papillary musclesA TrabeculaeB ArteriesC Chordae tendineaeD Tiny valves

10 Which coronary artery is nicknamed the widow maker because an occlusion in this vessel will deprive the majority of the heartrsquos muscle mass of oxygen-rich bloodA Left mainB CircumflexC Right coronary arteryD Inferior vena cava

15

The Nuts and Bolts of Implantable Device Therapy Pacemakers First Edition Tom Kenny copy 2015 John Wiley amp Sons Ltd Published 2015 by John Wiley amp Sons Ltd

Chapter 2

Cardiac conduction system

Introduction

The sinoatrial (SA) node is the heartrsquos natural pacemaker In the healthy heart the SA node fires and the electrical stimulus conducts along special pathways through the heart that cause cells in specific areas to depolarize After the stimulus passes the area the cells repolarize Myocardial cell depolarization and repolarization govern the heartrsquos contraction and relaxation sequencemdashin other words the heart rhythm Rhythm disorders occur when this healthy conduction system is in some way disrupted or impaired

the conduction pathways

When the SA node fires and sends out electrical energy the stimulus travels from the SA node in the high right atrium out over the atria via path-ways known as internodal tracts Most of these internodal tracts are located in the right atrium the

main tract over to the left atrium is called Bachmannrsquos bundle It is because of Bachmannrsquos bundle linking the right atrium to the left atrium that the atria can contract simultaneously that is as these two upper chambers contract as one coherent unit After traversing the atria the impulse comes down through conduction tracts to the atrioventricular (AV) node

The AV node is an area of highly specialized tissue located below the atria and above the ventri-cles on the right side of the septum Anatomically the AV node rests in the AV groove The tissue in the healthy AV node has the ability to change the speed of the electrical impulse specifically it slows the stimulus down The slowdown serves two important purposes First slowing the impulse allows the atria time to contract and provide the very valuable atrial kick to ventricular filling Second the slowed impulse protects the ventricles from being forced to beat too quickly that is to beat before they have been adequately filled with blood

Once the impulse crosses the AV node it travels down into the ventricles and accelerates its speed The impulse travels through the bundle of His (pronounced hiss) to the bundle branches (right and left) and then out to the network of Purkinje fibers This allows both ventricles to depolarize and contract simultaneously (Figure 21)

polarization depolarization and repolarization

Cardiac conduction is driven by the ability of myo-cardial cells to be depolarized and repolarized Depolarization occurs at the cellular level and is

2

Learning objectives

bullensp Describe the electrical conduction system of the healthy heart

bullensp List the main anatomical landmarks of normal cardiac conduction

bullensp Name the anatomical location of the heartrsquos natural pacemaker

bullensp Explain depolarization and repolarization in electrical terms and how it relates to the cardiac contraction

bullensp Define the term automaticity and how it relates to cardiac conduction

16 The nuts and bolts of implantable device therapy pacemakers

in turn driven by electrolytes or positively and negatively charged ions (potassium sodium and calcium mainly) which are able to move in and out of the cardiac cells A myocardial cell may be said to be polarized when in its resting state that is there is no electrical activity going on In the rest-ing state the interior of the myocardial cell is somewhat more negatively charged that the outside This state is called the resting state or sometimes phase 0 of the action potential

An electrical stimulus changes the resting state abruptly The electrical energy allows for the movement of ions to the point that the interior of the myocardial cell is now more positive than the outside In other words the charge reverses and the cell is said to be depolarized When this happens the cell contracts when this happens to a large area of the myocardium the heart muscle contracts Cardiac contraction occurs as a result of cellular depolarization

When the electrical stimulus moves past the myocardial cell the ions move in reverse and the interior of the cell assumes its previous negative charge with respect to the outside This process is

called repolarization and it brings the myocar-dial cell back to the resting state When this happens the cell relaxes When a large area of the myocardium repolarizes the heart resumes its previous shape (loses its contraction)

Intrinsic pacemakers

The healthy heartrsquos natural pacemaker is the SA node but the SA node is not the only area of the heart that can generate an electrical pulse The SA node serves as the heartrsquos pacemaker because it generates these pulses faster than other areas In the healthy heart the SA node can fire at around 60ndash100 times a minute corresponding to a healthy heart rate The AV junction can also fire but it is much slower at around 40ndash60 times a minute Even the Purkinje fibers (the ventri-cles) can act as a pacemaker but they fire at only 20ndash40 times a minute Thus in a healthy individual the SA node outpaces these other potential pacemakers The AV junction and ven-tricles are sometimes called backup pacemakers because they take over the heart rate in the event

Sinoatrial (SA) node

Internodal tracts

Right bundle branch

Left bundle branch

Purkinje fibers

Bachmannrsquos bundle

Bundle of His

Atrioventricular (AV) node

Figure 21 The conduction pathways in the healthy heart

Chapter 2 Cardiac conduction system 17

that the SA node is not able to pace the heart While these backup pacemakers may help keep the heart beating in the event of a dysfunctional SA node they are slow and not as reliable as the SA node

The healthy heart is able to pace itself because of a property called automaticity Automaticity is an inherent property of individual cardiac conductive cells that allows them to depolarize spontaneously In other words some cells in the heart (such as the cells of the SA node) are able to generate an electrical stimulus on their own

refractoriness

Refractoriness refers to the temporary inability of a cardiac cell to respond to an electrical stimulus Refractoriness can be the normal appropriate and expected behavior of cardiac cells For example immediately after electrical energy has passed through cardiac cells they go through a phase where they are unresponsive to electrical energy no matter how much voltage is delivered This normal refractoriness is called the physiological refractory period and it is very brief (it is mea-sured in thousandths of a second) Of course the exact duration of a refractory period is based in part on the heart rate the faster the heart rate the shorter the refractory period and vice versa

Refractoriness may also occur when certain cells become dysfunctional that is cardiac cells may lose their ability to conduct properly Furthermore the heartrsquos inherent pacemakers may change their rate for instance the AV junction may speed up and start to deliver impulses at 80 or 100 times a minute When this happensmdashthat is when another pacemaker takes over the role of the SA nodemdashthat pacemaker is called a usurping pacemaker because it usurps the rightful role of the SA node In the event that other pacemakers of the heart start to compete with the SA node the fastest pacemaker always wins

Conductivity

Conductivity is the cardiac cellrsquos ability to transmit an electrical impulse to an adjacent cell Typically cardiac conduction is antegrade that

is it moves forward Electrical impulses actually travel out in all different directions but refracto-riness prevents those cells that were just stimu-lated from depolarization This keeps conduction moving in a single direction through the heart Normal antegrade conduction in a healthy heart goes from SA node to atria to AV node over the bundle of His and down to the ventricles Some individuals have the ability to conduct retro-grade retrograde conduction is abnormal but not uncommon

autonomic nervous system

The heart does not need direct stimulation from the nervous system to generate electricity and to beat but that is not to say that the nervous system does not control the heart rate The autonomic nervous system indirectly affects cardiac conduction by regulating how fast the heart will beat and how hard the heart muscle will squeeze (contractility) The autonomic ner-vous system communicates with the heart through a network of baroreceptors which respond to chemical signals

The autonomic nervous system of the body like certain other physiological systems exists as two systems in balance The autonomic nervous system consists of a sympathetic system and a parasympathetic system They both work together and in a healthy individual one may overdrive the other during specific times The sympathetic ner-vous system is mainly associated with epinephrine (adrenaline) and it has the ability to speed the heart rate and increase cardiac contractility and can increase the speed by which electrical impulses can cross the AV node The sympathetic nervous system takes over when we are stressed frightened (fight-or-flight response) or exerting ourselves

On the other hand the autonomic nervous system also has a parasympathetic system that is associated with acetylcholine This system can decrease the heart rate decrease cardiac contrac-tility and cause impulses crossing the AV node to be slowed The parasympathetic system might take over when we are sleeping or at rest

18 The nuts and bolts of implantable device therapy pacemakers

the nuts and bolts of the cardiac conduction system

bull The heartrsquos natural pacemaker is the sinoatrial (SA) node which generates an electrical impulse spontaneously about 60ndash100 times a minute in a healthy individual

bull The normal conduction path is from the SA node over the atria to the atrioventricular (AV) node and then via the bundle of His down the right and left bundle branches and into the Purkinje fibers

bull The AV node has the ability to slow down the electrical impulse This serves two main purposes it allows for atrial kick and it protects the ventricles from beating too rapidly

bull In the healthy heart the right and left atria contract simultaneously and then the right and left ventricles contract simultaneously

bull Depolarization and repolarization of myocar-dial cells are driven by positively and negatively charged ions which cross the cell membrane A resting cardiac cell is more negative on the inside than the surrounding media but when it depolarizes it becomes more positive on the inside relative to the surrounding media Following depolarization the ions move and the cell returns to being more negative on the inside than the outside in a process known as repolarization

bull Depolarization causes the cardiac cell to contract

bull The main ions involved in depolarization and repolarization are sodium potassium and calcium ions

bull Although the heartrsquos natural pacemaker is the SA node the AV junction and the Purkinje fibers (ventricles) can also assume (usurp) this role if the SA node cannot generate electricity properly The AV junction and Purkinje fibers are not as reliable as a healthy AV node and typically generate electricity more slowly (the AV junction about 40ndash60 times a minute and the Purkinje fibers 20ndash40 times a minute)

bull However sometimes the AV junction or Purkinje fibers may generate electricity very

quickly this is abnormal When they do they may take over the heart When pacemakers of the heart compete the fastest one drives the heart

bull Automaticity is an inherent property of certain cardiac conductive cells to depolarize spontaneously Not all cardiac conduction cells possess automaticity but the SA node AV junction and Purkinje fibers possess automaticity

bull Refractoriness is the natural and expected behavior of cardiac conductive cells that makes them unresponsive to electrical impulses immediately after they have depolarized The natural refractory period is very brief

bull The faster the heart rate the shorter the refractory period the slower the heart rate the longer the refractory period

bull Refractoriness can also occur as a dysfunction bull Conductivity is the cardiac cellrsquos ability to

transmit an electrical impulse to adjacent cells Refractoriness means that cells that just depolarized will not depolarize again This keeps conduction moving forward (antegrade conduction)

bull Antegrade conduction is normal retrograde conduction is abnormal but is not rare

bull The autonomic nervous system regulates the heart rate by controlling how fast the heart beats and how vigorously it contracts

bull The autonomic nervous system consists of a sympathetic and parasympathetic nervous system which work together but at certain times one or the other is dominant The sympathetic nervous system makes the heart beat faster and more vigorously it takes over during times of stress exertion or fight or flight The parasympathetic ner-vous system makes the heart beat slowly and less vigorously it takes over during periods of rest

bull The neurotransmitters associated with these two systems are epinephrine or adrenaline (sympathetic nervous system) and acetylcho-line (parasympathetic nervous system)

Chapter 2 Cardiac conduction system 19

test your knowledge

Please write in the landmarks of the cardiac conduction system Select from the following (each is used only once)

AV node Bachmanrsquos bundle Bundle of His

Internodal tracts Left bundle branch Purkinje fibers

Right bundle branch Ventricles

1 Which of the following is not considered one of the heartrsquos intrinsic pacemakersA AV junctionB SA nodeC Purkinje fibersD Aorta

2 Depolarization and repolarization involve the movement of charged ions across cardiac cell membranes What types of ions are primarily involvedA Calcium potassium and sodiumB Acetylcholine and epinephrineC Magnesium and folateD All of the above

3 Refractoriness refers toA The ability of cardiac conduction cells to

contract

B The inability of cardiac conduction cells to respond to an electrical stimulus

C The unique permeability of the cardiac cellsD Abnormal cardiac conduction

4 When a cardiac cell depolarizes the interior of the cell becomes what with respect to the surrounding mediaA More positiveB More negativeC More acidicD Hotter

5 Fill in the blank In the healthy heart the faster the heart rate the ___ the refractory periodA More vigorousB Less reliableC LongerD Shorter

20 The nuts and bolts of implantable device therapy pacemakers

6 Which specific part of the nervous system governs the fight-or-flight response and is associated with epinephrine (adrenaline) (Pick the most exact answer)A Autonomic nervous systemB Parasympathetic nervous systemC Sympathetic nervous systemD Central nervous system

7 In normal antegrade conduction the electrical impulse travels from the SA node over the atria to the AV node and to what structure before reaching the ventriclesA The intermodal tractsB The Purkinje fibersC The bundle of HisD The ventricular apex

8 Which of the following is true about retrograde cardiac conductionA Everyone has itB It is perfectly normal and can be helpful in

stressful situations

C It is abnormal but not uncommonD It is impossible to diagnose definitively

except in an autopsy

9 What chemical is associated with the parasympathetic nervous systemA AcetylcholineB PotassiumC EpinephrineD Dopamine

10 The stimulation of the myocardial cells of which of the following structures is most closely associated with ventricular depolarizationA SA nodeB AV junctionC Bundle of HisD Purkinje fibers

Chapter 1 Cardiovascular anatomy and physiology 3

fairly simple Deoxygenated blood enters the right side of the heart and is pumped over to the lungs via the pulmonary arteries and is returned back (as oxygen-rich blood) to the left side of the heart by way of the pulmonary veins (PV) While both right and left sides of the heart contract at the same time as a single unit the right side is busy pumping deoxygenated blood to the lungs while the left side is pumping reoxygenated blood out to the rest of the body

Deoxygenated blood enters the right side of the heart via the superior vena cava (SVC) but once it has become oxygenated again blood is pumped back out from the left side of the heart into the aorta The aorta is the largest vessel in the body and it forms a U shape at the top of the heart These portions of the aorta are called the ascending the descending and the arch Coming off the aortic arch are three main arteries the left subclavian artery the left common carotid artery and the brachiocephalic trunk

To better understand the blood flow through the heart it is important to review the structure of the heart The atria or upper chambers of the heart are smaller have thinner walls and are smoother on the inside than the ventricles Within the ventri-cles is a network of fibrous strands known as trabeculae These structural differences become important in lead implantation within the heart it is much easier to affix or lodge a lead in the trabeculae of the ventricles than to try to anchor the lead to a smooth atrial wall Historically atrial leads have almost always been active-fixation screw-in-type leads while ventricular leads were almost always passive-fixation leads (fins or tines that lodge in the trabeculae) Today active-fixation leads are often used in both chambers since they facilitate lead removal (Figure 12)

Overall blood flow to the heart is discussed right and left sides although it is important to recognize that what happens in the heart that is systole (contraction) and diastole (relaxation) are happening on both sides at the same time The right atrium of the heart receives blood from the SVC the inferior vena cava (IVC) and the coronary sinus (CS) The CS is technically a vein and it has an opening or ostium (sometimes just called os) at the base of the right atrium slightly posterior The CS delivers oxygen-depleted blood

to the right atrium from the coronary arteries that encircle the exterior of the heart The CS is of interest in cardiac resynchronization therapy (CRT) because the left ventricular lead is passed through the CS (counter to the flow of blood) in order to be placed into the coronary vessels to pace the left ventricle CRT is used in patients with heart failure whose hearts have remodeled that is enlarged and changed shape (It may be said that with heart failure the heart changes from the shape of a football to the shape of a basketball) The CS may be relocated in this remodeling which can be challenging in implanting a CRT lead because the physician must first locate the os of the CS and then navigate through it in order to implant the left ventricular lead

Anatomically the heart is dominated by the large muscle mass of the left ventricle which makes up about two-thirds of the heart in terms of weight and volume This greater size is typically ascribed to the fact that the left ventricle must pump blood throughout the whole body whereas the right ventricle only has to pump blood to the lungs The left and right ventricles pump blood to different destinations but the left ventricle is larger and more muscular for a reasonmdashpressure It is impor-tant to review the pressures against which the heart must work to understand cardiac blood flow (Figure 13)

Deoxygenated blood in the right side of the heart must travel over the lungs to pick up oxygen This means that blood in the right ventricle travels across the pulmonary valve into the pulmonary artery and then out toward the lungs The pulmonary valve opens automatically when pressure from the contracting right ventricle forces it open This occurs when the pressure in the right ventricle exceeds the pressure in the pulmonary artery Pressure gradients are key concepts in understanding blood flow Valves are like gates that open and close in response to pressure In general the pressure in the PV is fairly low around 12 mmHg Thus the right ventricle does not need to create a lot of force to open the pulmonary valve

Meanwhile as the left ventricle contracts it cre-ates pressure on the aortic valve leading to the aorta In order to open the aortic valve and pump blood out into the aorta the heart must overcome the pressure in the aortic valve Pressure in the

4 The nuts and bolts of implantable device therapy pacemakers

Right atrium

Left atrium

Left ventricle

Right ventricle

Figure 12 Note that the atria are smooth walled while the ventricles contain a spongelike fibrous network of trabeculae

Left pulmonaryarteries

Left pulmonaryveins

Aorta

Aortic valve

Pulmonaryvalve

Right pulmonaryarteries

Superiorvena cava

Inferiorvena cava

Right pulmonaryveins

Figure 13 The blood flow within the heart takes oxygen-depleted blood from the body into the right atrium where it flows to the right ventricle and is pumped out over the lungs the reoxygenated blood from the lungs is pumped into the left atrium where it flows to the left ventricle and is pumped out via the aorta to the body

Chapter 1 Cardiovascular anatomy and physiology 5

aorta is high around 120 mmHg or ten times higher than the pressure in the PV The left ventricle must therefore work much harder to pump blood than the right ventricle This requires the left ventricle to be larger and more muscular than the right ven-tricle (see Figure 14)

On the right side of the heart blood travels from the right atrium into the right ventricle via the tricuspid valve The tricuspid valve gets its name from its characteristic shape involving three leaves or cusps Attached to these cusps are cords that anchor into the base of the ventricle known as chordae tendineae they look almost like little parachutes The strands of the chordae tendineae attach to tiny papillary muscles These chords attach to the valve leaves at one end and a papil-lary muscle at the other end On the right side of the heart the tricuspid valve is associated with three papillary muscles The purpose of these chords and muscles is to assure that the valve is

effectively closed and opened at the proper times (Figure 15)

The heart can rightly be thought of as a pump but it must be remembered that the heart is also a muscle and all muscles need a steady supply of oxy-gen-rich blood The heart muscle is supplied with blood through a network of coronary arteries that surround the outside of the heart Blockage in a coronary artery results in ischemia which can lead to death of cardiac muscle including the chordae tendineae and papillary muscles While the patient may survive such an ischemic event the damage to the heart may lead to an incompetent valve that is a valve that is no longer able to function effectively

In tracing the blood flow from the right ventricle to the pulmonary artery it should be clear that the blood has to go from down in the right ventricle to up through the pulmonary valve and into the pulmonary artery The blood is able to make this journey because of the pumping pressure of the heart The route the blood takes as it exits the right ventricle and journeys up toward the pulmonary valve is known as the right ventricular outflow tract (RVOT) On the other side of the heart there is also a corresponding left ventricular outflow tract (LVOT) of approximately the same size Cardiac leads are sometimes fixated in the RVOT (see Figure 16)

Blood pumped out of the right ventricle crosses the pulmonary valve and enters the pulmonary artery which splits into two branches right and left The right pulmonary artery takes blood to the right lung while the left pulmonary artery takes blood to the left lung In this respect the pulmonary artery is unique in the body in that it is an artery but it carries deoxygenated blood Blood travels through the lung to the alveoli where it gains oxygen and loses carbon dioxide Once it is reoxygenated the blood gathers into the PV (which are also unique being the only veins to carry oxygenated blood) There are four PV in total the two right-sided PV take oxygen-ated blood from the right lung while the two left-sided PV take oxygenated blood from the left lung and they all bring this reoxygenated blood to the left atrium

The left atrium is smooth walled like the right atrium and although the left atrium is much

Figure 14 The left ventricular is far more muscular than the right ventricle because it must overcome 10 times the pressure of the right ventricle in order to pump blood out via the aortic valve and into the aorta

6 The nuts and bolts of implantable device therapy pacemakers

Right atrium

Left atrium

Left ventricle

Right ventricle

Figure 15 The leaflets of the valves are attached by chordae tendineae at one end and papillary muscles on the other which assure the effective opening and closing of the valves

Right-ventricularoutflow tract (RVOT)

Figure 16 There are two outflow tracts in the heart one associated with the right ventricle and the other the left ventricle These outflow tracts are roughly the same size The illustration shows the right ventricular outflow tract (RVOT) a preferred location for right ventricular lead fixation The left ventricular outflow tract (LVOT) cannot be seen in this illustration as it is posterior

Chapter 1 Cardiovascular anatomy and physiology 7

smaller than the left ventricle it is larger than the right atrium The blood will travel from the left atrium into the left ventricle by way of the mitral valve The mitral valve gets its name from the miter a bishoprsquos hat The mitral valve has two cusps and papillary muscles connected by chordae tendineae Once blood is in the left ventricle a contraction will force it toward the aorta by way of the LVOT In persons with hypertrophic obstructive cardiomy-opathy (HOCM) the septum or wall in the heart can become thick and enlarged effectively narrow-ing the LVOT In the healthy heart the blood flows up the LVOT across the aortic valve and into the aorta From the aorta blood will be directed upward toward the head downward toward the legs and feet and some blood will be redirected back to the coronary arteries of the heart

The blood flow from the left ventricle to the aorta and back to the coronary arteries is unique in the body in that most of that flow occurs during diastole rather than systole Think of it this way as blood is pumped by the left ventricle during systole upward and downward to the brain and the rest of the body some of that blood flows back during diastole and drains into the coronary arteries Although it may seem counterintuitive it is impor-tant to remember that coronary artery perfusion occurs primarily during diastole

Volume valves and pressure

While it is tempting to think of the heart as simply a pump with an electrical system it is more accu-rate to say that the heart moves blood because of variations in volume and pressure

The heart has four valves two are atrioventricular (AV) valves because they connect the atrium to the ventricle and two are semilunar valves connecting the heart with pulmonary arteries or the aorta The right AV valve is the tricuspid while the left AV valve is the mitral valve The flow or movement of blood is guided and controlled by the heartrsquos muscular action in the form of systole (contraction) or diastole (relaxation)

When blood pours into the right atrium it dis-tends and expands the right atrium When this volume of blood creates sufficient pressure it forces the tricuspid valve open Think of it as volume creating pressure and pressure opening

valves When the tricuspid valve opens the blood dumps into the right ventricle During this period of diastole blood pours into the right ventricle creating volume and expanding it As blood pours in and ventricular diastole nears the end the atria contract while the AV valves are still open in something nicknamed atrial kick Atrial systole occurs at the end of ventricular diastole Atrial kick forces the maximum amount of blood from the atria into the ventricles It is estimated that atrial kick delivers 20ndash30 of cardiac output

The atrial contribution to ventricular filling (atrial kick) is of enormous clinical significance Patients with certain atrial arrhythmias such as atrial fibrillation (AF) lose atrial kick Even milder forms of atrial tachyarrhythmias may compromise atrial kick Moreover patients who do not have AV synchrony (one atrial beat corresponding to one ventricular beat) will lose atrial kick The loss of atrial kick can reduce cardiac output by 20ndash30

Once the atria have contracted and the ventricles are filled with blood the AV valves snap closed The ventricles now start to contract and push open the semilunar valves For example as the right ven-tricle contracts (systole) it opens the PV (The PV is sometimes called the pulmonic valve itrsquos the same thing) The whole process works on volume pressure and contractionrelaxation

Of the four valves in the heart the tricuspid valve is of most interest to device specialists because at least one lead will be placed into the right ventricle This means that pacing and defi-brillation leads typically cross the tricuspid valve These leads may interfere with the proper closing of the tricuspid valve particularly if the lead is large diameter or several leads are passed over the tricuspid valve A single lead through the tricuspid valve is likely to cause minimal to no dysfunction However it is not unusual to see patients with three or more leads in the right ventricle and these leads may hamper the tricuspid valve in its closing leading to tricuspid regurgitation or backflow of blood Patients with multiple leads (such as chil-dren who grow up with pacemakers) may need to have some of them removed to avoid tricuspid regurgitation

Right ventricular leads are typically fixated at the right ventricular apex or in the RVOT and may be

8 The nuts and bolts of implantable device therapy pacemakers

lodged in the trabeculae Trabeculae are a fibrous network that could be described as similar to the pores of a sponge (see Figure 17)

On the left side of the heart the blood flow pattern is similar Blood flows into the left atrium over the open mitral valve and into the left atrium At the end of ventricular diastole as the ventricle is distended and stretched atrial kick delivers more blood into left ventricle The volume of blood cre-ates pressure The mitral valve closes As the left ventricle contracts the growing pressure over-comes the resistance on the aortic valve and it opens allowing blood to flow out into the aorta and beyond

While any valve can become diseased or dys-functional valvular disease more commonly affects the mitral and the aortic valves (left-sided valves) than the right-sided valves (tricuspid and pul-monary) These left valves are more likely to suffer damage or disease because of the high-pressure environment in which they function exposing them to more potential damage Other damage can occur when plaque builds up in the coronary arteries and the aorta The aorta can become

diseased typically in the case of an aortic aneurysm or dissection Carotid arteries in the neck are also frequently a site for atherosclerotic disease

the right atrial appendage

The right atrial appendage (RAA) is an area near the right atrium which is a preferred site for attach-ing active-fixation atrial leads (see Figure 18) The RAA serves no obvious purpose and may be sacri-ficed in certain heart surgeries such as a coronary artery bypass graft (CABG) procedure CABG or bypass surgery involves stopping the heart and running the blood through a machine to reoxygen-ate it during the course of the procedure This is done by attaching a large-diameter hose to the RAA when surgery concludes and this hose is removed it can damage the RAA to the point that it is completely or partially surgically removed In patients who have had a CABG procedure the RAA may not be available for right atrial lead placement For such patients the right atrial lead is often placed against the lateral wall of the right atrium It is sometimes possible to fixate a right

Figure 17 Trabeculae form a fibrous network within the ventricles Passive-fixation ventricular leads are held in place by lodging their tines or fins in the trabeculae

Chapter 1 Cardiovascular anatomy and physiology 9

atrial lead in the remnants of the RAA if a portion of it is preserved

arteries and veins

Any discussion about the heart necessarily involves the vasculature which is why we commonly refer to our circulatory system as the cardiovascular

system As most of us remember from anatomy classes arteries carry blood away from the heart while veins transport blood back to the heart (The exceptions are the PV and arteries described earlier) Arteries carry oxygen-rich blood while veins carry deoxygenated blood What is of importance to device clinicians is that arteries are high-pressure vessels while veins are low-pressure vessels Arteries tend to be more muscular in structure and muscular contractions of the arteries help to move blood outward into the body Veins tend to be less muscular than arteries but unlike arteries they contain a system of tiny interior valves The purpose of these little valves is to maintain a unidirectional flow of blood (see Figure 19)

Some of the most important veins in the body are the SVC and its counterpart the IVC The SVC takes blood from the upper part of the body and delivers it into the right atrium with the IVC doing the same for blood from the lower part of the body The coronary veins networked around the outside of the heart carry blood from the heart tissue and deliver it back to the right atrium they are closely linked to the coronary arteries which carry reoxygenated blood from the left ventricle back to the heart muscle The CS is the site where

Right atrial appendage

Figure 18 The right atrial appendage serves no obvious hemodynamic purpose but is often the site of fixation of atrial leads

Artery Vein

Figure 19 Veins and arteries have different purposes different anatomical structure and different pressures Overall veins are low-pressure systems that move blood by a series of little valves that keep blood flowing in the same direction Arteries are high-pressure systems and are more muscular so that they force blood forward by squeezing

10 The nuts and bolts of implantable device therapy pacemakers

the great middle and small cardiac veins all drain the CS takes this blood and delivers it back to the right atrium

The PV carry reoxygenated blood from the lungs and deliver it to the left atrium so that it can be pumped back out to the body by the left ventricle

The most important artery in the body is by far the aorta The aorta is connected to the left ven-tricle and it is the first and main conduit that takes reoxygenated blood from the left ventricle and sends it out to the body As this reoxygenated blood makes its way out into the body its first two stops are the head (brain) and the heart muscle Inadequate oxygenated blood to the brain can very quickly provoke symptoms This is the reason that depressed cardiac output is associated with symptoms like dizziness light-headedness feeling woozy and fatigue Severe symptoms might include syncope

The coronary arteries arise from the root of the aorta just above the cusps of the aortic valve Coronary arteries receive most blood during dias-tole so blood is more or less pouring back into these vessels The coronary artery system is a networkmdashalmost a meshmdashof middle-sized to very small vessels that cover the exterior surface of the heart (see Figure 110)

Coronary arteries get their name from the Latin word for crown (such as in our word coronation) because this network of vessels sits like a crown atop the heart and encircles it The heart gets blood from outside in as oxygen-rich blood in these coro-nary arteries is delivered to the exterior of the heart muscle Coronary arteries are not within the heart they are epicardial structures The two main coro-nary arteries are the right coronary artery (RCA) and the left coronary artery (LCA) The RCA and LCA come off the aorta and are located in the AV groove an exterior structure below the atria and above the ventricles The RCA runs from the aorta in the right AV groove while the LCA comes off the aorta and quickly branches to the left main The left main is a very short artery (about the size of a thumbnail) which branches quickly into the left anterior descending artery (LAD) and the circum-flex artery The LAD travels down the front of the

left side of the heart The circumflex artery can be found in the left AV groove

The RCA bifurcates to the posterior descending artery The right atrium and right ventricle get their supply of oxygen-rich blood from the RCA Much of the cardiac conduction system is right sided that is it commences with the sinoatrial (SA) node in the high right atrium and travels down to the AV node on the right side of the septum These important electrical structures get their oxygen-rich blood supply from the RCA The electrical system travels down the ventricles to the inferior wall of the left ventricle (located at the bottom of the heart but belonging to the left ventricle) the inferior wall is also fed oxygen-rich blood by the RCA An occlusion of the RCA such as might occur in a myocardial infarction (MI) may result in death of the heart muscle it feeds which includes the right atrium the right ventricle and the inferior wall of the left ventricle and it might affect the conduction system including the SA node and the AV node An inferior wall MI is typically the result of a clogged RCA

RCA

LCA

Aorta

Figure 110 The coronary arteries of the heart branch out to surround the entire heart muscle The two most prominent coronary arteries are the right coronary artery (RCA) and the left coronary artery (LCA)

Chapter 1 Cardiovascular anatomy and physiology 11

For an MI the more distal the occlusion (that is further from the aorta) the less potential damage the heart attack will have In other words the worst occlusions are proximal Whether or not a patient survives an MI comes down to the numbers The loss of over 40 of the left ventricular muscle mass results in death However that 40 is cumulative For example a person can survive an MI that costs him 30 of left ventricular muscle mass but if that heart attack is followed by another that kills 15 of the left ventricular muscle mass the second (milder) heart attack will prove fatal This is the reason many people survive a first heart attack but die of a second or third attack even though their fatal MI may be relatively mild

The LCA branches quickly to the left main which branches almost at once to the LAD The LAD runs on the outside of the heart along the front of the heart along the septum The left main supplies the left atrium and the majority of the left ventricle in other words the major pumping por-tion of the heart An occlusion in the left main can be particularly disastrous because it is so far upstream and affects the heartrsquos left ventricular muscle mass No wonder they call the left main the widow maker

The coronary arteries do not run in straight lines they twist and turn and sometimes make very sharp bends People with coronary artery disease (CAD) typically have plaque buildup in the areas where the coronary arteries bend at sharp angles These are areas where the blood flow creates a lot of turbulence and plaque and other substances can collect and build up It is typically to find blockage and occlusions at these places

The LAD supplies the anterior wall of the left ventricle and the septum with oxygen-rich blood Since part of the heartrsquos conduction system runs through the septum (the right and left bundle branches) an LAD occlusion can damage the heartrsquos ability to conduct electricity properly

The circumflex artery runs along the AV groove laterally and thus supplies oxygen-rich blood to the lateral wall of the left ventricle along with the left atrium The circumflex runs along the AV groove to the left heading toward the back of the heart where it meets with the RCA which

travels along the AV groove to the left toward the back of the heart Both the circumflex and the RCA are responsible for providing oxygenated blood to the posterior wall of the ventricle But which is the more important provider That depends on crux and whether the heart is right or left dominant

The crux refers to an imaginary line drawn exactly down the middle of the ventricular poste-rior wall without any regard to anatomical landmarks If the RCA crosses the crux then the heart is said to be right dominant and the RCA is the greater supplier of oxygenated blood to the ven-tricular posterior wall On the other hand if the circumflex crosses the crux then the heart is said to be left dominant and the circumflex is the more significant provider Roughly 60 of the population is right dominant

While this information about the vasculature is important it is even more important for clini-cians to recognize that there is tremendous interpatient variability in terms of venous anatomy While we can describe major structures and typical venous formations anomalies are very common These anomalies may be minor variations or they can be very pronounced such as missing veins and arteries Clinicians who deal with the cardiovascular system must be prepared for large and small differences in vascular anatomy The cardiovascular system is incredibly resilient and can adapt to some anomalies by building up collateral circulation For example a person with a missing or incompetent vessel may over time build up a network of smaller collateral vessels that compensates by doing the same job It is not unusual to see vessels grow larger or branch out to compensate Sometimes when an artery is blocked collateral circulation will build up and compensate by delivering oxygen-rich blood to the heart muscle

Every artery in the body has a matching or corresponding vein usually located in close proximity This applies to the coronary system as well Note that there are great differences in coro-nary venous anatomy among patients which becomes important when implanting left ventric-ular leads for CRT devices (see Figure 111)

12 The nuts and bolts of implantable device therapy pacemakers

the nuts and bolts of cardiovascular anatomy and physiology

bull The heart consists of four chambers which may be considered the upper chambers (right atrium and left atrium) and lower chambers (right ven-tricle and left ventricle) or may be thought of as the right heart (right atrium and right ventricle) and left heart (left atrium and left ventricle)

bull Deoxygenated blood enters the right atrium flows to the right ventricle and is pumped out via the pulmonary valve to the lungs The blood is reoxygenated in the lungs and then enters the left atrium and flows to the left atrium and out of the aortic valve into the aorta and the rest of the body

bull The heart has two outflow tracts a right ven-tricular outflow tract (RVOT) and a left ven-tricular outflow tract (LVOT)

bull The largest chamber of the heart is the left ven-tricle making up about 23 of the cardiac mass The left ventricle is muscular and massive because it must pump against approximately 10 times the pressure against which the right ven-tricle pumps (120 vs 12 mmHg)

bull The two cardiac valves most susceptible to dysfunction and disease are the mitral and aortic valve because they function in a high-pressure environment

bull The chordae tendineae connect the leaflets of the valves to the papillary muscles and look like little parachutes These help the valves open and close properly

bull In general arteries transport blood away from the heart are muscular and contract to help move the blood on its way and have thicker walls than veins Veins have thinner less muscular walls and carry oxygen-depleted blood back to the heart using tiny interior valves to keep the blood flow moving forward Arteries transport oxygenated blood and can be considered high-pressure systems while veins transported deoxygenated blood in a low-pressure system

bull Cardiac contraction is systole cardiac relaxation is diastole Atrial systole occurs at the very end of ventricular diastole in something nicknamed atrial kick In a healthy individual about 20ndash30 of cardiac output comes from atrial kick

bull The heart is a muscle that is fed with oxygenated blood through a network of coronary arteries Coronary arteries get oxygen-rich blood from the aorta and they perfuse the heart muscle during diastole as blood drains back

bull Although arteries and veins generally take blood away from or to the heart respectively

Rightventricle

Leftventricle

Leftatrium

Aorta

Rightatrium

Rightcoronaryartery

Leftcoronaryartery

Circumflexartery

Superiorvena cava

Inferiorvena cava

Arterial system

Rightventricle

Leftventricle

Greatcardiac vein

Smallcardiac vein

Middlecardiac vein

Leftatrium

Aorta

Rightatrium

Superiorvena cava

Inferiorvena cava

Venous system

Coronarysinus

Figure 111 The coronary artery system is matched by a coronary venous system

Chapter 1 Cardiovascular anatomy and physiology 13

test your knowledge

Fill in these anatomical landmarks of the heart They include the four chambers the four valves and the bodyrsquos main artery

the pulmonary vessels are the exception The pulmonary artery transports deoxygenated blood while the pulmonary veins carry oxygen-ated blood This is because of their special loca-tion and the fact that their work is taking blood from the right side to the left side of the heart

bull The main coronary arteries are the right coro-nary artery and the left coronary artery The left coronary artery branches quickly to a short vessel known as the left main (the widow maker) and the circumflex artery

bull A myocardial infarction can kill the heart muscle The clogged arteries determine the loc ation of the affected muscle A right coro-nary artery occlusion will affect the right atrium and ventricle and the inferior wall of the left ventricle while a left coronary artery occlusion will affect the left atrium and most of the left ventricle By far the most dangerous occlusion is in the left main because it feeds the majority of the heart muscle with oxygen-rich blood

answer the following questions

1 What are the names of the two atrioventricular valves of the heartA Atrial and ventricularB Tricuspid and mitralC Pulmonary and pulmonicD Aortic and superior vena cava

2 Which chambers of the heart contain trabeculaeA AtriaB VentriclesC Right-sided chambers onlyD Left-sided chambers only

14 The nuts and bolts of implantable device therapy pacemakers

3 Why can trabeculae be important to device cliniciansA They are good places to secure a passive-fixa-

tion leadB They help regulate blood pressureC They prevent atherosclerosisD All of the above

4 Which of the following is not true about the aortaA It is an arteryB It consists of an ascending portion a descend-

ing portion and an archC It carries only oxygenated bloodD It is the largest vein in the body

5 From which artery does blood enter the right atriumA Superior vena cavaB Inferior vena cavaC Coronary sinusD All of the above

6 What is atrial kickA Ventricular pressure on the tricuspid valve

forcing it shutB Electrical impulses originating in the atrium

to cause the heart to beat fasterC Atrial systole that forces the most blood possible

into the already-full and diastolic ventriclesD A type of arrhythmia

7 Why would a person with chronic AF likely have decreased cardiac outputA AF is associated with low blood volumeB AF causes the heart to beat very slowlyC AF may cause the heart to experience fre-

quent pausesD AF reduces or eliminates atrial kickmdashand

atrial kick contributes to cardiac output

8 Which of the following is often a preferred location for affixing a cardiac pacing leadA The right ventricular outflow tract (RVOT)B The left ventricular outflow tract (LVOT)C The excised right atrial appendageD The tricuspid valve

9 What connects the valve leaflets to the papillary musclesA TrabeculaeB ArteriesC Chordae tendineaeD Tiny valves

10 Which coronary artery is nicknamed the widow maker because an occlusion in this vessel will deprive the majority of the heartrsquos muscle mass of oxygen-rich bloodA Left mainB CircumflexC Right coronary arteryD Inferior vena cava

15

The Nuts and Bolts of Implantable Device Therapy Pacemakers First Edition Tom Kenny copy 2015 John Wiley amp Sons Ltd Published 2015 by John Wiley amp Sons Ltd

Chapter 2

Cardiac conduction system

Introduction

The sinoatrial (SA) node is the heartrsquos natural pacemaker In the healthy heart the SA node fires and the electrical stimulus conducts along special pathways through the heart that cause cells in specific areas to depolarize After the stimulus passes the area the cells repolarize Myocardial cell depolarization and repolarization govern the heartrsquos contraction and relaxation sequencemdashin other words the heart rhythm Rhythm disorders occur when this healthy conduction system is in some way disrupted or impaired

the conduction pathways

When the SA node fires and sends out electrical energy the stimulus travels from the SA node in the high right atrium out over the atria via path-ways known as internodal tracts Most of these internodal tracts are located in the right atrium the

main tract over to the left atrium is called Bachmannrsquos bundle It is because of Bachmannrsquos bundle linking the right atrium to the left atrium that the atria can contract simultaneously that is as these two upper chambers contract as one coherent unit After traversing the atria the impulse comes down through conduction tracts to the atrioventricular (AV) node

The AV node is an area of highly specialized tissue located below the atria and above the ventri-cles on the right side of the septum Anatomically the AV node rests in the AV groove The tissue in the healthy AV node has the ability to change the speed of the electrical impulse specifically it slows the stimulus down The slowdown serves two important purposes First slowing the impulse allows the atria time to contract and provide the very valuable atrial kick to ventricular filling Second the slowed impulse protects the ventricles from being forced to beat too quickly that is to beat before they have been adequately filled with blood

Once the impulse crosses the AV node it travels down into the ventricles and accelerates its speed The impulse travels through the bundle of His (pronounced hiss) to the bundle branches (right and left) and then out to the network of Purkinje fibers This allows both ventricles to depolarize and contract simultaneously (Figure 21)

polarization depolarization and repolarization

Cardiac conduction is driven by the ability of myo-cardial cells to be depolarized and repolarized Depolarization occurs at the cellular level and is

2

Learning objectives

bullensp Describe the electrical conduction system of the healthy heart

bullensp List the main anatomical landmarks of normal cardiac conduction

bullensp Name the anatomical location of the heartrsquos natural pacemaker

bullensp Explain depolarization and repolarization in electrical terms and how it relates to the cardiac contraction

bullensp Define the term automaticity and how it relates to cardiac conduction

16 The nuts and bolts of implantable device therapy pacemakers

in turn driven by electrolytes or positively and negatively charged ions (potassium sodium and calcium mainly) which are able to move in and out of the cardiac cells A myocardial cell may be said to be polarized when in its resting state that is there is no electrical activity going on In the rest-ing state the interior of the myocardial cell is somewhat more negatively charged that the outside This state is called the resting state or sometimes phase 0 of the action potential

An electrical stimulus changes the resting state abruptly The electrical energy allows for the movement of ions to the point that the interior of the myocardial cell is now more positive than the outside In other words the charge reverses and the cell is said to be depolarized When this happens the cell contracts when this happens to a large area of the myocardium the heart muscle contracts Cardiac contraction occurs as a result of cellular depolarization

When the electrical stimulus moves past the myocardial cell the ions move in reverse and the interior of the cell assumes its previous negative charge with respect to the outside This process is

called repolarization and it brings the myocar-dial cell back to the resting state When this happens the cell relaxes When a large area of the myocardium repolarizes the heart resumes its previous shape (loses its contraction)

Intrinsic pacemakers

The healthy heartrsquos natural pacemaker is the SA node but the SA node is not the only area of the heart that can generate an electrical pulse The SA node serves as the heartrsquos pacemaker because it generates these pulses faster than other areas In the healthy heart the SA node can fire at around 60ndash100 times a minute corresponding to a healthy heart rate The AV junction can also fire but it is much slower at around 40ndash60 times a minute Even the Purkinje fibers (the ventri-cles) can act as a pacemaker but they fire at only 20ndash40 times a minute Thus in a healthy individual the SA node outpaces these other potential pacemakers The AV junction and ven-tricles are sometimes called backup pacemakers because they take over the heart rate in the event

Sinoatrial (SA) node

Internodal tracts

Right bundle branch

Left bundle branch

Purkinje fibers

Bachmannrsquos bundle

Bundle of His

Atrioventricular (AV) node

Figure 21 The conduction pathways in the healthy heart

Chapter 2 Cardiac conduction system 17

that the SA node is not able to pace the heart While these backup pacemakers may help keep the heart beating in the event of a dysfunctional SA node they are slow and not as reliable as the SA node

The healthy heart is able to pace itself because of a property called automaticity Automaticity is an inherent property of individual cardiac conductive cells that allows them to depolarize spontaneously In other words some cells in the heart (such as the cells of the SA node) are able to generate an electrical stimulus on their own

refractoriness

Refractoriness refers to the temporary inability of a cardiac cell to respond to an electrical stimulus Refractoriness can be the normal appropriate and expected behavior of cardiac cells For example immediately after electrical energy has passed through cardiac cells they go through a phase where they are unresponsive to electrical energy no matter how much voltage is delivered This normal refractoriness is called the physiological refractory period and it is very brief (it is mea-sured in thousandths of a second) Of course the exact duration of a refractory period is based in part on the heart rate the faster the heart rate the shorter the refractory period and vice versa

Refractoriness may also occur when certain cells become dysfunctional that is cardiac cells may lose their ability to conduct properly Furthermore the heartrsquos inherent pacemakers may change their rate for instance the AV junction may speed up and start to deliver impulses at 80 or 100 times a minute When this happensmdashthat is when another pacemaker takes over the role of the SA nodemdashthat pacemaker is called a usurping pacemaker because it usurps the rightful role of the SA node In the event that other pacemakers of the heart start to compete with the SA node the fastest pacemaker always wins

Conductivity

Conductivity is the cardiac cellrsquos ability to transmit an electrical impulse to an adjacent cell Typically cardiac conduction is antegrade that

is it moves forward Electrical impulses actually travel out in all different directions but refracto-riness prevents those cells that were just stimu-lated from depolarization This keeps conduction moving in a single direction through the heart Normal antegrade conduction in a healthy heart goes from SA node to atria to AV node over the bundle of His and down to the ventricles Some individuals have the ability to conduct retro-grade retrograde conduction is abnormal but not uncommon

autonomic nervous system

The heart does not need direct stimulation from the nervous system to generate electricity and to beat but that is not to say that the nervous system does not control the heart rate The autonomic nervous system indirectly affects cardiac conduction by regulating how fast the heart will beat and how hard the heart muscle will squeeze (contractility) The autonomic ner-vous system communicates with the heart through a network of baroreceptors which respond to chemical signals

The autonomic nervous system of the body like certain other physiological systems exists as two systems in balance The autonomic nervous system consists of a sympathetic system and a parasympathetic system They both work together and in a healthy individual one may overdrive the other during specific times The sympathetic ner-vous system is mainly associated with epinephrine (adrenaline) and it has the ability to speed the heart rate and increase cardiac contractility and can increase the speed by which electrical impulses can cross the AV node The sympathetic nervous system takes over when we are stressed frightened (fight-or-flight response) or exerting ourselves

On the other hand the autonomic nervous system also has a parasympathetic system that is associated with acetylcholine This system can decrease the heart rate decrease cardiac contrac-tility and cause impulses crossing the AV node to be slowed The parasympathetic system might take over when we are sleeping or at rest

18 The nuts and bolts of implantable device therapy pacemakers

the nuts and bolts of the cardiac conduction system

bull The heartrsquos natural pacemaker is the sinoatrial (SA) node which generates an electrical impulse spontaneously about 60ndash100 times a minute in a healthy individual

bull The normal conduction path is from the SA node over the atria to the atrioventricular (AV) node and then via the bundle of His down the right and left bundle branches and into the Purkinje fibers

bull The AV node has the ability to slow down the electrical impulse This serves two main purposes it allows for atrial kick and it protects the ventricles from beating too rapidly

bull In the healthy heart the right and left atria contract simultaneously and then the right and left ventricles contract simultaneously

bull Depolarization and repolarization of myocar-dial cells are driven by positively and negatively charged ions which cross the cell membrane A resting cardiac cell is more negative on the inside than the surrounding media but when it depolarizes it becomes more positive on the inside relative to the surrounding media Following depolarization the ions move and the cell returns to being more negative on the inside than the outside in a process known as repolarization

bull Depolarization causes the cardiac cell to contract

bull The main ions involved in depolarization and repolarization are sodium potassium and calcium ions

bull Although the heartrsquos natural pacemaker is the SA node the AV junction and the Purkinje fibers (ventricles) can also assume (usurp) this role if the SA node cannot generate electricity properly The AV junction and Purkinje fibers are not as reliable as a healthy AV node and typically generate electricity more slowly (the AV junction about 40ndash60 times a minute and the Purkinje fibers 20ndash40 times a minute)

bull However sometimes the AV junction or Purkinje fibers may generate electricity very

quickly this is abnormal When they do they may take over the heart When pacemakers of the heart compete the fastest one drives the heart

bull Automaticity is an inherent property of certain cardiac conductive cells to depolarize spontaneously Not all cardiac conduction cells possess automaticity but the SA node AV junction and Purkinje fibers possess automaticity

bull Refractoriness is the natural and expected behavior of cardiac conductive cells that makes them unresponsive to electrical impulses immediately after they have depolarized The natural refractory period is very brief

bull The faster the heart rate the shorter the refractory period the slower the heart rate the longer the refractory period

bull Refractoriness can also occur as a dysfunction bull Conductivity is the cardiac cellrsquos ability to

transmit an electrical impulse to adjacent cells Refractoriness means that cells that just depolarized will not depolarize again This keeps conduction moving forward (antegrade conduction)

bull Antegrade conduction is normal retrograde conduction is abnormal but is not rare

bull The autonomic nervous system regulates the heart rate by controlling how fast the heart beats and how vigorously it contracts

bull The autonomic nervous system consists of a sympathetic and parasympathetic nervous system which work together but at certain times one or the other is dominant The sympathetic nervous system makes the heart beat faster and more vigorously it takes over during times of stress exertion or fight or flight The parasympathetic ner-vous system makes the heart beat slowly and less vigorously it takes over during periods of rest

bull The neurotransmitters associated with these two systems are epinephrine or adrenaline (sympathetic nervous system) and acetylcho-line (parasympathetic nervous system)

Chapter 2 Cardiac conduction system 19

test your knowledge

Please write in the landmarks of the cardiac conduction system Select from the following (each is used only once)

AV node Bachmanrsquos bundle Bundle of His

Internodal tracts Left bundle branch Purkinje fibers

Right bundle branch Ventricles

1 Which of the following is not considered one of the heartrsquos intrinsic pacemakersA AV junctionB SA nodeC Purkinje fibersD Aorta

2 Depolarization and repolarization involve the movement of charged ions across cardiac cell membranes What types of ions are primarily involvedA Calcium potassium and sodiumB Acetylcholine and epinephrineC Magnesium and folateD All of the above

3 Refractoriness refers toA The ability of cardiac conduction cells to

contract

B The inability of cardiac conduction cells to respond to an electrical stimulus

C The unique permeability of the cardiac cellsD Abnormal cardiac conduction

4 When a cardiac cell depolarizes the interior of the cell becomes what with respect to the surrounding mediaA More positiveB More negativeC More acidicD Hotter

5 Fill in the blank In the healthy heart the faster the heart rate the ___ the refractory periodA More vigorousB Less reliableC LongerD Shorter

20 The nuts and bolts of implantable device therapy pacemakers

6 Which specific part of the nervous system governs the fight-or-flight response and is associated with epinephrine (adrenaline) (Pick the most exact answer)A Autonomic nervous systemB Parasympathetic nervous systemC Sympathetic nervous systemD Central nervous system

7 In normal antegrade conduction the electrical impulse travels from the SA node over the atria to the AV node and to what structure before reaching the ventriclesA The intermodal tractsB The Purkinje fibersC The bundle of HisD The ventricular apex

8 Which of the following is true about retrograde cardiac conductionA Everyone has itB It is perfectly normal and can be helpful in

stressful situations

C It is abnormal but not uncommonD It is impossible to diagnose definitively

except in an autopsy

9 What chemical is associated with the parasympathetic nervous systemA AcetylcholineB PotassiumC EpinephrineD Dopamine

10 The stimulation of the myocardial cells of which of the following structures is most closely associated with ventricular depolarizationA SA nodeB AV junctionC Bundle of HisD Purkinje fibers

4 The nuts and bolts of implantable device therapy pacemakers

Right atrium

Left atrium

Left ventricle

Right ventricle

Figure 12 Note that the atria are smooth walled while the ventricles contain a spongelike fibrous network of trabeculae

Left pulmonaryarteries

Left pulmonaryveins

Aorta

Aortic valve

Pulmonaryvalve

Right pulmonaryarteries

Superiorvena cava

Inferiorvena cava

Right pulmonaryveins

Figure 13 The blood flow within the heart takes oxygen-depleted blood from the body into the right atrium where it flows to the right ventricle and is pumped out over the lungs the reoxygenated blood from the lungs is pumped into the left atrium where it flows to the left ventricle and is pumped out via the aorta to the body

Chapter 1 Cardiovascular anatomy and physiology 5

aorta is high around 120 mmHg or ten times higher than the pressure in the PV The left ventricle must therefore work much harder to pump blood than the right ventricle This requires the left ventricle to be larger and more muscular than the right ven-tricle (see Figure 14)

On the right side of the heart blood travels from the right atrium into the right ventricle via the tricuspid valve The tricuspid valve gets its name from its characteristic shape involving three leaves or cusps Attached to these cusps are cords that anchor into the base of the ventricle known as chordae tendineae they look almost like little parachutes The strands of the chordae tendineae attach to tiny papillary muscles These chords attach to the valve leaves at one end and a papil-lary muscle at the other end On the right side of the heart the tricuspid valve is associated with three papillary muscles The purpose of these chords and muscles is to assure that the valve is

effectively closed and opened at the proper times (Figure 15)

The heart can rightly be thought of as a pump but it must be remembered that the heart is also a muscle and all muscles need a steady supply of oxy-gen-rich blood The heart muscle is supplied with blood through a network of coronary arteries that surround the outside of the heart Blockage in a coronary artery results in ischemia which can lead to death of cardiac muscle including the chordae tendineae and papillary muscles While the patient may survive such an ischemic event the damage to the heart may lead to an incompetent valve that is a valve that is no longer able to function effectively

In tracing the blood flow from the right ventricle to the pulmonary artery it should be clear that the blood has to go from down in the right ventricle to up through the pulmonary valve and into the pulmonary artery The blood is able to make this journey because of the pumping pressure of the heart The route the blood takes as it exits the right ventricle and journeys up toward the pulmonary valve is known as the right ventricular outflow tract (RVOT) On the other side of the heart there is also a corresponding left ventricular outflow tract (LVOT) of approximately the same size Cardiac leads are sometimes fixated in the RVOT (see Figure 16)

Blood pumped out of the right ventricle crosses the pulmonary valve and enters the pulmonary artery which splits into two branches right and left The right pulmonary artery takes blood to the right lung while the left pulmonary artery takes blood to the left lung In this respect the pulmonary artery is unique in the body in that it is an artery but it carries deoxygenated blood Blood travels through the lung to the alveoli where it gains oxygen and loses carbon dioxide Once it is reoxygenated the blood gathers into the PV (which are also unique being the only veins to carry oxygenated blood) There are four PV in total the two right-sided PV take oxygen-ated blood from the right lung while the two left-sided PV take oxygenated blood from the left lung and they all bring this reoxygenated blood to the left atrium

The left atrium is smooth walled like the right atrium and although the left atrium is much

Figure 14 The left ventricular is far more muscular than the right ventricle because it must overcome 10 times the pressure of the right ventricle in order to pump blood out via the aortic valve and into the aorta

6 The nuts and bolts of implantable device therapy pacemakers

Right atrium

Left atrium

Left ventricle

Right ventricle

Figure 15 The leaflets of the valves are attached by chordae tendineae at one end and papillary muscles on the other which assure the effective opening and closing of the valves

Right-ventricularoutflow tract (RVOT)

Figure 16 There are two outflow tracts in the heart one associated with the right ventricle and the other the left ventricle These outflow tracts are roughly the same size The illustration shows the right ventricular outflow tract (RVOT) a preferred location for right ventricular lead fixation The left ventricular outflow tract (LVOT) cannot be seen in this illustration as it is posterior

Chapter 1 Cardiovascular anatomy and physiology 7

smaller than the left ventricle it is larger than the right atrium The blood will travel from the left atrium into the left ventricle by way of the mitral valve The mitral valve gets its name from the miter a bishoprsquos hat The mitral valve has two cusps and papillary muscles connected by chordae tendineae Once blood is in the left ventricle a contraction will force it toward the aorta by way of the LVOT In persons with hypertrophic obstructive cardiomy-opathy (HOCM) the septum or wall in the heart can become thick and enlarged effectively narrow-ing the LVOT In the healthy heart the blood flows up the LVOT across the aortic valve and into the aorta From the aorta blood will be directed upward toward the head downward toward the legs and feet and some blood will be redirected back to the coronary arteries of the heart

The blood flow from the left ventricle to the aorta and back to the coronary arteries is unique in the body in that most of that flow occurs during diastole rather than systole Think of it this way as blood is pumped by the left ventricle during systole upward and downward to the brain and the rest of the body some of that blood flows back during diastole and drains into the coronary arteries Although it may seem counterintuitive it is impor-tant to remember that coronary artery perfusion occurs primarily during diastole

Volume valves and pressure

While it is tempting to think of the heart as simply a pump with an electrical system it is more accu-rate to say that the heart moves blood because of variations in volume and pressure

The heart has four valves two are atrioventricular (AV) valves because they connect the atrium to the ventricle and two are semilunar valves connecting the heart with pulmonary arteries or the aorta The right AV valve is the tricuspid while the left AV valve is the mitral valve The flow or movement of blood is guided and controlled by the heartrsquos muscular action in the form of systole (contraction) or diastole (relaxation)

When blood pours into the right atrium it dis-tends and expands the right atrium When this volume of blood creates sufficient pressure it forces the tricuspid valve open Think of it as volume creating pressure and pressure opening

valves When the tricuspid valve opens the blood dumps into the right ventricle During this period of diastole blood pours into the right ventricle creating volume and expanding it As blood pours in and ventricular diastole nears the end the atria contract while the AV valves are still open in something nicknamed atrial kick Atrial systole occurs at the end of ventricular diastole Atrial kick forces the maximum amount of blood from the atria into the ventricles It is estimated that atrial kick delivers 20ndash30 of cardiac output

The atrial contribution to ventricular filling (atrial kick) is of enormous clinical significance Patients with certain atrial arrhythmias such as atrial fibrillation (AF) lose atrial kick Even milder forms of atrial tachyarrhythmias may compromise atrial kick Moreover patients who do not have AV synchrony (one atrial beat corresponding to one ventricular beat) will lose atrial kick The loss of atrial kick can reduce cardiac output by 20ndash30

Once the atria have contracted and the ventricles are filled with blood the AV valves snap closed The ventricles now start to contract and push open the semilunar valves For example as the right ven-tricle contracts (systole) it opens the PV (The PV is sometimes called the pulmonic valve itrsquos the same thing) The whole process works on volume pressure and contractionrelaxation

Of the four valves in the heart the tricuspid valve is of most interest to device specialists because at least one lead will be placed into the right ventricle This means that pacing and defi-brillation leads typically cross the tricuspid valve These leads may interfere with the proper closing of the tricuspid valve particularly if the lead is large diameter or several leads are passed over the tricuspid valve A single lead through the tricuspid valve is likely to cause minimal to no dysfunction However it is not unusual to see patients with three or more leads in the right ventricle and these leads may hamper the tricuspid valve in its closing leading to tricuspid regurgitation or backflow of blood Patients with multiple leads (such as chil-dren who grow up with pacemakers) may need to have some of them removed to avoid tricuspid regurgitation

Right ventricular leads are typically fixated at the right ventricular apex or in the RVOT and may be

8 The nuts and bolts of implantable device therapy pacemakers

lodged in the trabeculae Trabeculae are a fibrous network that could be described as similar to the pores of a sponge (see Figure 17)

On the left side of the heart the blood flow pattern is similar Blood flows into the left atrium over the open mitral valve and into the left atrium At the end of ventricular diastole as the ventricle is distended and stretched atrial kick delivers more blood into left ventricle The volume of blood cre-ates pressure The mitral valve closes As the left ventricle contracts the growing pressure over-comes the resistance on the aortic valve and it opens allowing blood to flow out into the aorta and beyond

While any valve can become diseased or dys-functional valvular disease more commonly affects the mitral and the aortic valves (left-sided valves) than the right-sided valves (tricuspid and pul-monary) These left valves are more likely to suffer damage or disease because of the high-pressure environment in which they function exposing them to more potential damage Other damage can occur when plaque builds up in the coronary arteries and the aorta The aorta can become

diseased typically in the case of an aortic aneurysm or dissection Carotid arteries in the neck are also frequently a site for atherosclerotic disease

the right atrial appendage

The right atrial appendage (RAA) is an area near the right atrium which is a preferred site for attach-ing active-fixation atrial leads (see Figure 18) The RAA serves no obvious purpose and may be sacri-ficed in certain heart surgeries such as a coronary artery bypass graft (CABG) procedure CABG or bypass surgery involves stopping the heart and running the blood through a machine to reoxygen-ate it during the course of the procedure This is done by attaching a large-diameter hose to the RAA when surgery concludes and this hose is removed it can damage the RAA to the point that it is completely or partially surgically removed In patients who have had a CABG procedure the RAA may not be available for right atrial lead placement For such patients the right atrial lead is often placed against the lateral wall of the right atrium It is sometimes possible to fixate a right

Figure 17 Trabeculae form a fibrous network within the ventricles Passive-fixation ventricular leads are held in place by lodging their tines or fins in the trabeculae

Chapter 1 Cardiovascular anatomy and physiology 9

atrial lead in the remnants of the RAA if a portion of it is preserved

arteries and veins

Any discussion about the heart necessarily involves the vasculature which is why we commonly refer to our circulatory system as the cardiovascular

system As most of us remember from anatomy classes arteries carry blood away from the heart while veins transport blood back to the heart (The exceptions are the PV and arteries described earlier) Arteries carry oxygen-rich blood while veins carry deoxygenated blood What is of importance to device clinicians is that arteries are high-pressure vessels while veins are low-pressure vessels Arteries tend to be more muscular in structure and muscular contractions of the arteries help to move blood outward into the body Veins tend to be less muscular than arteries but unlike arteries they contain a system of tiny interior valves The purpose of these little valves is to maintain a unidirectional flow of blood (see Figure 19)

Some of the most important veins in the body are the SVC and its counterpart the IVC The SVC takes blood from the upper part of the body and delivers it into the right atrium with the IVC doing the same for blood from the lower part of the body The coronary veins networked around the outside of the heart carry blood from the heart tissue and deliver it back to the right atrium they are closely linked to the coronary arteries which carry reoxygenated blood from the left ventricle back to the heart muscle The CS is the site where

Right atrial appendage

Figure 18 The right atrial appendage serves no obvious hemodynamic purpose but is often the site of fixation of atrial leads

Artery Vein

Figure 19 Veins and arteries have different purposes different anatomical structure and different pressures Overall veins are low-pressure systems that move blood by a series of little valves that keep blood flowing in the same direction Arteries are high-pressure systems and are more muscular so that they force blood forward by squeezing

10 The nuts and bolts of implantable device therapy pacemakers

the great middle and small cardiac veins all drain the CS takes this blood and delivers it back to the right atrium

The PV carry reoxygenated blood from the lungs and deliver it to the left atrium so that it can be pumped back out to the body by the left ventricle

The most important artery in the body is by far the aorta The aorta is connected to the left ven-tricle and it is the first and main conduit that takes reoxygenated blood from the left ventricle and sends it out to the body As this reoxygenated blood makes its way out into the body its first two stops are the head (brain) and the heart muscle Inadequate oxygenated blood to the brain can very quickly provoke symptoms This is the reason that depressed cardiac output is associated with symptoms like dizziness light-headedness feeling woozy and fatigue Severe symptoms might include syncope

The coronary arteries arise from the root of the aorta just above the cusps of the aortic valve Coronary arteries receive most blood during dias-tole so blood is more or less pouring back into these vessels The coronary artery system is a networkmdashalmost a meshmdashof middle-sized to very small vessels that cover the exterior surface of the heart (see Figure 110)

Coronary arteries get their name from the Latin word for crown (such as in our word coronation) because this network of vessels sits like a crown atop the heart and encircles it The heart gets blood from outside in as oxygen-rich blood in these coro-nary arteries is delivered to the exterior of the heart muscle Coronary arteries are not within the heart they are epicardial structures The two main coro-nary arteries are the right coronary artery (RCA) and the left coronary artery (LCA) The RCA and LCA come off the aorta and are located in the AV groove an exterior structure below the atria and above the ventricles The RCA runs from the aorta in the right AV groove while the LCA comes off the aorta and quickly branches to the left main The left main is a very short artery (about the size of a thumbnail) which branches quickly into the left anterior descending artery (LAD) and the circum-flex artery The LAD travels down the front of the

left side of the heart The circumflex artery can be found in the left AV groove

The RCA bifurcates to the posterior descending artery The right atrium and right ventricle get their supply of oxygen-rich blood from the RCA Much of the cardiac conduction system is right sided that is it commences with the sinoatrial (SA) node in the high right atrium and travels down to the AV node on the right side of the septum These important electrical structures get their oxygen-rich blood supply from the RCA The electrical system travels down the ventricles to the inferior wall of the left ventricle (located at the bottom of the heart but belonging to the left ventricle) the inferior wall is also fed oxygen-rich blood by the RCA An occlusion of the RCA such as might occur in a myocardial infarction (MI) may result in death of the heart muscle it feeds which includes the right atrium the right ventricle and the inferior wall of the left ventricle and it might affect the conduction system including the SA node and the AV node An inferior wall MI is typically the result of a clogged RCA

RCA

LCA

Aorta

Figure 110 The coronary arteries of the heart branch out to surround the entire heart muscle The two most prominent coronary arteries are the right coronary artery (RCA) and the left coronary artery (LCA)

Chapter 1 Cardiovascular anatomy and physiology 11

For an MI the more distal the occlusion (that is further from the aorta) the less potential damage the heart attack will have In other words the worst occlusions are proximal Whether or not a patient survives an MI comes down to the numbers The loss of over 40 of the left ventricular muscle mass results in death However that 40 is cumulative For example a person can survive an MI that costs him 30 of left ventricular muscle mass but if that heart attack is followed by another that kills 15 of the left ventricular muscle mass the second (milder) heart attack will prove fatal This is the reason many people survive a first heart attack but die of a second or third attack even though their fatal MI may be relatively mild

The LCA branches quickly to the left main which branches almost at once to the LAD The LAD runs on the outside of the heart along the front of the heart along the septum The left main supplies the left atrium and the majority of the left ventricle in other words the major pumping por-tion of the heart An occlusion in the left main can be particularly disastrous because it is so far upstream and affects the heartrsquos left ventricular muscle mass No wonder they call the left main the widow maker

The coronary arteries do not run in straight lines they twist and turn and sometimes make very sharp bends People with coronary artery disease (CAD) typically have plaque buildup in the areas where the coronary arteries bend at sharp angles These are areas where the blood flow creates a lot of turbulence and plaque and other substances can collect and build up It is typically to find blockage and occlusions at these places

The LAD supplies the anterior wall of the left ventricle and the septum with oxygen-rich blood Since part of the heartrsquos conduction system runs through the septum (the right and left bundle branches) an LAD occlusion can damage the heartrsquos ability to conduct electricity properly

The circumflex artery runs along the AV groove laterally and thus supplies oxygen-rich blood to the lateral wall of the left ventricle along with the left atrium The circumflex runs along the AV groove to the left heading toward the back of the heart where it meets with the RCA which

travels along the AV groove to the left toward the back of the heart Both the circumflex and the RCA are responsible for providing oxygenated blood to the posterior wall of the ventricle But which is the more important provider That depends on crux and whether the heart is right or left dominant

The crux refers to an imaginary line drawn exactly down the middle of the ventricular poste-rior wall without any regard to anatomical landmarks If the RCA crosses the crux then the heart is said to be right dominant and the RCA is the greater supplier of oxygenated blood to the ven-tricular posterior wall On the other hand if the circumflex crosses the crux then the heart is said to be left dominant and the circumflex is the more significant provider Roughly 60 of the population is right dominant

While this information about the vasculature is important it is even more important for clini-cians to recognize that there is tremendous interpatient variability in terms of venous anatomy While we can describe major structures and typical venous formations anomalies are very common These anomalies may be minor variations or they can be very pronounced such as missing veins and arteries Clinicians who deal with the cardiovascular system must be prepared for large and small differences in vascular anatomy The cardiovascular system is incredibly resilient and can adapt to some anomalies by building up collateral circulation For example a person with a missing or incompetent vessel may over time build up a network of smaller collateral vessels that compensates by doing the same job It is not unusual to see vessels grow larger or branch out to compensate Sometimes when an artery is blocked collateral circulation will build up and compensate by delivering oxygen-rich blood to the heart muscle

Every artery in the body has a matching or corresponding vein usually located in close proximity This applies to the coronary system as well Note that there are great differences in coro-nary venous anatomy among patients which becomes important when implanting left ventric-ular leads for CRT devices (see Figure 111)

12 The nuts and bolts of implantable device therapy pacemakers

the nuts and bolts of cardiovascular anatomy and physiology

bull The heart consists of four chambers which may be considered the upper chambers (right atrium and left atrium) and lower chambers (right ven-tricle and left ventricle) or may be thought of as the right heart (right atrium and right ventricle) and left heart (left atrium and left ventricle)

bull Deoxygenated blood enters the right atrium flows to the right ventricle and is pumped out via the pulmonary valve to the lungs The blood is reoxygenated in the lungs and then enters the left atrium and flows to the left atrium and out of the aortic valve into the aorta and the rest of the body

bull The heart has two outflow tracts a right ven-tricular outflow tract (RVOT) and a left ven-tricular outflow tract (LVOT)

bull The largest chamber of the heart is the left ven-tricle making up about 23 of the cardiac mass The left ventricle is muscular and massive because it must pump against approximately 10 times the pressure against which the right ven-tricle pumps (120 vs 12 mmHg)

bull The two cardiac valves most susceptible to dysfunction and disease are the mitral and aortic valve because they function in a high-pressure environment

bull The chordae tendineae connect the leaflets of the valves to the papillary muscles and look like little parachutes These help the valves open and close properly

bull In general arteries transport blood away from the heart are muscular and contract to help move the blood on its way and have thicker walls than veins Veins have thinner less muscular walls and carry oxygen-depleted blood back to the heart using tiny interior valves to keep the blood flow moving forward Arteries transport oxygenated blood and can be considered high-pressure systems while veins transported deoxygenated blood in a low-pressure system

bull Cardiac contraction is systole cardiac relaxation is diastole Atrial systole occurs at the very end of ventricular diastole in something nicknamed atrial kick In a healthy individual about 20ndash30 of cardiac output comes from atrial kick

bull The heart is a muscle that is fed with oxygenated blood through a network of coronary arteries Coronary arteries get oxygen-rich blood from the aorta and they perfuse the heart muscle during diastole as blood drains back

bull Although arteries and veins generally take blood away from or to the heart respectively

Rightventricle

Leftventricle

Leftatrium

Aorta

Rightatrium

Rightcoronaryartery

Leftcoronaryartery

Circumflexartery

Superiorvena cava

Inferiorvena cava

Arterial system

Rightventricle

Leftventricle

Greatcardiac vein

Smallcardiac vein

Middlecardiac vein

Leftatrium

Aorta

Rightatrium

Superiorvena cava

Inferiorvena cava

Venous system

Coronarysinus

Figure 111 The coronary artery system is matched by a coronary venous system

Chapter 1 Cardiovascular anatomy and physiology 13

test your knowledge

Fill in these anatomical landmarks of the heart They include the four chambers the four valves and the bodyrsquos main artery

the pulmonary vessels are the exception The pulmonary artery transports deoxygenated blood while the pulmonary veins carry oxygen-ated blood This is because of their special loca-tion and the fact that their work is taking blood from the right side to the left side of the heart

bull The main coronary arteries are the right coro-nary artery and the left coronary artery The left coronary artery branches quickly to a short vessel known as the left main (the widow maker) and the circumflex artery

bull A myocardial infarction can kill the heart muscle The clogged arteries determine the loc ation of the affected muscle A right coro-nary artery occlusion will affect the right atrium and ventricle and the inferior wall of the left ventricle while a left coronary artery occlusion will affect the left atrium and most of the left ventricle By far the most dangerous occlusion is in the left main because it feeds the majority of the heart muscle with oxygen-rich blood

answer the following questions

1 What are the names of the two atrioventricular valves of the heartA Atrial and ventricularB Tricuspid and mitralC Pulmonary and pulmonicD Aortic and superior vena cava

2 Which chambers of the heart contain trabeculaeA AtriaB VentriclesC Right-sided chambers onlyD Left-sided chambers only

14 The nuts and bolts of implantable device therapy pacemakers

3 Why can trabeculae be important to device cliniciansA They are good places to secure a passive-fixa-

tion leadB They help regulate blood pressureC They prevent atherosclerosisD All of the above

4 Which of the following is not true about the aortaA It is an arteryB It consists of an ascending portion a descend-

ing portion and an archC It carries only oxygenated bloodD It is the largest vein in the body

5 From which artery does blood enter the right atriumA Superior vena cavaB Inferior vena cavaC Coronary sinusD All of the above

6 What is atrial kickA Ventricular pressure on the tricuspid valve

forcing it shutB Electrical impulses originating in the atrium

to cause the heart to beat fasterC Atrial systole that forces the most blood possible

into the already-full and diastolic ventriclesD A type of arrhythmia

7 Why would a person with chronic AF likely have decreased cardiac outputA AF is associated with low blood volumeB AF causes the heart to beat very slowlyC AF may cause the heart to experience fre-

quent pausesD AF reduces or eliminates atrial kickmdashand

atrial kick contributes to cardiac output

8 Which of the following is often a preferred location for affixing a cardiac pacing leadA The right ventricular outflow tract (RVOT)B The left ventricular outflow tract (LVOT)C The excised right atrial appendageD The tricuspid valve

9 What connects the valve leaflets to the papillary musclesA TrabeculaeB ArteriesC Chordae tendineaeD Tiny valves

10 Which coronary artery is nicknamed the widow maker because an occlusion in this vessel will deprive the majority of the heartrsquos muscle mass of oxygen-rich bloodA Left mainB CircumflexC Right coronary arteryD Inferior vena cava

15

The Nuts and Bolts of Implantable Device Therapy Pacemakers First Edition Tom Kenny copy 2015 John Wiley amp Sons Ltd Published 2015 by John Wiley amp Sons Ltd

Chapter 2

Cardiac conduction system

Introduction

The sinoatrial (SA) node is the heartrsquos natural pacemaker In the healthy heart the SA node fires and the electrical stimulus conducts along special pathways through the heart that cause cells in specific areas to depolarize After the stimulus passes the area the cells repolarize Myocardial cell depolarization and repolarization govern the heartrsquos contraction and relaxation sequencemdashin other words the heart rhythm Rhythm disorders occur when this healthy conduction system is in some way disrupted or impaired

the conduction pathways

When the SA node fires and sends out electrical energy the stimulus travels from the SA node in the high right atrium out over the atria via path-ways known as internodal tracts Most of these internodal tracts are located in the right atrium the

main tract over to the left atrium is called Bachmannrsquos bundle It is because of Bachmannrsquos bundle linking the right atrium to the left atrium that the atria can contract simultaneously that is as these two upper chambers contract as one coherent unit After traversing the atria the impulse comes down through conduction tracts to the atrioventricular (AV) node

The AV node is an area of highly specialized tissue located below the atria and above the ventri-cles on the right side of the septum Anatomically the AV node rests in the AV groove The tissue in the healthy AV node has the ability to change the speed of the electrical impulse specifically it slows the stimulus down The slowdown serves two important purposes First slowing the impulse allows the atria time to contract and provide the very valuable atrial kick to ventricular filling Second the slowed impulse protects the ventricles from being forced to beat too quickly that is to beat before they have been adequately filled with blood

Once the impulse crosses the AV node it travels down into the ventricles and accelerates its speed The impulse travels through the bundle of His (pronounced hiss) to the bundle branches (right and left) and then out to the network of Purkinje fibers This allows both ventricles to depolarize and contract simultaneously (Figure 21)

polarization depolarization and repolarization

Cardiac conduction is driven by the ability of myo-cardial cells to be depolarized and repolarized Depolarization occurs at the cellular level and is

2

Learning objectives

bullensp Describe the electrical conduction system of the healthy heart

bullensp List the main anatomical landmarks of normal cardiac conduction

bullensp Name the anatomical location of the heartrsquos natural pacemaker

bullensp Explain depolarization and repolarization in electrical terms and how it relates to the cardiac contraction

bullensp Define the term automaticity and how it relates to cardiac conduction

16 The nuts and bolts of implantable device therapy pacemakers

in turn driven by electrolytes or positively and negatively charged ions (potassium sodium and calcium mainly) which are able to move in and out of the cardiac cells A myocardial cell may be said to be polarized when in its resting state that is there is no electrical activity going on In the rest-ing state the interior of the myocardial cell is somewhat more negatively charged that the outside This state is called the resting state or sometimes phase 0 of the action potential

An electrical stimulus changes the resting state abruptly The electrical energy allows for the movement of ions to the point that the interior of the myocardial cell is now more positive than the outside In other words the charge reverses and the cell is said to be depolarized When this happens the cell contracts when this happens to a large area of the myocardium the heart muscle contracts Cardiac contraction occurs as a result of cellular depolarization

When the electrical stimulus moves past the myocardial cell the ions move in reverse and the interior of the cell assumes its previous negative charge with respect to the outside This process is

called repolarization and it brings the myocar-dial cell back to the resting state When this happens the cell relaxes When a large area of the myocardium repolarizes the heart resumes its previous shape (loses its contraction)

Intrinsic pacemakers

The healthy heartrsquos natural pacemaker is the SA node but the SA node is not the only area of the heart that can generate an electrical pulse The SA node serves as the heartrsquos pacemaker because it generates these pulses faster than other areas In the healthy heart the SA node can fire at around 60ndash100 times a minute corresponding to a healthy heart rate The AV junction can also fire but it is much slower at around 40ndash60 times a minute Even the Purkinje fibers (the ventri-cles) can act as a pacemaker but they fire at only 20ndash40 times a minute Thus in a healthy individual the SA node outpaces these other potential pacemakers The AV junction and ven-tricles are sometimes called backup pacemakers because they take over the heart rate in the event

Sinoatrial (SA) node

Internodal tracts

Right bundle branch

Left bundle branch

Purkinje fibers

Bachmannrsquos bundle

Bundle of His

Atrioventricular (AV) node

Figure 21 The conduction pathways in the healthy heart

Chapter 2 Cardiac conduction system 17

that the SA node is not able to pace the heart While these backup pacemakers may help keep the heart beating in the event of a dysfunctional SA node they are slow and not as reliable as the SA node

The healthy heart is able to pace itself because of a property called automaticity Automaticity is an inherent property of individual cardiac conductive cells that allows them to depolarize spontaneously In other words some cells in the heart (such as the cells of the SA node) are able to generate an electrical stimulus on their own

refractoriness

Refractoriness refers to the temporary inability of a cardiac cell to respond to an electrical stimulus Refractoriness can be the normal appropriate and expected behavior of cardiac cells For example immediately after electrical energy has passed through cardiac cells they go through a phase where they are unresponsive to electrical energy no matter how much voltage is delivered This normal refractoriness is called the physiological refractory period and it is very brief (it is mea-sured in thousandths of a second) Of course the exact duration of a refractory period is based in part on the heart rate the faster the heart rate the shorter the refractory period and vice versa

Refractoriness may also occur when certain cells become dysfunctional that is cardiac cells may lose their ability to conduct properly Furthermore the heartrsquos inherent pacemakers may change their rate for instance the AV junction may speed up and start to deliver impulses at 80 or 100 times a minute When this happensmdashthat is when another pacemaker takes over the role of the SA nodemdashthat pacemaker is called a usurping pacemaker because it usurps the rightful role of the SA node In the event that other pacemakers of the heart start to compete with the SA node the fastest pacemaker always wins

Conductivity

Conductivity is the cardiac cellrsquos ability to transmit an electrical impulse to an adjacent cell Typically cardiac conduction is antegrade that

is it moves forward Electrical impulses actually travel out in all different directions but refracto-riness prevents those cells that were just stimu-lated from depolarization This keeps conduction moving in a single direction through the heart Normal antegrade conduction in a healthy heart goes from SA node to atria to AV node over the bundle of His and down to the ventricles Some individuals have the ability to conduct retro-grade retrograde conduction is abnormal but not uncommon

autonomic nervous system

The heart does not need direct stimulation from the nervous system to generate electricity and to beat but that is not to say that the nervous system does not control the heart rate The autonomic nervous system indirectly affects cardiac conduction by regulating how fast the heart will beat and how hard the heart muscle will squeeze (contractility) The autonomic ner-vous system communicates with the heart through a network of baroreceptors which respond to chemical signals

The autonomic nervous system of the body like certain other physiological systems exists as two systems in balance The autonomic nervous system consists of a sympathetic system and a parasympathetic system They both work together and in a healthy individual one may overdrive the other during specific times The sympathetic ner-vous system is mainly associated with epinephrine (adrenaline) and it has the ability to speed the heart rate and increase cardiac contractility and can increase the speed by which electrical impulses can cross the AV node The sympathetic nervous system takes over when we are stressed frightened (fight-or-flight response) or exerting ourselves

On the other hand the autonomic nervous system also has a parasympathetic system that is associated with acetylcholine This system can decrease the heart rate decrease cardiac contrac-tility and cause impulses crossing the AV node to be slowed The parasympathetic system might take over when we are sleeping or at rest

18 The nuts and bolts of implantable device therapy pacemakers

the nuts and bolts of the cardiac conduction system

bull The heartrsquos natural pacemaker is the sinoatrial (SA) node which generates an electrical impulse spontaneously about 60ndash100 times a minute in a healthy individual

bull The normal conduction path is from the SA node over the atria to the atrioventricular (AV) node and then via the bundle of His down the right and left bundle branches and into the Purkinje fibers

bull The AV node has the ability to slow down the electrical impulse This serves two main purposes it allows for atrial kick and it protects the ventricles from beating too rapidly

bull In the healthy heart the right and left atria contract simultaneously and then the right and left ventricles contract simultaneously

bull Depolarization and repolarization of myocar-dial cells are driven by positively and negatively charged ions which cross the cell membrane A resting cardiac cell is more negative on the inside than the surrounding media but when it depolarizes it becomes more positive on the inside relative to the surrounding media Following depolarization the ions move and the cell returns to being more negative on the inside than the outside in a process known as repolarization

bull Depolarization causes the cardiac cell to contract

bull The main ions involved in depolarization and repolarization are sodium potassium and calcium ions

bull Although the heartrsquos natural pacemaker is the SA node the AV junction and the Purkinje fibers (ventricles) can also assume (usurp) this role if the SA node cannot generate electricity properly The AV junction and Purkinje fibers are not as reliable as a healthy AV node and typically generate electricity more slowly (the AV junction about 40ndash60 times a minute and the Purkinje fibers 20ndash40 times a minute)

bull However sometimes the AV junction or Purkinje fibers may generate electricity very

quickly this is abnormal When they do they may take over the heart When pacemakers of the heart compete the fastest one drives the heart

bull Automaticity is an inherent property of certain cardiac conductive cells to depolarize spontaneously Not all cardiac conduction cells possess automaticity but the SA node AV junction and Purkinje fibers possess automaticity

bull Refractoriness is the natural and expected behavior of cardiac conductive cells that makes them unresponsive to electrical impulses immediately after they have depolarized The natural refractory period is very brief

bull The faster the heart rate the shorter the refractory period the slower the heart rate the longer the refractory period

bull Refractoriness can also occur as a dysfunction bull Conductivity is the cardiac cellrsquos ability to

transmit an electrical impulse to adjacent cells Refractoriness means that cells that just depolarized will not depolarize again This keeps conduction moving forward (antegrade conduction)

bull Antegrade conduction is normal retrograde conduction is abnormal but is not rare

bull The autonomic nervous system regulates the heart rate by controlling how fast the heart beats and how vigorously it contracts

bull The autonomic nervous system consists of a sympathetic and parasympathetic nervous system which work together but at certain times one or the other is dominant The sympathetic nervous system makes the heart beat faster and more vigorously it takes over during times of stress exertion or fight or flight The parasympathetic ner-vous system makes the heart beat slowly and less vigorously it takes over during periods of rest

bull The neurotransmitters associated with these two systems are epinephrine or adrenaline (sympathetic nervous system) and acetylcho-line (parasympathetic nervous system)

Chapter 2 Cardiac conduction system 19

test your knowledge

Please write in the landmarks of the cardiac conduction system Select from the following (each is used only once)

AV node Bachmanrsquos bundle Bundle of His

Internodal tracts Left bundle branch Purkinje fibers

Right bundle branch Ventricles

1 Which of the following is not considered one of the heartrsquos intrinsic pacemakersA AV junctionB SA nodeC Purkinje fibersD Aorta

2 Depolarization and repolarization involve the movement of charged ions across cardiac cell membranes What types of ions are primarily involvedA Calcium potassium and sodiumB Acetylcholine and epinephrineC Magnesium and folateD All of the above

3 Refractoriness refers toA The ability of cardiac conduction cells to

contract

B The inability of cardiac conduction cells to respond to an electrical stimulus

C The unique permeability of the cardiac cellsD Abnormal cardiac conduction

4 When a cardiac cell depolarizes the interior of the cell becomes what with respect to the surrounding mediaA More positiveB More negativeC More acidicD Hotter

5 Fill in the blank In the healthy heart the faster the heart rate the ___ the refractory periodA More vigorousB Less reliableC LongerD Shorter

20 The nuts and bolts of implantable device therapy pacemakers

6 Which specific part of the nervous system governs the fight-or-flight response and is associated with epinephrine (adrenaline) (Pick the most exact answer)A Autonomic nervous systemB Parasympathetic nervous systemC Sympathetic nervous systemD Central nervous system

7 In normal antegrade conduction the electrical impulse travels from the SA node over the atria to the AV node and to what structure before reaching the ventriclesA The intermodal tractsB The Purkinje fibersC The bundle of HisD The ventricular apex

8 Which of the following is true about retrograde cardiac conductionA Everyone has itB It is perfectly normal and can be helpful in

stressful situations

C It is abnormal but not uncommonD It is impossible to diagnose definitively

except in an autopsy

9 What chemical is associated with the parasympathetic nervous systemA AcetylcholineB PotassiumC EpinephrineD Dopamine

10 The stimulation of the myocardial cells of which of the following structures is most closely associated with ventricular depolarizationA SA nodeB AV junctionC Bundle of HisD Purkinje fibers

Chapter 1 Cardiovascular anatomy and physiology 5

aorta is high around 120 mmHg or ten times higher than the pressure in the PV The left ventricle must therefore work much harder to pump blood than the right ventricle This requires the left ventricle to be larger and more muscular than the right ven-tricle (see Figure 14)

On the right side of the heart blood travels from the right atrium into the right ventricle via the tricuspid valve The tricuspid valve gets its name from its characteristic shape involving three leaves or cusps Attached to these cusps are cords that anchor into the base of the ventricle known as chordae tendineae they look almost like little parachutes The strands of the chordae tendineae attach to tiny papillary muscles These chords attach to the valve leaves at one end and a papil-lary muscle at the other end On the right side of the heart the tricuspid valve is associated with three papillary muscles The purpose of these chords and muscles is to assure that the valve is

effectively closed and opened at the proper times (Figure 15)

The heart can rightly be thought of as a pump but it must be remembered that the heart is also a muscle and all muscles need a steady supply of oxy-gen-rich blood The heart muscle is supplied with blood through a network of coronary arteries that surround the outside of the heart Blockage in a coronary artery results in ischemia which can lead to death of cardiac muscle including the chordae tendineae and papillary muscles While the patient may survive such an ischemic event the damage to the heart may lead to an incompetent valve that is a valve that is no longer able to function effectively

In tracing the blood flow from the right ventricle to the pulmonary artery it should be clear that the blood has to go from down in the right ventricle to up through the pulmonary valve and into the pulmonary artery The blood is able to make this journey because of the pumping pressure of the heart The route the blood takes as it exits the right ventricle and journeys up toward the pulmonary valve is known as the right ventricular outflow tract (RVOT) On the other side of the heart there is also a corresponding left ventricular outflow tract (LVOT) of approximately the same size Cardiac leads are sometimes fixated in the RVOT (see Figure 16)

Blood pumped out of the right ventricle crosses the pulmonary valve and enters the pulmonary artery which splits into two branches right and left The right pulmonary artery takes blood to the right lung while the left pulmonary artery takes blood to the left lung In this respect the pulmonary artery is unique in the body in that it is an artery but it carries deoxygenated blood Blood travels through the lung to the alveoli where it gains oxygen and loses carbon dioxide Once it is reoxygenated the blood gathers into the PV (which are also unique being the only veins to carry oxygenated blood) There are four PV in total the two right-sided PV take oxygen-ated blood from the right lung while the two left-sided PV take oxygenated blood from the left lung and they all bring this reoxygenated blood to the left atrium

The left atrium is smooth walled like the right atrium and although the left atrium is much

Figure 14 The left ventricular is far more muscular than the right ventricle because it must overcome 10 times the pressure of the right ventricle in order to pump blood out via the aortic valve and into the aorta

6 The nuts and bolts of implantable device therapy pacemakers

Right atrium

Left atrium

Left ventricle

Right ventricle

Figure 15 The leaflets of the valves are attached by chordae tendineae at one end and papillary muscles on the other which assure the effective opening and closing of the valves

Right-ventricularoutflow tract (RVOT)

Figure 16 There are two outflow tracts in the heart one associated with the right ventricle and the other the left ventricle These outflow tracts are roughly the same size The illustration shows the right ventricular outflow tract (RVOT) a preferred location for right ventricular lead fixation The left ventricular outflow tract (LVOT) cannot be seen in this illustration as it is posterior

Chapter 1 Cardiovascular anatomy and physiology 7

smaller than the left ventricle it is larger than the right atrium The blood will travel from the left atrium into the left ventricle by way of the mitral valve The mitral valve gets its name from the miter a bishoprsquos hat The mitral valve has two cusps and papillary muscles connected by chordae tendineae Once blood is in the left ventricle a contraction will force it toward the aorta by way of the LVOT In persons with hypertrophic obstructive cardiomy-opathy (HOCM) the septum or wall in the heart can become thick and enlarged effectively narrow-ing the LVOT In the healthy heart the blood flows up the LVOT across the aortic valve and into the aorta From the aorta blood will be directed upward toward the head downward toward the legs and feet and some blood will be redirected back to the coronary arteries of the heart

The blood flow from the left ventricle to the aorta and back to the coronary arteries is unique in the body in that most of that flow occurs during diastole rather than systole Think of it this way as blood is pumped by the left ventricle during systole upward and downward to the brain and the rest of the body some of that blood flows back during diastole and drains into the coronary arteries Although it may seem counterintuitive it is impor-tant to remember that coronary artery perfusion occurs primarily during diastole

Volume valves and pressure

While it is tempting to think of the heart as simply a pump with an electrical system it is more accu-rate to say that the heart moves blood because of variations in volume and pressure

The heart has four valves two are atrioventricular (AV) valves because they connect the atrium to the ventricle and two are semilunar valves connecting the heart with pulmonary arteries or the aorta The right AV valve is the tricuspid while the left AV valve is the mitral valve The flow or movement of blood is guided and controlled by the heartrsquos muscular action in the form of systole (contraction) or diastole (relaxation)

When blood pours into the right atrium it dis-tends and expands the right atrium When this volume of blood creates sufficient pressure it forces the tricuspid valve open Think of it as volume creating pressure and pressure opening

valves When the tricuspid valve opens the blood dumps into the right ventricle During this period of diastole blood pours into the right ventricle creating volume and expanding it As blood pours in and ventricular diastole nears the end the atria contract while the AV valves are still open in something nicknamed atrial kick Atrial systole occurs at the end of ventricular diastole Atrial kick forces the maximum amount of blood from the atria into the ventricles It is estimated that atrial kick delivers 20ndash30 of cardiac output

The atrial contribution to ventricular filling (atrial kick) is of enormous clinical significance Patients with certain atrial arrhythmias such as atrial fibrillation (AF) lose atrial kick Even milder forms of atrial tachyarrhythmias may compromise atrial kick Moreover patients who do not have AV synchrony (one atrial beat corresponding to one ventricular beat) will lose atrial kick The loss of atrial kick can reduce cardiac output by 20ndash30

Once the atria have contracted and the ventricles are filled with blood the AV valves snap closed The ventricles now start to contract and push open the semilunar valves For example as the right ven-tricle contracts (systole) it opens the PV (The PV is sometimes called the pulmonic valve itrsquos the same thing) The whole process works on volume pressure and contractionrelaxation

Of the four valves in the heart the tricuspid valve is of most interest to device specialists because at least one lead will be placed into the right ventricle This means that pacing and defi-brillation leads typically cross the tricuspid valve These leads may interfere with the proper closing of the tricuspid valve particularly if the lead is large diameter or several leads are passed over the tricuspid valve A single lead through the tricuspid valve is likely to cause minimal to no dysfunction However it is not unusual to see patients with three or more leads in the right ventricle and these leads may hamper the tricuspid valve in its closing leading to tricuspid regurgitation or backflow of blood Patients with multiple leads (such as chil-dren who grow up with pacemakers) may need to have some of them removed to avoid tricuspid regurgitation

Right ventricular leads are typically fixated at the right ventricular apex or in the RVOT and may be

8 The nuts and bolts of implantable device therapy pacemakers

lodged in the trabeculae Trabeculae are a fibrous network that could be described as similar to the pores of a sponge (see Figure 17)

On the left side of the heart the blood flow pattern is similar Blood flows into the left atrium over the open mitral valve and into the left atrium At the end of ventricular diastole as the ventricle is distended and stretched atrial kick delivers more blood into left ventricle The volume of blood cre-ates pressure The mitral valve closes As the left ventricle contracts the growing pressure over-comes the resistance on the aortic valve and it opens allowing blood to flow out into the aorta and beyond

While any valve can become diseased or dys-functional valvular disease more commonly affects the mitral and the aortic valves (left-sided valves) than the right-sided valves (tricuspid and pul-monary) These left valves are more likely to suffer damage or disease because of the high-pressure environment in which they function exposing them to more potential damage Other damage can occur when plaque builds up in the coronary arteries and the aorta The aorta can become

diseased typically in the case of an aortic aneurysm or dissection Carotid arteries in the neck are also frequently a site for atherosclerotic disease

the right atrial appendage

The right atrial appendage (RAA) is an area near the right atrium which is a preferred site for attach-ing active-fixation atrial leads (see Figure 18) The RAA serves no obvious purpose and may be sacri-ficed in certain heart surgeries such as a coronary artery bypass graft (CABG) procedure CABG or bypass surgery involves stopping the heart and running the blood through a machine to reoxygen-ate it during the course of the procedure This is done by attaching a large-diameter hose to the RAA when surgery concludes and this hose is removed it can damage the RAA to the point that it is completely or partially surgically removed In patients who have had a CABG procedure the RAA may not be available for right atrial lead placement For such patients the right atrial lead is often placed against the lateral wall of the right atrium It is sometimes possible to fixate a right

Figure 17 Trabeculae form a fibrous network within the ventricles Passive-fixation ventricular leads are held in place by lodging their tines or fins in the trabeculae

Chapter 1 Cardiovascular anatomy and physiology 9

atrial lead in the remnants of the RAA if a portion of it is preserved

arteries and veins

Any discussion about the heart necessarily involves the vasculature which is why we commonly refer to our circulatory system as the cardiovascular

system As most of us remember from anatomy classes arteries carry blood away from the heart while veins transport blood back to the heart (The exceptions are the PV and arteries described earlier) Arteries carry oxygen-rich blood while veins carry deoxygenated blood What is of importance to device clinicians is that arteries are high-pressure vessels while veins are low-pressure vessels Arteries tend to be more muscular in structure and muscular contractions of the arteries help to move blood outward into the body Veins tend to be less muscular than arteries but unlike arteries they contain a system of tiny interior valves The purpose of these little valves is to maintain a unidirectional flow of blood (see Figure 19)

Some of the most important veins in the body are the SVC and its counterpart the IVC The SVC takes blood from the upper part of the body and delivers it into the right atrium with the IVC doing the same for blood from the lower part of the body The coronary veins networked around the outside of the heart carry blood from the heart tissue and deliver it back to the right atrium they are closely linked to the coronary arteries which carry reoxygenated blood from the left ventricle back to the heart muscle The CS is the site where

Right atrial appendage

Figure 18 The right atrial appendage serves no obvious hemodynamic purpose but is often the site of fixation of atrial leads

Artery Vein

Figure 19 Veins and arteries have different purposes different anatomical structure and different pressures Overall veins are low-pressure systems that move blood by a series of little valves that keep blood flowing in the same direction Arteries are high-pressure systems and are more muscular so that they force blood forward by squeezing

10 The nuts and bolts of implantable device therapy pacemakers

the great middle and small cardiac veins all drain the CS takes this blood and delivers it back to the right atrium

The PV carry reoxygenated blood from the lungs and deliver it to the left atrium so that it can be pumped back out to the body by the left ventricle

The most important artery in the body is by far the aorta The aorta is connected to the left ven-tricle and it is the first and main conduit that takes reoxygenated blood from the left ventricle and sends it out to the body As this reoxygenated blood makes its way out into the body its first two stops are the head (brain) and the heart muscle Inadequate oxygenated blood to the brain can very quickly provoke symptoms This is the reason that depressed cardiac output is associated with symptoms like dizziness light-headedness feeling woozy and fatigue Severe symptoms might include syncope

The coronary arteries arise from the root of the aorta just above the cusps of the aortic valve Coronary arteries receive most blood during dias-tole so blood is more or less pouring back into these vessels The coronary artery system is a networkmdashalmost a meshmdashof middle-sized to very small vessels that cover the exterior surface of the heart (see Figure 110)

Coronary arteries get their name from the Latin word for crown (such as in our word coronation) because this network of vessels sits like a crown atop the heart and encircles it The heart gets blood from outside in as oxygen-rich blood in these coro-nary arteries is delivered to the exterior of the heart muscle Coronary arteries are not within the heart they are epicardial structures The two main coro-nary arteries are the right coronary artery (RCA) and the left coronary artery (LCA) The RCA and LCA come off the aorta and are located in the AV groove an exterior structure below the atria and above the ventricles The RCA runs from the aorta in the right AV groove while the LCA comes off the aorta and quickly branches to the left main The left main is a very short artery (about the size of a thumbnail) which branches quickly into the left anterior descending artery (LAD) and the circum-flex artery The LAD travels down the front of the

left side of the heart The circumflex artery can be found in the left AV groove

The RCA bifurcates to the posterior descending artery The right atrium and right ventricle get their supply of oxygen-rich blood from the RCA Much of the cardiac conduction system is right sided that is it commences with the sinoatrial (SA) node in the high right atrium and travels down to the AV node on the right side of the septum These important electrical structures get their oxygen-rich blood supply from the RCA The electrical system travels down the ventricles to the inferior wall of the left ventricle (located at the bottom of the heart but belonging to the left ventricle) the inferior wall is also fed oxygen-rich blood by the RCA An occlusion of the RCA such as might occur in a myocardial infarction (MI) may result in death of the heart muscle it feeds which includes the right atrium the right ventricle and the inferior wall of the left ventricle and it might affect the conduction system including the SA node and the AV node An inferior wall MI is typically the result of a clogged RCA

RCA

LCA

Aorta

Figure 110 The coronary arteries of the heart branch out to surround the entire heart muscle The two most prominent coronary arteries are the right coronary artery (RCA) and the left coronary artery (LCA)

Chapter 1 Cardiovascular anatomy and physiology 11

For an MI the more distal the occlusion (that is further from the aorta) the less potential damage the heart attack will have In other words the worst occlusions are proximal Whether or not a patient survives an MI comes down to the numbers The loss of over 40 of the left ventricular muscle mass results in death However that 40 is cumulative For example a person can survive an MI that costs him 30 of left ventricular muscle mass but if that heart attack is followed by another that kills 15 of the left ventricular muscle mass the second (milder) heart attack will prove fatal This is the reason many people survive a first heart attack but die of a second or third attack even though their fatal MI may be relatively mild

The LCA branches quickly to the left main which branches almost at once to the LAD The LAD runs on the outside of the heart along the front of the heart along the septum The left main supplies the left atrium and the majority of the left ventricle in other words the major pumping por-tion of the heart An occlusion in the left main can be particularly disastrous because it is so far upstream and affects the heartrsquos left ventricular muscle mass No wonder they call the left main the widow maker

The coronary arteries do not run in straight lines they twist and turn and sometimes make very sharp bends People with coronary artery disease (CAD) typically have plaque buildup in the areas where the coronary arteries bend at sharp angles These are areas where the blood flow creates a lot of turbulence and plaque and other substances can collect and build up It is typically to find blockage and occlusions at these places

The LAD supplies the anterior wall of the left ventricle and the septum with oxygen-rich blood Since part of the heartrsquos conduction system runs through the septum (the right and left bundle branches) an LAD occlusion can damage the heartrsquos ability to conduct electricity properly

The circumflex artery runs along the AV groove laterally and thus supplies oxygen-rich blood to the lateral wall of the left ventricle along with the left atrium The circumflex runs along the AV groove to the left heading toward the back of the heart where it meets with the RCA which

travels along the AV groove to the left toward the back of the heart Both the circumflex and the RCA are responsible for providing oxygenated blood to the posterior wall of the ventricle But which is the more important provider That depends on crux and whether the heart is right or left dominant

The crux refers to an imaginary line drawn exactly down the middle of the ventricular poste-rior wall without any regard to anatomical landmarks If the RCA crosses the crux then the heart is said to be right dominant and the RCA is the greater supplier of oxygenated blood to the ven-tricular posterior wall On the other hand if the circumflex crosses the crux then the heart is said to be left dominant and the circumflex is the more significant provider Roughly 60 of the population is right dominant

While this information about the vasculature is important it is even more important for clini-cians to recognize that there is tremendous interpatient variability in terms of venous anatomy While we can describe major structures and typical venous formations anomalies are very common These anomalies may be minor variations or they can be very pronounced such as missing veins and arteries Clinicians who deal with the cardiovascular system must be prepared for large and small differences in vascular anatomy The cardiovascular system is incredibly resilient and can adapt to some anomalies by building up collateral circulation For example a person with a missing or incompetent vessel may over time build up a network of smaller collateral vessels that compensates by doing the same job It is not unusual to see vessels grow larger or branch out to compensate Sometimes when an artery is blocked collateral circulation will build up and compensate by delivering oxygen-rich blood to the heart muscle

Every artery in the body has a matching or corresponding vein usually located in close proximity This applies to the coronary system as well Note that there are great differences in coro-nary venous anatomy among patients which becomes important when implanting left ventric-ular leads for CRT devices (see Figure 111)

12 The nuts and bolts of implantable device therapy pacemakers

the nuts and bolts of cardiovascular anatomy and physiology

bull The heart consists of four chambers which may be considered the upper chambers (right atrium and left atrium) and lower chambers (right ven-tricle and left ventricle) or may be thought of as the right heart (right atrium and right ventricle) and left heart (left atrium and left ventricle)

bull Deoxygenated blood enters the right atrium flows to the right ventricle and is pumped out via the pulmonary valve to the lungs The blood is reoxygenated in the lungs and then enters the left atrium and flows to the left atrium and out of the aortic valve into the aorta and the rest of the body

bull The heart has two outflow tracts a right ven-tricular outflow tract (RVOT) and a left ven-tricular outflow tract (LVOT)

bull The largest chamber of the heart is the left ven-tricle making up about 23 of the cardiac mass The left ventricle is muscular and massive because it must pump against approximately 10 times the pressure against which the right ven-tricle pumps (120 vs 12 mmHg)

bull The two cardiac valves most susceptible to dysfunction and disease are the mitral and aortic valve because they function in a high-pressure environment

bull The chordae tendineae connect the leaflets of the valves to the papillary muscles and look like little parachutes These help the valves open and close properly

bull In general arteries transport blood away from the heart are muscular and contract to help move the blood on its way and have thicker walls than veins Veins have thinner less muscular walls and carry oxygen-depleted blood back to the heart using tiny interior valves to keep the blood flow moving forward Arteries transport oxygenated blood and can be considered high-pressure systems while veins transported deoxygenated blood in a low-pressure system

bull Cardiac contraction is systole cardiac relaxation is diastole Atrial systole occurs at the very end of ventricular diastole in something nicknamed atrial kick In a healthy individual about 20ndash30 of cardiac output comes from atrial kick

bull The heart is a muscle that is fed with oxygenated blood through a network of coronary arteries Coronary arteries get oxygen-rich blood from the aorta and they perfuse the heart muscle during diastole as blood drains back

bull Although arteries and veins generally take blood away from or to the heart respectively

Rightventricle

Leftventricle

Leftatrium

Aorta

Rightatrium

Rightcoronaryartery

Leftcoronaryartery

Circumflexartery

Superiorvena cava

Inferiorvena cava

Arterial system

Rightventricle

Leftventricle

Greatcardiac vein

Smallcardiac vein

Middlecardiac vein

Leftatrium

Aorta

Rightatrium

Superiorvena cava

Inferiorvena cava

Venous system

Coronarysinus

Figure 111 The coronary artery system is matched by a coronary venous system

Chapter 1 Cardiovascular anatomy and physiology 13

test your knowledge

Fill in these anatomical landmarks of the heart They include the four chambers the four valves and the bodyrsquos main artery

the pulmonary vessels are the exception The pulmonary artery transports deoxygenated blood while the pulmonary veins carry oxygen-ated blood This is because of their special loca-tion and the fact that their work is taking blood from the right side to the left side of the heart

bull The main coronary arteries are the right coro-nary artery and the left coronary artery The left coronary artery branches quickly to a short vessel known as the left main (the widow maker) and the circumflex artery

bull A myocardial infarction can kill the heart muscle The clogged arteries determine the loc ation of the affected muscle A right coro-nary artery occlusion will affect the right atrium and ventricle and the inferior wall of the left ventricle while a left coronary artery occlusion will affect the left atrium and most of the left ventricle By far the most dangerous occlusion is in the left main because it feeds the majority of the heart muscle with oxygen-rich blood

answer the following questions

1 What are the names of the two atrioventricular valves of the heartA Atrial and ventricularB Tricuspid and mitralC Pulmonary and pulmonicD Aortic and superior vena cava

2 Which chambers of the heart contain trabeculaeA AtriaB VentriclesC Right-sided chambers onlyD Left-sided chambers only

14 The nuts and bolts of implantable device therapy pacemakers

3 Why can trabeculae be important to device cliniciansA They are good places to secure a passive-fixa-

tion leadB They help regulate blood pressureC They prevent atherosclerosisD All of the above

4 Which of the following is not true about the aortaA It is an arteryB It consists of an ascending portion a descend-

ing portion and an archC It carries only oxygenated bloodD It is the largest vein in the body

5 From which artery does blood enter the right atriumA Superior vena cavaB Inferior vena cavaC Coronary sinusD All of the above

6 What is atrial kickA Ventricular pressure on the tricuspid valve

forcing it shutB Electrical impulses originating in the atrium

to cause the heart to beat fasterC Atrial systole that forces the most blood possible

into the already-full and diastolic ventriclesD A type of arrhythmia

7 Why would a person with chronic AF likely have decreased cardiac outputA AF is associated with low blood volumeB AF causes the heart to beat very slowlyC AF may cause the heart to experience fre-

quent pausesD AF reduces or eliminates atrial kickmdashand

atrial kick contributes to cardiac output

8 Which of the following is often a preferred location for affixing a cardiac pacing leadA The right ventricular outflow tract (RVOT)B The left ventricular outflow tract (LVOT)C The excised right atrial appendageD The tricuspid valve

9 What connects the valve leaflets to the papillary musclesA TrabeculaeB ArteriesC Chordae tendineaeD Tiny valves

10 Which coronary artery is nicknamed the widow maker because an occlusion in this vessel will deprive the majority of the heartrsquos muscle mass of oxygen-rich bloodA Left mainB CircumflexC Right coronary arteryD Inferior vena cava

15

The Nuts and Bolts of Implantable Device Therapy Pacemakers First Edition Tom Kenny copy 2015 John Wiley amp Sons Ltd Published 2015 by John Wiley amp Sons Ltd

Chapter 2

Cardiac conduction system

Introduction

The sinoatrial (SA) node is the heartrsquos natural pacemaker In the healthy heart the SA node fires and the electrical stimulus conducts along special pathways through the heart that cause cells in specific areas to depolarize After the stimulus passes the area the cells repolarize Myocardial cell depolarization and repolarization govern the heartrsquos contraction and relaxation sequencemdashin other words the heart rhythm Rhythm disorders occur when this healthy conduction system is in some way disrupted or impaired

the conduction pathways

When the SA node fires and sends out electrical energy the stimulus travels from the SA node in the high right atrium out over the atria via path-ways known as internodal tracts Most of these internodal tracts are located in the right atrium the

main tract over to the left atrium is called Bachmannrsquos bundle It is because of Bachmannrsquos bundle linking the right atrium to the left atrium that the atria can contract simultaneously that is as these two upper chambers contract as one coherent unit After traversing the atria the impulse comes down through conduction tracts to the atrioventricular (AV) node

The AV node is an area of highly specialized tissue located below the atria and above the ventri-cles on the right side of the septum Anatomically the AV node rests in the AV groove The tissue in the healthy AV node has the ability to change the speed of the electrical impulse specifically it slows the stimulus down The slowdown serves two important purposes First slowing the impulse allows the atria time to contract and provide the very valuable atrial kick to ventricular filling Second the slowed impulse protects the ventricles from being forced to beat too quickly that is to beat before they have been adequately filled with blood

Once the impulse crosses the AV node it travels down into the ventricles and accelerates its speed The impulse travels through the bundle of His (pronounced hiss) to the bundle branches (right and left) and then out to the network of Purkinje fibers This allows both ventricles to depolarize and contract simultaneously (Figure 21)

polarization depolarization and repolarization

Cardiac conduction is driven by the ability of myo-cardial cells to be depolarized and repolarized Depolarization occurs at the cellular level and is

2

Learning objectives

bullensp Describe the electrical conduction system of the healthy heart

bullensp List the main anatomical landmarks of normal cardiac conduction

bullensp Name the anatomical location of the heartrsquos natural pacemaker

bullensp Explain depolarization and repolarization in electrical terms and how it relates to the cardiac contraction

bullensp Define the term automaticity and how it relates to cardiac conduction

16 The nuts and bolts of implantable device therapy pacemakers

in turn driven by electrolytes or positively and negatively charged ions (potassium sodium and calcium mainly) which are able to move in and out of the cardiac cells A myocardial cell may be said to be polarized when in its resting state that is there is no electrical activity going on In the rest-ing state the interior of the myocardial cell is somewhat more negatively charged that the outside This state is called the resting state or sometimes phase 0 of the action potential

An electrical stimulus changes the resting state abruptly The electrical energy allows for the movement of ions to the point that the interior of the myocardial cell is now more positive than the outside In other words the charge reverses and the cell is said to be depolarized When this happens the cell contracts when this happens to a large area of the myocardium the heart muscle contracts Cardiac contraction occurs as a result of cellular depolarization

When the electrical stimulus moves past the myocardial cell the ions move in reverse and the interior of the cell assumes its previous negative charge with respect to the outside This process is

called repolarization and it brings the myocar-dial cell back to the resting state When this happens the cell relaxes When a large area of the myocardium repolarizes the heart resumes its previous shape (loses its contraction)

Intrinsic pacemakers

The healthy heartrsquos natural pacemaker is the SA node but the SA node is not the only area of the heart that can generate an electrical pulse The SA node serves as the heartrsquos pacemaker because it generates these pulses faster than other areas In the healthy heart the SA node can fire at around 60ndash100 times a minute corresponding to a healthy heart rate The AV junction can also fire but it is much slower at around 40ndash60 times a minute Even the Purkinje fibers (the ventri-cles) can act as a pacemaker but they fire at only 20ndash40 times a minute Thus in a healthy individual the SA node outpaces these other potential pacemakers The AV junction and ven-tricles are sometimes called backup pacemakers because they take over the heart rate in the event

Sinoatrial (SA) node

Internodal tracts

Right bundle branch

Left bundle branch

Purkinje fibers

Bachmannrsquos bundle

Bundle of His

Atrioventricular (AV) node

Figure 21 The conduction pathways in the healthy heart

Chapter 2 Cardiac conduction system 17

that the SA node is not able to pace the heart While these backup pacemakers may help keep the heart beating in the event of a dysfunctional SA node they are slow and not as reliable as the SA node

The healthy heart is able to pace itself because of a property called automaticity Automaticity is an inherent property of individual cardiac conductive cells that allows them to depolarize spontaneously In other words some cells in the heart (such as the cells of the SA node) are able to generate an electrical stimulus on their own

refractoriness

Refractoriness refers to the temporary inability of a cardiac cell to respond to an electrical stimulus Refractoriness can be the normal appropriate and expected behavior of cardiac cells For example immediately after electrical energy has passed through cardiac cells they go through a phase where they are unresponsive to electrical energy no matter how much voltage is delivered This normal refractoriness is called the physiological refractory period and it is very brief (it is mea-sured in thousandths of a second) Of course the exact duration of a refractory period is based in part on the heart rate the faster the heart rate the shorter the refractory period and vice versa

Refractoriness may also occur when certain cells become dysfunctional that is cardiac cells may lose their ability to conduct properly Furthermore the heartrsquos inherent pacemakers may change their rate for instance the AV junction may speed up and start to deliver impulses at 80 or 100 times a minute When this happensmdashthat is when another pacemaker takes over the role of the SA nodemdashthat pacemaker is called a usurping pacemaker because it usurps the rightful role of the SA node In the event that other pacemakers of the heart start to compete with the SA node the fastest pacemaker always wins

Conductivity

Conductivity is the cardiac cellrsquos ability to transmit an electrical impulse to an adjacent cell Typically cardiac conduction is antegrade that

is it moves forward Electrical impulses actually travel out in all different directions but refracto-riness prevents those cells that were just stimu-lated from depolarization This keeps conduction moving in a single direction through the heart Normal antegrade conduction in a healthy heart goes from SA node to atria to AV node over the bundle of His and down to the ventricles Some individuals have the ability to conduct retro-grade retrograde conduction is abnormal but not uncommon

autonomic nervous system

The heart does not need direct stimulation from the nervous system to generate electricity and to beat but that is not to say that the nervous system does not control the heart rate The autonomic nervous system indirectly affects cardiac conduction by regulating how fast the heart will beat and how hard the heart muscle will squeeze (contractility) The autonomic ner-vous system communicates with the heart through a network of baroreceptors which respond to chemical signals

The autonomic nervous system of the body like certain other physiological systems exists as two systems in balance The autonomic nervous system consists of a sympathetic system and a parasympathetic system They both work together and in a healthy individual one may overdrive the other during specific times The sympathetic ner-vous system is mainly associated with epinephrine (adrenaline) and it has the ability to speed the heart rate and increase cardiac contractility and can increase the speed by which electrical impulses can cross the AV node The sympathetic nervous system takes over when we are stressed frightened (fight-or-flight response) or exerting ourselves

On the other hand the autonomic nervous system also has a parasympathetic system that is associated with acetylcholine This system can decrease the heart rate decrease cardiac contrac-tility and cause impulses crossing the AV node to be slowed The parasympathetic system might take over when we are sleeping or at rest

18 The nuts and bolts of implantable device therapy pacemakers

the nuts and bolts of the cardiac conduction system

bull The heartrsquos natural pacemaker is the sinoatrial (SA) node which generates an electrical impulse spontaneously about 60ndash100 times a minute in a healthy individual

bull The normal conduction path is from the SA node over the atria to the atrioventricular (AV) node and then via the bundle of His down the right and left bundle branches and into the Purkinje fibers

bull The AV node has the ability to slow down the electrical impulse This serves two main purposes it allows for atrial kick and it protects the ventricles from beating too rapidly

bull In the healthy heart the right and left atria contract simultaneously and then the right and left ventricles contract simultaneously

bull Depolarization and repolarization of myocar-dial cells are driven by positively and negatively charged ions which cross the cell membrane A resting cardiac cell is more negative on the inside than the surrounding media but when it depolarizes it becomes more positive on the inside relative to the surrounding media Following depolarization the ions move and the cell returns to being more negative on the inside than the outside in a process known as repolarization

bull Depolarization causes the cardiac cell to contract

bull The main ions involved in depolarization and repolarization are sodium potassium and calcium ions

bull Although the heartrsquos natural pacemaker is the SA node the AV junction and the Purkinje fibers (ventricles) can also assume (usurp) this role if the SA node cannot generate electricity properly The AV junction and Purkinje fibers are not as reliable as a healthy AV node and typically generate electricity more slowly (the AV junction about 40ndash60 times a minute and the Purkinje fibers 20ndash40 times a minute)

bull However sometimes the AV junction or Purkinje fibers may generate electricity very

quickly this is abnormal When they do they may take over the heart When pacemakers of the heart compete the fastest one drives the heart

bull Automaticity is an inherent property of certain cardiac conductive cells to depolarize spontaneously Not all cardiac conduction cells possess automaticity but the SA node AV junction and Purkinje fibers possess automaticity

bull Refractoriness is the natural and expected behavior of cardiac conductive cells that makes them unresponsive to electrical impulses immediately after they have depolarized The natural refractory period is very brief

bull The faster the heart rate the shorter the refractory period the slower the heart rate the longer the refractory period

bull Refractoriness can also occur as a dysfunction bull Conductivity is the cardiac cellrsquos ability to

transmit an electrical impulse to adjacent cells Refractoriness means that cells that just depolarized will not depolarize again This keeps conduction moving forward (antegrade conduction)

bull Antegrade conduction is normal retrograde conduction is abnormal but is not rare

bull The autonomic nervous system regulates the heart rate by controlling how fast the heart beats and how vigorously it contracts

bull The autonomic nervous system consists of a sympathetic and parasympathetic nervous system which work together but at certain times one or the other is dominant The sympathetic nervous system makes the heart beat faster and more vigorously it takes over during times of stress exertion or fight or flight The parasympathetic ner-vous system makes the heart beat slowly and less vigorously it takes over during periods of rest

bull The neurotransmitters associated with these two systems are epinephrine or adrenaline (sympathetic nervous system) and acetylcho-line (parasympathetic nervous system)

Chapter 2 Cardiac conduction system 19

test your knowledge

Please write in the landmarks of the cardiac conduction system Select from the following (each is used only once)

AV node Bachmanrsquos bundle Bundle of His

Internodal tracts Left bundle branch Purkinje fibers

Right bundle branch Ventricles

1 Which of the following is not considered one of the heartrsquos intrinsic pacemakersA AV junctionB SA nodeC Purkinje fibersD Aorta

2 Depolarization and repolarization involve the movement of charged ions across cardiac cell membranes What types of ions are primarily involvedA Calcium potassium and sodiumB Acetylcholine and epinephrineC Magnesium and folateD All of the above

3 Refractoriness refers toA The ability of cardiac conduction cells to

contract

B The inability of cardiac conduction cells to respond to an electrical stimulus

C The unique permeability of the cardiac cellsD Abnormal cardiac conduction

4 When a cardiac cell depolarizes the interior of the cell becomes what with respect to the surrounding mediaA More positiveB More negativeC More acidicD Hotter

5 Fill in the blank In the healthy heart the faster the heart rate the ___ the refractory periodA More vigorousB Less reliableC LongerD Shorter

20 The nuts and bolts of implantable device therapy pacemakers

6 Which specific part of the nervous system governs the fight-or-flight response and is associated with epinephrine (adrenaline) (Pick the most exact answer)A Autonomic nervous systemB Parasympathetic nervous systemC Sympathetic nervous systemD Central nervous system

7 In normal antegrade conduction the electrical impulse travels from the SA node over the atria to the AV node and to what structure before reaching the ventriclesA The intermodal tractsB The Purkinje fibersC The bundle of HisD The ventricular apex

8 Which of the following is true about retrograde cardiac conductionA Everyone has itB It is perfectly normal and can be helpful in

stressful situations

C It is abnormal but not uncommonD It is impossible to diagnose definitively

except in an autopsy

9 What chemical is associated with the parasympathetic nervous systemA AcetylcholineB PotassiumC EpinephrineD Dopamine

10 The stimulation of the myocardial cells of which of the following structures is most closely associated with ventricular depolarizationA SA nodeB AV junctionC Bundle of HisD Purkinje fibers

6 The nuts and bolts of implantable device therapy pacemakers

Right atrium

Left atrium

Left ventricle

Right ventricle

Figure 15 The leaflets of the valves are attached by chordae tendineae at one end and papillary muscles on the other which assure the effective opening and closing of the valves

Right-ventricularoutflow tract (RVOT)

Figure 16 There are two outflow tracts in the heart one associated with the right ventricle and the other the left ventricle These outflow tracts are roughly the same size The illustration shows the right ventricular outflow tract (RVOT) a preferred location for right ventricular lead fixation The left ventricular outflow tract (LVOT) cannot be seen in this illustration as it is posterior

Chapter 1 Cardiovascular anatomy and physiology 7

smaller than the left ventricle it is larger than the right atrium The blood will travel from the left atrium into the left ventricle by way of the mitral valve The mitral valve gets its name from the miter a bishoprsquos hat The mitral valve has two cusps and papillary muscles connected by chordae tendineae Once blood is in the left ventricle a contraction will force it toward the aorta by way of the LVOT In persons with hypertrophic obstructive cardiomy-opathy (HOCM) the septum or wall in the heart can become thick and enlarged effectively narrow-ing the LVOT In the healthy heart the blood flows up the LVOT across the aortic valve and into the aorta From the aorta blood will be directed upward toward the head downward toward the legs and feet and some blood will be redirected back to the coronary arteries of the heart

The blood flow from the left ventricle to the aorta and back to the coronary arteries is unique in the body in that most of that flow occurs during diastole rather than systole Think of it this way as blood is pumped by the left ventricle during systole upward and downward to the brain and the rest of the body some of that blood flows back during diastole and drains into the coronary arteries Although it may seem counterintuitive it is impor-tant to remember that coronary artery perfusion occurs primarily during diastole

Volume valves and pressure

While it is tempting to think of the heart as simply a pump with an electrical system it is more accu-rate to say that the heart moves blood because of variations in volume and pressure

The heart has four valves two are atrioventricular (AV) valves because they connect the atrium to the ventricle and two are semilunar valves connecting the heart with pulmonary arteries or the aorta The right AV valve is the tricuspid while the left AV valve is the mitral valve The flow or movement of blood is guided and controlled by the heartrsquos muscular action in the form of systole (contraction) or diastole (relaxation)

When blood pours into the right atrium it dis-tends and expands the right atrium When this volume of blood creates sufficient pressure it forces the tricuspid valve open Think of it as volume creating pressure and pressure opening

valves When the tricuspid valve opens the blood dumps into the right ventricle During this period of diastole blood pours into the right ventricle creating volume and expanding it As blood pours in and ventricular diastole nears the end the atria contract while the AV valves are still open in something nicknamed atrial kick Atrial systole occurs at the end of ventricular diastole Atrial kick forces the maximum amount of blood from the atria into the ventricles It is estimated that atrial kick delivers 20ndash30 of cardiac output

The atrial contribution to ventricular filling (atrial kick) is of enormous clinical significance Patients with certain atrial arrhythmias such as atrial fibrillation (AF) lose atrial kick Even milder forms of atrial tachyarrhythmias may compromise atrial kick Moreover patients who do not have AV synchrony (one atrial beat corresponding to one ventricular beat) will lose atrial kick The loss of atrial kick can reduce cardiac output by 20ndash30

Once the atria have contracted and the ventricles are filled with blood the AV valves snap closed The ventricles now start to contract and push open the semilunar valves For example as the right ven-tricle contracts (systole) it opens the PV (The PV is sometimes called the pulmonic valve itrsquos the same thing) The whole process works on volume pressure and contractionrelaxation

Of the four valves in the heart the tricuspid valve is of most interest to device specialists because at least one lead will be placed into the right ventricle This means that pacing and defi-brillation leads typically cross the tricuspid valve These leads may interfere with the proper closing of the tricuspid valve particularly if the lead is large diameter or several leads are passed over the tricuspid valve A single lead through the tricuspid valve is likely to cause minimal to no dysfunction However it is not unusual to see patients with three or more leads in the right ventricle and these leads may hamper the tricuspid valve in its closing leading to tricuspid regurgitation or backflow of blood Patients with multiple leads (such as chil-dren who grow up with pacemakers) may need to have some of them removed to avoid tricuspid regurgitation

Right ventricular leads are typically fixated at the right ventricular apex or in the RVOT and may be

8 The nuts and bolts of implantable device therapy pacemakers

lodged in the trabeculae Trabeculae are a fibrous network that could be described as similar to the pores of a sponge (see Figure 17)

On the left side of the heart the blood flow pattern is similar Blood flows into the left atrium over the open mitral valve and into the left atrium At the end of ventricular diastole as the ventricle is distended and stretched atrial kick delivers more blood into left ventricle The volume of blood cre-ates pressure The mitral valve closes As the left ventricle contracts the growing pressure over-comes the resistance on the aortic valve and it opens allowing blood to flow out into the aorta and beyond

While any valve can become diseased or dys-functional valvular disease more commonly affects the mitral and the aortic valves (left-sided valves) than the right-sided valves (tricuspid and pul-monary) These left valves are more likely to suffer damage or disease because of the high-pressure environment in which they function exposing them to more potential damage Other damage can occur when plaque builds up in the coronary arteries and the aorta The aorta can become

diseased typically in the case of an aortic aneurysm or dissection Carotid arteries in the neck are also frequently a site for atherosclerotic disease

the right atrial appendage

The right atrial appendage (RAA) is an area near the right atrium which is a preferred site for attach-ing active-fixation atrial leads (see Figure 18) The RAA serves no obvious purpose and may be sacri-ficed in certain heart surgeries such as a coronary artery bypass graft (CABG) procedure CABG or bypass surgery involves stopping the heart and running the blood through a machine to reoxygen-ate it during the course of the procedure This is done by attaching a large-diameter hose to the RAA when surgery concludes and this hose is removed it can damage the RAA to the point that it is completely or partially surgically removed In patients who have had a CABG procedure the RAA may not be available for right atrial lead placement For such patients the right atrial lead is often placed against the lateral wall of the right atrium It is sometimes possible to fixate a right

Figure 17 Trabeculae form a fibrous network within the ventricles Passive-fixation ventricular leads are held in place by lodging their tines or fins in the trabeculae

Chapter 1 Cardiovascular anatomy and physiology 9

atrial lead in the remnants of the RAA if a portion of it is preserved

arteries and veins

Any discussion about the heart necessarily involves the vasculature which is why we commonly refer to our circulatory system as the cardiovascular

system As most of us remember from anatomy classes arteries carry blood away from the heart while veins transport blood back to the heart (The exceptions are the PV and arteries described earlier) Arteries carry oxygen-rich blood while veins carry deoxygenated blood What is of importance to device clinicians is that arteries are high-pressure vessels while veins are low-pressure vessels Arteries tend to be more muscular in structure and muscular contractions of the arteries help to move blood outward into the body Veins tend to be less muscular than arteries but unlike arteries they contain a system of tiny interior valves The purpose of these little valves is to maintain a unidirectional flow of blood (see Figure 19)

Some of the most important veins in the body are the SVC and its counterpart the IVC The SVC takes blood from the upper part of the body and delivers it into the right atrium with the IVC doing the same for blood from the lower part of the body The coronary veins networked around the outside of the heart carry blood from the heart tissue and deliver it back to the right atrium they are closely linked to the coronary arteries which carry reoxygenated blood from the left ventricle back to the heart muscle The CS is the site where

Right atrial appendage

Figure 18 The right atrial appendage serves no obvious hemodynamic purpose but is often the site of fixation of atrial leads

Artery Vein

Figure 19 Veins and arteries have different purposes different anatomical structure and different pressures Overall veins are low-pressure systems that move blood by a series of little valves that keep blood flowing in the same direction Arteries are high-pressure systems and are more muscular so that they force blood forward by squeezing

10 The nuts and bolts of implantable device therapy pacemakers

the great middle and small cardiac veins all drain the CS takes this blood and delivers it back to the right atrium

The PV carry reoxygenated blood from the lungs and deliver it to the left atrium so that it can be pumped back out to the body by the left ventricle

The most important artery in the body is by far the aorta The aorta is connected to the left ven-tricle and it is the first and main conduit that takes reoxygenated blood from the left ventricle and sends it out to the body As this reoxygenated blood makes its way out into the body its first two stops are the head (brain) and the heart muscle Inadequate oxygenated blood to the brain can very quickly provoke symptoms This is the reason that depressed cardiac output is associated with symptoms like dizziness light-headedness feeling woozy and fatigue Severe symptoms might include syncope

The coronary arteries arise from the root of the aorta just above the cusps of the aortic valve Coronary arteries receive most blood during dias-tole so blood is more or less pouring back into these vessels The coronary artery system is a networkmdashalmost a meshmdashof middle-sized to very small vessels that cover the exterior surface of the heart (see Figure 110)

Coronary arteries get their name from the Latin word for crown (such as in our word coronation) because this network of vessels sits like a crown atop the heart and encircles it The heart gets blood from outside in as oxygen-rich blood in these coro-nary arteries is delivered to the exterior of the heart muscle Coronary arteries are not within the heart they are epicardial structures The two main coro-nary arteries are the right coronary artery (RCA) and the left coronary artery (LCA) The RCA and LCA come off the aorta and are located in the AV groove an exterior structure below the atria and above the ventricles The RCA runs from the aorta in the right AV groove while the LCA comes off the aorta and quickly branches to the left main The left main is a very short artery (about the size of a thumbnail) which branches quickly into the left anterior descending artery (LAD) and the circum-flex artery The LAD travels down the front of the

left side of the heart The circumflex artery can be found in the left AV groove

The RCA bifurcates to the posterior descending artery The right atrium and right ventricle get their supply of oxygen-rich blood from the RCA Much of the cardiac conduction system is right sided that is it commences with the sinoatrial (SA) node in the high right atrium and travels down to the AV node on the right side of the septum These important electrical structures get their oxygen-rich blood supply from the RCA The electrical system travels down the ventricles to the inferior wall of the left ventricle (located at the bottom of the heart but belonging to the left ventricle) the inferior wall is also fed oxygen-rich blood by the RCA An occlusion of the RCA such as might occur in a myocardial infarction (MI) may result in death of the heart muscle it feeds which includes the right atrium the right ventricle and the inferior wall of the left ventricle and it might affect the conduction system including the SA node and the AV node An inferior wall MI is typically the result of a clogged RCA

RCA

LCA

Aorta

Figure 110 The coronary arteries of the heart branch out to surround the entire heart muscle The two most prominent coronary arteries are the right coronary artery (RCA) and the left coronary artery (LCA)

Chapter 1 Cardiovascular anatomy and physiology 11

For an MI the more distal the occlusion (that is further from the aorta) the less potential damage the heart attack will have In other words the worst occlusions are proximal Whether or not a patient survives an MI comes down to the numbers The loss of over 40 of the left ventricular muscle mass results in death However that 40 is cumulative For example a person can survive an MI that costs him 30 of left ventricular muscle mass but if that heart attack is followed by another that kills 15 of the left ventricular muscle mass the second (milder) heart attack will prove fatal This is the reason many people survive a first heart attack but die of a second or third attack even though their fatal MI may be relatively mild

The LCA branches quickly to the left main which branches almost at once to the LAD The LAD runs on the outside of the heart along the front of the heart along the septum The left main supplies the left atrium and the majority of the left ventricle in other words the major pumping por-tion of the heart An occlusion in the left main can be particularly disastrous because it is so far upstream and affects the heartrsquos left ventricular muscle mass No wonder they call the left main the widow maker

The coronary arteries do not run in straight lines they twist and turn and sometimes make very sharp bends People with coronary artery disease (CAD) typically have plaque buildup in the areas where the coronary arteries bend at sharp angles These are areas where the blood flow creates a lot of turbulence and plaque and other substances can collect and build up It is typically to find blockage and occlusions at these places

The LAD supplies the anterior wall of the left ventricle and the septum with oxygen-rich blood Since part of the heartrsquos conduction system runs through the septum (the right and left bundle branches) an LAD occlusion can damage the heartrsquos ability to conduct electricity properly

The circumflex artery runs along the AV groove laterally and thus supplies oxygen-rich blood to the lateral wall of the left ventricle along with the left atrium The circumflex runs along the AV groove to the left heading toward the back of the heart where it meets with the RCA which

travels along the AV groove to the left toward the back of the heart Both the circumflex and the RCA are responsible for providing oxygenated blood to the posterior wall of the ventricle But which is the more important provider That depends on crux and whether the heart is right or left dominant

The crux refers to an imaginary line drawn exactly down the middle of the ventricular poste-rior wall without any regard to anatomical landmarks If the RCA crosses the crux then the heart is said to be right dominant and the RCA is the greater supplier of oxygenated blood to the ven-tricular posterior wall On the other hand if the circumflex crosses the crux then the heart is said to be left dominant and the circumflex is the more significant provider Roughly 60 of the population is right dominant

While this information about the vasculature is important it is even more important for clini-cians to recognize that there is tremendous interpatient variability in terms of venous anatomy While we can describe major structures and typical venous formations anomalies are very common These anomalies may be minor variations or they can be very pronounced such as missing veins and arteries Clinicians who deal with the cardiovascular system must be prepared for large and small differences in vascular anatomy The cardiovascular system is incredibly resilient and can adapt to some anomalies by building up collateral circulation For example a person with a missing or incompetent vessel may over time build up a network of smaller collateral vessels that compensates by doing the same job It is not unusual to see vessels grow larger or branch out to compensate Sometimes when an artery is blocked collateral circulation will build up and compensate by delivering oxygen-rich blood to the heart muscle

Every artery in the body has a matching or corresponding vein usually located in close proximity This applies to the coronary system as well Note that there are great differences in coro-nary venous anatomy among patients which becomes important when implanting left ventric-ular leads for CRT devices (see Figure 111)

12 The nuts and bolts of implantable device therapy pacemakers

the nuts and bolts of cardiovascular anatomy and physiology

bull The heart consists of four chambers which may be considered the upper chambers (right atrium and left atrium) and lower chambers (right ven-tricle and left ventricle) or may be thought of as the right heart (right atrium and right ventricle) and left heart (left atrium and left ventricle)

bull Deoxygenated blood enters the right atrium flows to the right ventricle and is pumped out via the pulmonary valve to the lungs The blood is reoxygenated in the lungs and then enters the left atrium and flows to the left atrium and out of the aortic valve into the aorta and the rest of the body

bull The heart has two outflow tracts a right ven-tricular outflow tract (RVOT) and a left ven-tricular outflow tract (LVOT)

bull The largest chamber of the heart is the left ven-tricle making up about 23 of the cardiac mass The left ventricle is muscular and massive because it must pump against approximately 10 times the pressure against which the right ven-tricle pumps (120 vs 12 mmHg)

bull The two cardiac valves most susceptible to dysfunction and disease are the mitral and aortic valve because they function in a high-pressure environment

bull The chordae tendineae connect the leaflets of the valves to the papillary muscles and look like little parachutes These help the valves open and close properly

bull In general arteries transport blood away from the heart are muscular and contract to help move the blood on its way and have thicker walls than veins Veins have thinner less muscular walls and carry oxygen-depleted blood back to the heart using tiny interior valves to keep the blood flow moving forward Arteries transport oxygenated blood and can be considered high-pressure systems while veins transported deoxygenated blood in a low-pressure system

bull Cardiac contraction is systole cardiac relaxation is diastole Atrial systole occurs at the very end of ventricular diastole in something nicknamed atrial kick In a healthy individual about 20ndash30 of cardiac output comes from atrial kick

bull The heart is a muscle that is fed with oxygenated blood through a network of coronary arteries Coronary arteries get oxygen-rich blood from the aorta and they perfuse the heart muscle during diastole as blood drains back

bull Although arteries and veins generally take blood away from or to the heart respectively

Rightventricle

Leftventricle

Leftatrium

Aorta

Rightatrium

Rightcoronaryartery

Leftcoronaryartery

Circumflexartery

Superiorvena cava

Inferiorvena cava

Arterial system

Rightventricle

Leftventricle

Greatcardiac vein

Smallcardiac vein

Middlecardiac vein

Leftatrium

Aorta

Rightatrium

Superiorvena cava

Inferiorvena cava

Venous system

Coronarysinus

Figure 111 The coronary artery system is matched by a coronary venous system

Chapter 1 Cardiovascular anatomy and physiology 13

test your knowledge

Fill in these anatomical landmarks of the heart They include the four chambers the four valves and the bodyrsquos main artery

the pulmonary vessels are the exception The pulmonary artery transports deoxygenated blood while the pulmonary veins carry oxygen-ated blood This is because of their special loca-tion and the fact that their work is taking blood from the right side to the left side of the heart

bull The main coronary arteries are the right coro-nary artery and the left coronary artery The left coronary artery branches quickly to a short vessel known as the left main (the widow maker) and the circumflex artery

bull A myocardial infarction can kill the heart muscle The clogged arteries determine the loc ation of the affected muscle A right coro-nary artery occlusion will affect the right atrium and ventricle and the inferior wall of the left ventricle while a left coronary artery occlusion will affect the left atrium and most of the left ventricle By far the most dangerous occlusion is in the left main because it feeds the majority of the heart muscle with oxygen-rich blood

answer the following questions

1 What are the names of the two atrioventricular valves of the heartA Atrial and ventricularB Tricuspid and mitralC Pulmonary and pulmonicD Aortic and superior vena cava

2 Which chambers of the heart contain trabeculaeA AtriaB VentriclesC Right-sided chambers onlyD Left-sided chambers only

14 The nuts and bolts of implantable device therapy pacemakers

3 Why can trabeculae be important to device cliniciansA They are good places to secure a passive-fixa-

tion leadB They help regulate blood pressureC They prevent atherosclerosisD All of the above

4 Which of the following is not true about the aortaA It is an arteryB It consists of an ascending portion a descend-

ing portion and an archC It carries only oxygenated bloodD It is the largest vein in the body

5 From which artery does blood enter the right atriumA Superior vena cavaB Inferior vena cavaC Coronary sinusD All of the above

6 What is atrial kickA Ventricular pressure on the tricuspid valve

forcing it shutB Electrical impulses originating in the atrium

to cause the heart to beat fasterC Atrial systole that forces the most blood possible

into the already-full and diastolic ventriclesD A type of arrhythmia

7 Why would a person with chronic AF likely have decreased cardiac outputA AF is associated with low blood volumeB AF causes the heart to beat very slowlyC AF may cause the heart to experience fre-

quent pausesD AF reduces or eliminates atrial kickmdashand

atrial kick contributes to cardiac output

8 Which of the following is often a preferred location for affixing a cardiac pacing leadA The right ventricular outflow tract (RVOT)B The left ventricular outflow tract (LVOT)C The excised right atrial appendageD The tricuspid valve

9 What connects the valve leaflets to the papillary musclesA TrabeculaeB ArteriesC Chordae tendineaeD Tiny valves

10 Which coronary artery is nicknamed the widow maker because an occlusion in this vessel will deprive the majority of the heartrsquos muscle mass of oxygen-rich bloodA Left mainB CircumflexC Right coronary arteryD Inferior vena cava

15

The Nuts and Bolts of Implantable Device Therapy Pacemakers First Edition Tom Kenny copy 2015 John Wiley amp Sons Ltd Published 2015 by John Wiley amp Sons Ltd

Chapter 2

Cardiac conduction system

Introduction

The sinoatrial (SA) node is the heartrsquos natural pacemaker In the healthy heart the SA node fires and the electrical stimulus conducts along special pathways through the heart that cause cells in specific areas to depolarize After the stimulus passes the area the cells repolarize Myocardial cell depolarization and repolarization govern the heartrsquos contraction and relaxation sequencemdashin other words the heart rhythm Rhythm disorders occur when this healthy conduction system is in some way disrupted or impaired

the conduction pathways

When the SA node fires and sends out electrical energy the stimulus travels from the SA node in the high right atrium out over the atria via path-ways known as internodal tracts Most of these internodal tracts are located in the right atrium the

main tract over to the left atrium is called Bachmannrsquos bundle It is because of Bachmannrsquos bundle linking the right atrium to the left atrium that the atria can contract simultaneously that is as these two upper chambers contract as one coherent unit After traversing the atria the impulse comes down through conduction tracts to the atrioventricular (AV) node

The AV node is an area of highly specialized tissue located below the atria and above the ventri-cles on the right side of the septum Anatomically the AV node rests in the AV groove The tissue in the healthy AV node has the ability to change the speed of the electrical impulse specifically it slows the stimulus down The slowdown serves two important purposes First slowing the impulse allows the atria time to contract and provide the very valuable atrial kick to ventricular filling Second the slowed impulse protects the ventricles from being forced to beat too quickly that is to beat before they have been adequately filled with blood

Once the impulse crosses the AV node it travels down into the ventricles and accelerates its speed The impulse travels through the bundle of His (pronounced hiss) to the bundle branches (right and left) and then out to the network of Purkinje fibers This allows both ventricles to depolarize and contract simultaneously (Figure 21)

polarization depolarization and repolarization

Cardiac conduction is driven by the ability of myo-cardial cells to be depolarized and repolarized Depolarization occurs at the cellular level and is

2

Learning objectives

bullensp Describe the electrical conduction system of the healthy heart

bullensp List the main anatomical landmarks of normal cardiac conduction

bullensp Name the anatomical location of the heartrsquos natural pacemaker

bullensp Explain depolarization and repolarization in electrical terms and how it relates to the cardiac contraction

bullensp Define the term automaticity and how it relates to cardiac conduction

16 The nuts and bolts of implantable device therapy pacemakers

in turn driven by electrolytes or positively and negatively charged ions (potassium sodium and calcium mainly) which are able to move in and out of the cardiac cells A myocardial cell may be said to be polarized when in its resting state that is there is no electrical activity going on In the rest-ing state the interior of the myocardial cell is somewhat more negatively charged that the outside This state is called the resting state or sometimes phase 0 of the action potential

An electrical stimulus changes the resting state abruptly The electrical energy allows for the movement of ions to the point that the interior of the myocardial cell is now more positive than the outside In other words the charge reverses and the cell is said to be depolarized When this happens the cell contracts when this happens to a large area of the myocardium the heart muscle contracts Cardiac contraction occurs as a result of cellular depolarization

When the electrical stimulus moves past the myocardial cell the ions move in reverse and the interior of the cell assumes its previous negative charge with respect to the outside This process is

called repolarization and it brings the myocar-dial cell back to the resting state When this happens the cell relaxes When a large area of the myocardium repolarizes the heart resumes its previous shape (loses its contraction)

Intrinsic pacemakers

The healthy heartrsquos natural pacemaker is the SA node but the SA node is not the only area of the heart that can generate an electrical pulse The SA node serves as the heartrsquos pacemaker because it generates these pulses faster than other areas In the healthy heart the SA node can fire at around 60ndash100 times a minute corresponding to a healthy heart rate The AV junction can also fire but it is much slower at around 40ndash60 times a minute Even the Purkinje fibers (the ventri-cles) can act as a pacemaker but they fire at only 20ndash40 times a minute Thus in a healthy individual the SA node outpaces these other potential pacemakers The AV junction and ven-tricles are sometimes called backup pacemakers because they take over the heart rate in the event

Sinoatrial (SA) node

Internodal tracts

Right bundle branch

Left bundle branch

Purkinje fibers

Bachmannrsquos bundle

Bundle of His

Atrioventricular (AV) node

Figure 21 The conduction pathways in the healthy heart

Chapter 2 Cardiac conduction system 17

that the SA node is not able to pace the heart While these backup pacemakers may help keep the heart beating in the event of a dysfunctional SA node they are slow and not as reliable as the SA node

The healthy heart is able to pace itself because of a property called automaticity Automaticity is an inherent property of individual cardiac conductive cells that allows them to depolarize spontaneously In other words some cells in the heart (such as the cells of the SA node) are able to generate an electrical stimulus on their own

refractoriness

Refractoriness refers to the temporary inability of a cardiac cell to respond to an electrical stimulus Refractoriness can be the normal appropriate and expected behavior of cardiac cells For example immediately after electrical energy has passed through cardiac cells they go through a phase where they are unresponsive to electrical energy no matter how much voltage is delivered This normal refractoriness is called the physiological refractory period and it is very brief (it is mea-sured in thousandths of a second) Of course the exact duration of a refractory period is based in part on the heart rate the faster the heart rate the shorter the refractory period and vice versa

Refractoriness may also occur when certain cells become dysfunctional that is cardiac cells may lose their ability to conduct properly Furthermore the heartrsquos inherent pacemakers may change their rate for instance the AV junction may speed up and start to deliver impulses at 80 or 100 times a minute When this happensmdashthat is when another pacemaker takes over the role of the SA nodemdashthat pacemaker is called a usurping pacemaker because it usurps the rightful role of the SA node In the event that other pacemakers of the heart start to compete with the SA node the fastest pacemaker always wins

Conductivity

Conductivity is the cardiac cellrsquos ability to transmit an electrical impulse to an adjacent cell Typically cardiac conduction is antegrade that

is it moves forward Electrical impulses actually travel out in all different directions but refracto-riness prevents those cells that were just stimu-lated from depolarization This keeps conduction moving in a single direction through the heart Normal antegrade conduction in a healthy heart goes from SA node to atria to AV node over the bundle of His and down to the ventricles Some individuals have the ability to conduct retro-grade retrograde conduction is abnormal but not uncommon

autonomic nervous system

The heart does not need direct stimulation from the nervous system to generate electricity and to beat but that is not to say that the nervous system does not control the heart rate The autonomic nervous system indirectly affects cardiac conduction by regulating how fast the heart will beat and how hard the heart muscle will squeeze (contractility) The autonomic ner-vous system communicates with the heart through a network of baroreceptors which respond to chemical signals

The autonomic nervous system of the body like certain other physiological systems exists as two systems in balance The autonomic nervous system consists of a sympathetic system and a parasympathetic system They both work together and in a healthy individual one may overdrive the other during specific times The sympathetic ner-vous system is mainly associated with epinephrine (adrenaline) and it has the ability to speed the heart rate and increase cardiac contractility and can increase the speed by which electrical impulses can cross the AV node The sympathetic nervous system takes over when we are stressed frightened (fight-or-flight response) or exerting ourselves

On the other hand the autonomic nervous system also has a parasympathetic system that is associated with acetylcholine This system can decrease the heart rate decrease cardiac contrac-tility and cause impulses crossing the AV node to be slowed The parasympathetic system might take over when we are sleeping or at rest

18 The nuts and bolts of implantable device therapy pacemakers

the nuts and bolts of the cardiac conduction system

bull The heartrsquos natural pacemaker is the sinoatrial (SA) node which generates an electrical impulse spontaneously about 60ndash100 times a minute in a healthy individual

bull The normal conduction path is from the SA node over the atria to the atrioventricular (AV) node and then via the bundle of His down the right and left bundle branches and into the Purkinje fibers

bull The AV node has the ability to slow down the electrical impulse This serves two main purposes it allows for atrial kick and it protects the ventricles from beating too rapidly

bull In the healthy heart the right and left atria contract simultaneously and then the right and left ventricles contract simultaneously

bull Depolarization and repolarization of myocar-dial cells are driven by positively and negatively charged ions which cross the cell membrane A resting cardiac cell is more negative on the inside than the surrounding media but when it depolarizes it becomes more positive on the inside relative to the surrounding media Following depolarization the ions move and the cell returns to being more negative on the inside than the outside in a process known as repolarization

bull Depolarization causes the cardiac cell to contract

bull The main ions involved in depolarization and repolarization are sodium potassium and calcium ions

bull Although the heartrsquos natural pacemaker is the SA node the AV junction and the Purkinje fibers (ventricles) can also assume (usurp) this role if the SA node cannot generate electricity properly The AV junction and Purkinje fibers are not as reliable as a healthy AV node and typically generate electricity more slowly (the AV junction about 40ndash60 times a minute and the Purkinje fibers 20ndash40 times a minute)

bull However sometimes the AV junction or Purkinje fibers may generate electricity very

quickly this is abnormal When they do they may take over the heart When pacemakers of the heart compete the fastest one drives the heart

bull Automaticity is an inherent property of certain cardiac conductive cells to depolarize spontaneously Not all cardiac conduction cells possess automaticity but the SA node AV junction and Purkinje fibers possess automaticity

bull Refractoriness is the natural and expected behavior of cardiac conductive cells that makes them unresponsive to electrical impulses immediately after they have depolarized The natural refractory period is very brief

bull The faster the heart rate the shorter the refractory period the slower the heart rate the longer the refractory period

bull Refractoriness can also occur as a dysfunction bull Conductivity is the cardiac cellrsquos ability to

transmit an electrical impulse to adjacent cells Refractoriness means that cells that just depolarized will not depolarize again This keeps conduction moving forward (antegrade conduction)

bull Antegrade conduction is normal retrograde conduction is abnormal but is not rare

bull The autonomic nervous system regulates the heart rate by controlling how fast the heart beats and how vigorously it contracts

bull The autonomic nervous system consists of a sympathetic and parasympathetic nervous system which work together but at certain times one or the other is dominant The sympathetic nervous system makes the heart beat faster and more vigorously it takes over during times of stress exertion or fight or flight The parasympathetic ner-vous system makes the heart beat slowly and less vigorously it takes over during periods of rest

bull The neurotransmitters associated with these two systems are epinephrine or adrenaline (sympathetic nervous system) and acetylcho-line (parasympathetic nervous system)

Chapter 2 Cardiac conduction system 19

test your knowledge

Please write in the landmarks of the cardiac conduction system Select from the following (each is used only once)

AV node Bachmanrsquos bundle Bundle of His

Internodal tracts Left bundle branch Purkinje fibers

Right bundle branch Ventricles

1 Which of the following is not considered one of the heartrsquos intrinsic pacemakersA AV junctionB SA nodeC Purkinje fibersD Aorta

2 Depolarization and repolarization involve the movement of charged ions across cardiac cell membranes What types of ions are primarily involvedA Calcium potassium and sodiumB Acetylcholine and epinephrineC Magnesium and folateD All of the above

3 Refractoriness refers toA The ability of cardiac conduction cells to

contract

B The inability of cardiac conduction cells to respond to an electrical stimulus

C The unique permeability of the cardiac cellsD Abnormal cardiac conduction

4 When a cardiac cell depolarizes the interior of the cell becomes what with respect to the surrounding mediaA More positiveB More negativeC More acidicD Hotter

5 Fill in the blank In the healthy heart the faster the heart rate the ___ the refractory periodA More vigorousB Less reliableC LongerD Shorter

20 The nuts and bolts of implantable device therapy pacemakers

6 Which specific part of the nervous system governs the fight-or-flight response and is associated with epinephrine (adrenaline) (Pick the most exact answer)A Autonomic nervous systemB Parasympathetic nervous systemC Sympathetic nervous systemD Central nervous system

7 In normal antegrade conduction the electrical impulse travels from the SA node over the atria to the AV node and to what structure before reaching the ventriclesA The intermodal tractsB The Purkinje fibersC The bundle of HisD The ventricular apex

8 Which of the following is true about retrograde cardiac conductionA Everyone has itB It is perfectly normal and can be helpful in

stressful situations

C It is abnormal but not uncommonD It is impossible to diagnose definitively

except in an autopsy

9 What chemical is associated with the parasympathetic nervous systemA AcetylcholineB PotassiumC EpinephrineD Dopamine

10 The stimulation of the myocardial cells of which of the following structures is most closely associated with ventricular depolarizationA SA nodeB AV junctionC Bundle of HisD Purkinje fibers

Chapter 1 Cardiovascular anatomy and physiology 7

smaller than the left ventricle it is larger than the right atrium The blood will travel from the left atrium into the left ventricle by way of the mitral valve The mitral valve gets its name from the miter a bishoprsquos hat The mitral valve has two cusps and papillary muscles connected by chordae tendineae Once blood is in the left ventricle a contraction will force it toward the aorta by way of the LVOT In persons with hypertrophic obstructive cardiomy-opathy (HOCM) the septum or wall in the heart can become thick and enlarged effectively narrow-ing the LVOT In the healthy heart the blood flows up the LVOT across the aortic valve and into the aorta From the aorta blood will be directed upward toward the head downward toward the legs and feet and some blood will be redirected back to the coronary arteries of the heart

The blood flow from the left ventricle to the aorta and back to the coronary arteries is unique in the body in that most of that flow occurs during diastole rather than systole Think of it this way as blood is pumped by the left ventricle during systole upward and downward to the brain and the rest of the body some of that blood flows back during diastole and drains into the coronary arteries Although it may seem counterintuitive it is impor-tant to remember that coronary artery perfusion occurs primarily during diastole

Volume valves and pressure

While it is tempting to think of the heart as simply a pump with an electrical system it is more accu-rate to say that the heart moves blood because of variations in volume and pressure

The heart has four valves two are atrioventricular (AV) valves because they connect the atrium to the ventricle and two are semilunar valves connecting the heart with pulmonary arteries or the aorta The right AV valve is the tricuspid while the left AV valve is the mitral valve The flow or movement of blood is guided and controlled by the heartrsquos muscular action in the form of systole (contraction) or diastole (relaxation)

When blood pours into the right atrium it dis-tends and expands the right atrium When this volume of blood creates sufficient pressure it forces the tricuspid valve open Think of it as volume creating pressure and pressure opening

valves When the tricuspid valve opens the blood dumps into the right ventricle During this period of diastole blood pours into the right ventricle creating volume and expanding it As blood pours in and ventricular diastole nears the end the atria contract while the AV valves are still open in something nicknamed atrial kick Atrial systole occurs at the end of ventricular diastole Atrial kick forces the maximum amount of blood from the atria into the ventricles It is estimated that atrial kick delivers 20ndash30 of cardiac output

The atrial contribution to ventricular filling (atrial kick) is of enormous clinical significance Patients with certain atrial arrhythmias such as atrial fibrillation (AF) lose atrial kick Even milder forms of atrial tachyarrhythmias may compromise atrial kick Moreover patients who do not have AV synchrony (one atrial beat corresponding to one ventricular beat) will lose atrial kick The loss of atrial kick can reduce cardiac output by 20ndash30

Once the atria have contracted and the ventricles are filled with blood the AV valves snap closed The ventricles now start to contract and push open the semilunar valves For example as the right ven-tricle contracts (systole) it opens the PV (The PV is sometimes called the pulmonic valve itrsquos the same thing) The whole process works on volume pressure and contractionrelaxation

Of the four valves in the heart the tricuspid valve is of most interest to device specialists because at least one lead will be placed into the right ventricle This means that pacing and defi-brillation leads typically cross the tricuspid valve These leads may interfere with the proper closing of the tricuspid valve particularly if the lead is large diameter or several leads are passed over the tricuspid valve A single lead through the tricuspid valve is likely to cause minimal to no dysfunction However it is not unusual to see patients with three or more leads in the right ventricle and these leads may hamper the tricuspid valve in its closing leading to tricuspid regurgitation or backflow of blood Patients with multiple leads (such as chil-dren who grow up with pacemakers) may need to have some of them removed to avoid tricuspid regurgitation

Right ventricular leads are typically fixated at the right ventricular apex or in the RVOT and may be

8 The nuts and bolts of implantable device therapy pacemakers

lodged in the trabeculae Trabeculae are a fibrous network that could be described as similar to the pores of a sponge (see Figure 17)

On the left side of the heart the blood flow pattern is similar Blood flows into the left atrium over the open mitral valve and into the left atrium At the end of ventricular diastole as the ventricle is distended and stretched atrial kick delivers more blood into left ventricle The volume of blood cre-ates pressure The mitral valve closes As the left ventricle contracts the growing pressure over-comes the resistance on the aortic valve and it opens allowing blood to flow out into the aorta and beyond

While any valve can become diseased or dys-functional valvular disease more commonly affects the mitral and the aortic valves (left-sided valves) than the right-sided valves (tricuspid and pul-monary) These left valves are more likely to suffer damage or disease because of the high-pressure environment in which they function exposing them to more potential damage Other damage can occur when plaque builds up in the coronary arteries and the aorta The aorta can become

diseased typically in the case of an aortic aneurysm or dissection Carotid arteries in the neck are also frequently a site for atherosclerotic disease

the right atrial appendage

The right atrial appendage (RAA) is an area near the right atrium which is a preferred site for attach-ing active-fixation atrial leads (see Figure 18) The RAA serves no obvious purpose and may be sacri-ficed in certain heart surgeries such as a coronary artery bypass graft (CABG) procedure CABG or bypass surgery involves stopping the heart and running the blood through a machine to reoxygen-ate it during the course of the procedure This is done by attaching a large-diameter hose to the RAA when surgery concludes and this hose is removed it can damage the RAA to the point that it is completely or partially surgically removed In patients who have had a CABG procedure the RAA may not be available for right atrial lead placement For such patients the right atrial lead is often placed against the lateral wall of the right atrium It is sometimes possible to fixate a right

Figure 17 Trabeculae form a fibrous network within the ventricles Passive-fixation ventricular leads are held in place by lodging their tines or fins in the trabeculae

Chapter 1 Cardiovascular anatomy and physiology 9

atrial lead in the remnants of the RAA if a portion of it is preserved

arteries and veins

Any discussion about the heart necessarily involves the vasculature which is why we commonly refer to our circulatory system as the cardiovascular

system As most of us remember from anatomy classes arteries carry blood away from the heart while veins transport blood back to the heart (The exceptions are the PV and arteries described earlier) Arteries carry oxygen-rich blood while veins carry deoxygenated blood What is of importance to device clinicians is that arteries are high-pressure vessels while veins are low-pressure vessels Arteries tend to be more muscular in structure and muscular contractions of the arteries help to move blood outward into the body Veins tend to be less muscular than arteries but unlike arteries they contain a system of tiny interior valves The purpose of these little valves is to maintain a unidirectional flow of blood (see Figure 19)

Some of the most important veins in the body are the SVC and its counterpart the IVC The SVC takes blood from the upper part of the body and delivers it into the right atrium with the IVC doing the same for blood from the lower part of the body The coronary veins networked around the outside of the heart carry blood from the heart tissue and deliver it back to the right atrium they are closely linked to the coronary arteries which carry reoxygenated blood from the left ventricle back to the heart muscle The CS is the site where

Right atrial appendage

Figure 18 The right atrial appendage serves no obvious hemodynamic purpose but is often the site of fixation of atrial leads

Artery Vein

Figure 19 Veins and arteries have different purposes different anatomical structure and different pressures Overall veins are low-pressure systems that move blood by a series of little valves that keep blood flowing in the same direction Arteries are high-pressure systems and are more muscular so that they force blood forward by squeezing

10 The nuts and bolts of implantable device therapy pacemakers

the great middle and small cardiac veins all drain the CS takes this blood and delivers it back to the right atrium

The PV carry reoxygenated blood from the lungs and deliver it to the left atrium so that it can be pumped back out to the body by the left ventricle

The most important artery in the body is by far the aorta The aorta is connected to the left ven-tricle and it is the first and main conduit that takes reoxygenated blood from the left ventricle and sends it out to the body As this reoxygenated blood makes its way out into the body its first two stops are the head (brain) and the heart muscle Inadequate oxygenated blood to the brain can very quickly provoke symptoms This is the reason that depressed cardiac output is associated with symptoms like dizziness light-headedness feeling woozy and fatigue Severe symptoms might include syncope

The coronary arteries arise from the root of the aorta just above the cusps of the aortic valve Coronary arteries receive most blood during dias-tole so blood is more or less pouring back into these vessels The coronary artery system is a networkmdashalmost a meshmdashof middle-sized to very small vessels that cover the exterior surface of the heart (see Figure 110)

Coronary arteries get their name from the Latin word for crown (such as in our word coronation) because this network of vessels sits like a crown atop the heart and encircles it The heart gets blood from outside in as oxygen-rich blood in these coro-nary arteries is delivered to the exterior of the heart muscle Coronary arteries are not within the heart they are epicardial structures The two main coro-nary arteries are the right coronary artery (RCA) and the left coronary artery (LCA) The RCA and LCA come off the aorta and are located in the AV groove an exterior structure below the atria and above the ventricles The RCA runs from the aorta in the right AV groove while the LCA comes off the aorta and quickly branches to the left main The left main is a very short artery (about the size of a thumbnail) which branches quickly into the left anterior descending artery (LAD) and the circum-flex artery The LAD travels down the front of the

left side of the heart The circumflex artery can be found in the left AV groove

The RCA bifurcates to the posterior descending artery The right atrium and right ventricle get their supply of oxygen-rich blood from the RCA Much of the cardiac conduction system is right sided that is it commences with the sinoatrial (SA) node in the high right atrium and travels down to the AV node on the right side of the septum These important electrical structures get their oxygen-rich blood supply from the RCA The electrical system travels down the ventricles to the inferior wall of the left ventricle (located at the bottom of the heart but belonging to the left ventricle) the inferior wall is also fed oxygen-rich blood by the RCA An occlusion of the RCA such as might occur in a myocardial infarction (MI) may result in death of the heart muscle it feeds which includes the right atrium the right ventricle and the inferior wall of the left ventricle and it might affect the conduction system including the SA node and the AV node An inferior wall MI is typically the result of a clogged RCA

RCA

LCA

Aorta

Figure 110 The coronary arteries of the heart branch out to surround the entire heart muscle The two most prominent coronary arteries are the right coronary artery (RCA) and the left coronary artery (LCA)

Chapter 1 Cardiovascular anatomy and physiology 11

For an MI the more distal the occlusion (that is further from the aorta) the less potential damage the heart attack will have In other words the worst occlusions are proximal Whether or not a patient survives an MI comes down to the numbers The loss of over 40 of the left ventricular muscle mass results in death However that 40 is cumulative For example a person can survive an MI that costs him 30 of left ventricular muscle mass but if that heart attack is followed by another that kills 15 of the left ventricular muscle mass the second (milder) heart attack will prove fatal This is the reason many people survive a first heart attack but die of a second or third attack even though their fatal MI may be relatively mild

The LCA branches quickly to the left main which branches almost at once to the LAD The LAD runs on the outside of the heart along the front of the heart along the septum The left main supplies the left atrium and the majority of the left ventricle in other words the major pumping por-tion of the heart An occlusion in the left main can be particularly disastrous because it is so far upstream and affects the heartrsquos left ventricular muscle mass No wonder they call the left main the widow maker

The coronary arteries do not run in straight lines they twist and turn and sometimes make very sharp bends People with coronary artery disease (CAD) typically have plaque buildup in the areas where the coronary arteries bend at sharp angles These are areas where the blood flow creates a lot of turbulence and plaque and other substances can collect and build up It is typically to find blockage and occlusions at these places

The LAD supplies the anterior wall of the left ventricle and the septum with oxygen-rich blood Since part of the heartrsquos conduction system runs through the septum (the right and left bundle branches) an LAD occlusion can damage the heartrsquos ability to conduct electricity properly

The circumflex artery runs along the AV groove laterally and thus supplies oxygen-rich blood to the lateral wall of the left ventricle along with the left atrium The circumflex runs along the AV groove to the left heading toward the back of the heart where it meets with the RCA which

travels along the AV groove to the left toward the back of the heart Both the circumflex and the RCA are responsible for providing oxygenated blood to the posterior wall of the ventricle But which is the more important provider That depends on crux and whether the heart is right or left dominant

The crux refers to an imaginary line drawn exactly down the middle of the ventricular poste-rior wall without any regard to anatomical landmarks If the RCA crosses the crux then the heart is said to be right dominant and the RCA is the greater supplier of oxygenated blood to the ven-tricular posterior wall On the other hand if the circumflex crosses the crux then the heart is said to be left dominant and the circumflex is the more significant provider Roughly 60 of the population is right dominant

While this information about the vasculature is important it is even more important for clini-cians to recognize that there is tremendous interpatient variability in terms of venous anatomy While we can describe major structures and typical venous formations anomalies are very common These anomalies may be minor variations or they can be very pronounced such as missing veins and arteries Clinicians who deal with the cardiovascular system must be prepared for large and small differences in vascular anatomy The cardiovascular system is incredibly resilient and can adapt to some anomalies by building up collateral circulation For example a person with a missing or incompetent vessel may over time build up a network of smaller collateral vessels that compensates by doing the same job It is not unusual to see vessels grow larger or branch out to compensate Sometimes when an artery is blocked collateral circulation will build up and compensate by delivering oxygen-rich blood to the heart muscle

Every artery in the body has a matching or corresponding vein usually located in close proximity This applies to the coronary system as well Note that there are great differences in coro-nary venous anatomy among patients which becomes important when implanting left ventric-ular leads for CRT devices (see Figure 111)

12 The nuts and bolts of implantable device therapy pacemakers

the nuts and bolts of cardiovascular anatomy and physiology

bull The heart consists of four chambers which may be considered the upper chambers (right atrium and left atrium) and lower chambers (right ven-tricle and left ventricle) or may be thought of as the right heart (right atrium and right ventricle) and left heart (left atrium and left ventricle)

bull Deoxygenated blood enters the right atrium flows to the right ventricle and is pumped out via the pulmonary valve to the lungs The blood is reoxygenated in the lungs and then enters the left atrium and flows to the left atrium and out of the aortic valve into the aorta and the rest of the body

bull The heart has two outflow tracts a right ven-tricular outflow tract (RVOT) and a left ven-tricular outflow tract (LVOT)

bull The largest chamber of the heart is the left ven-tricle making up about 23 of the cardiac mass The left ventricle is muscular and massive because it must pump against approximately 10 times the pressure against which the right ven-tricle pumps (120 vs 12 mmHg)

bull The two cardiac valves most susceptible to dysfunction and disease are the mitral and aortic valve because they function in a high-pressure environment

bull The chordae tendineae connect the leaflets of the valves to the papillary muscles and look like little parachutes These help the valves open and close properly

bull In general arteries transport blood away from the heart are muscular and contract to help move the blood on its way and have thicker walls than veins Veins have thinner less muscular walls and carry oxygen-depleted blood back to the heart using tiny interior valves to keep the blood flow moving forward Arteries transport oxygenated blood and can be considered high-pressure systems while veins transported deoxygenated blood in a low-pressure system

bull Cardiac contraction is systole cardiac relaxation is diastole Atrial systole occurs at the very end of ventricular diastole in something nicknamed atrial kick In a healthy individual about 20ndash30 of cardiac output comes from atrial kick

bull The heart is a muscle that is fed with oxygenated blood through a network of coronary arteries Coronary arteries get oxygen-rich blood from the aorta and they perfuse the heart muscle during diastole as blood drains back

bull Although arteries and veins generally take blood away from or to the heart respectively

Rightventricle

Leftventricle

Leftatrium

Aorta

Rightatrium

Rightcoronaryartery

Leftcoronaryartery

Circumflexartery

Superiorvena cava

Inferiorvena cava

Arterial system

Rightventricle

Leftventricle

Greatcardiac vein

Smallcardiac vein

Middlecardiac vein

Leftatrium

Aorta

Rightatrium

Superiorvena cava

Inferiorvena cava

Venous system

Coronarysinus

Figure 111 The coronary artery system is matched by a coronary venous system

Chapter 1 Cardiovascular anatomy and physiology 13

test your knowledge

Fill in these anatomical landmarks of the heart They include the four chambers the four valves and the bodyrsquos main artery

the pulmonary vessels are the exception The pulmonary artery transports deoxygenated blood while the pulmonary veins carry oxygen-ated blood This is because of their special loca-tion and the fact that their work is taking blood from the right side to the left side of the heart

bull The main coronary arteries are the right coro-nary artery and the left coronary artery The left coronary artery branches quickly to a short vessel known as the left main (the widow maker) and the circumflex artery

bull A myocardial infarction can kill the heart muscle The clogged arteries determine the loc ation of the affected muscle A right coro-nary artery occlusion will affect the right atrium and ventricle and the inferior wall of the left ventricle while a left coronary artery occlusion will affect the left atrium and most of the left ventricle By far the most dangerous occlusion is in the left main because it feeds the majority of the heart muscle with oxygen-rich blood

answer the following questions

1 What are the names of the two atrioventricular valves of the heartA Atrial and ventricularB Tricuspid and mitralC Pulmonary and pulmonicD Aortic and superior vena cava

2 Which chambers of the heart contain trabeculaeA AtriaB VentriclesC Right-sided chambers onlyD Left-sided chambers only

14 The nuts and bolts of implantable device therapy pacemakers

3 Why can trabeculae be important to device cliniciansA They are good places to secure a passive-fixa-

tion leadB They help regulate blood pressureC They prevent atherosclerosisD All of the above

4 Which of the following is not true about the aortaA It is an arteryB It consists of an ascending portion a descend-

ing portion and an archC It carries only oxygenated bloodD It is the largest vein in the body

5 From which artery does blood enter the right atriumA Superior vena cavaB Inferior vena cavaC Coronary sinusD All of the above

6 What is atrial kickA Ventricular pressure on the tricuspid valve

forcing it shutB Electrical impulses originating in the atrium

to cause the heart to beat fasterC Atrial systole that forces the most blood possible

into the already-full and diastolic ventriclesD A type of arrhythmia

7 Why would a person with chronic AF likely have decreased cardiac outputA AF is associated with low blood volumeB AF causes the heart to beat very slowlyC AF may cause the heart to experience fre-

quent pausesD AF reduces or eliminates atrial kickmdashand

atrial kick contributes to cardiac output

8 Which of the following is often a preferred location for affixing a cardiac pacing leadA The right ventricular outflow tract (RVOT)B The left ventricular outflow tract (LVOT)C The excised right atrial appendageD The tricuspid valve

9 What connects the valve leaflets to the papillary musclesA TrabeculaeB ArteriesC Chordae tendineaeD Tiny valves

10 Which coronary artery is nicknamed the widow maker because an occlusion in this vessel will deprive the majority of the heartrsquos muscle mass of oxygen-rich bloodA Left mainB CircumflexC Right coronary arteryD Inferior vena cava

15

The Nuts and Bolts of Implantable Device Therapy Pacemakers First Edition Tom Kenny copy 2015 John Wiley amp Sons Ltd Published 2015 by John Wiley amp Sons Ltd

Chapter 2

Cardiac conduction system

Introduction

The sinoatrial (SA) node is the heartrsquos natural pacemaker In the healthy heart the SA node fires and the electrical stimulus conducts along special pathways through the heart that cause cells in specific areas to depolarize After the stimulus passes the area the cells repolarize Myocardial cell depolarization and repolarization govern the heartrsquos contraction and relaxation sequencemdashin other words the heart rhythm Rhythm disorders occur when this healthy conduction system is in some way disrupted or impaired

the conduction pathways

When the SA node fires and sends out electrical energy the stimulus travels from the SA node in the high right atrium out over the atria via path-ways known as internodal tracts Most of these internodal tracts are located in the right atrium the

main tract over to the left atrium is called Bachmannrsquos bundle It is because of Bachmannrsquos bundle linking the right atrium to the left atrium that the atria can contract simultaneously that is as these two upper chambers contract as one coherent unit After traversing the atria the impulse comes down through conduction tracts to the atrioventricular (AV) node

The AV node is an area of highly specialized tissue located below the atria and above the ventri-cles on the right side of the septum Anatomically the AV node rests in the AV groove The tissue in the healthy AV node has the ability to change the speed of the electrical impulse specifically it slows the stimulus down The slowdown serves two important purposes First slowing the impulse allows the atria time to contract and provide the very valuable atrial kick to ventricular filling Second the slowed impulse protects the ventricles from being forced to beat too quickly that is to beat before they have been adequately filled with blood

Once the impulse crosses the AV node it travels down into the ventricles and accelerates its speed The impulse travels through the bundle of His (pronounced hiss) to the bundle branches (right and left) and then out to the network of Purkinje fibers This allows both ventricles to depolarize and contract simultaneously (Figure 21)

polarization depolarization and repolarization

Cardiac conduction is driven by the ability of myo-cardial cells to be depolarized and repolarized Depolarization occurs at the cellular level and is

2

Learning objectives

bullensp Describe the electrical conduction system of the healthy heart

bullensp List the main anatomical landmarks of normal cardiac conduction

bullensp Name the anatomical location of the heartrsquos natural pacemaker

bullensp Explain depolarization and repolarization in electrical terms and how it relates to the cardiac contraction

bullensp Define the term automaticity and how it relates to cardiac conduction

16 The nuts and bolts of implantable device therapy pacemakers

in turn driven by electrolytes or positively and negatively charged ions (potassium sodium and calcium mainly) which are able to move in and out of the cardiac cells A myocardial cell may be said to be polarized when in its resting state that is there is no electrical activity going on In the rest-ing state the interior of the myocardial cell is somewhat more negatively charged that the outside This state is called the resting state or sometimes phase 0 of the action potential

An electrical stimulus changes the resting state abruptly The electrical energy allows for the movement of ions to the point that the interior of the myocardial cell is now more positive than the outside In other words the charge reverses and the cell is said to be depolarized When this happens the cell contracts when this happens to a large area of the myocardium the heart muscle contracts Cardiac contraction occurs as a result of cellular depolarization

When the electrical stimulus moves past the myocardial cell the ions move in reverse and the interior of the cell assumes its previous negative charge with respect to the outside This process is

called repolarization and it brings the myocar-dial cell back to the resting state When this happens the cell relaxes When a large area of the myocardium repolarizes the heart resumes its previous shape (loses its contraction)

Intrinsic pacemakers

The healthy heartrsquos natural pacemaker is the SA node but the SA node is not the only area of the heart that can generate an electrical pulse The SA node serves as the heartrsquos pacemaker because it generates these pulses faster than other areas In the healthy heart the SA node can fire at around 60ndash100 times a minute corresponding to a healthy heart rate The AV junction can also fire but it is much slower at around 40ndash60 times a minute Even the Purkinje fibers (the ventri-cles) can act as a pacemaker but they fire at only 20ndash40 times a minute Thus in a healthy individual the SA node outpaces these other potential pacemakers The AV junction and ven-tricles are sometimes called backup pacemakers because they take over the heart rate in the event

Sinoatrial (SA) node

Internodal tracts

Right bundle branch

Left bundle branch

Purkinje fibers

Bachmannrsquos bundle

Bundle of His

Atrioventricular (AV) node

Figure 21 The conduction pathways in the healthy heart

Chapter 2 Cardiac conduction system 17

that the SA node is not able to pace the heart While these backup pacemakers may help keep the heart beating in the event of a dysfunctional SA node they are slow and not as reliable as the SA node

The healthy heart is able to pace itself because of a property called automaticity Automaticity is an inherent property of individual cardiac conductive cells that allows them to depolarize spontaneously In other words some cells in the heart (such as the cells of the SA node) are able to generate an electrical stimulus on their own

refractoriness

Refractoriness refers to the temporary inability of a cardiac cell to respond to an electrical stimulus Refractoriness can be the normal appropriate and expected behavior of cardiac cells For example immediately after electrical energy has passed through cardiac cells they go through a phase where they are unresponsive to electrical energy no matter how much voltage is delivered This normal refractoriness is called the physiological refractory period and it is very brief (it is mea-sured in thousandths of a second) Of course the exact duration of a refractory period is based in part on the heart rate the faster the heart rate the shorter the refractory period and vice versa

Refractoriness may also occur when certain cells become dysfunctional that is cardiac cells may lose their ability to conduct properly Furthermore the heartrsquos inherent pacemakers may change their rate for instance the AV junction may speed up and start to deliver impulses at 80 or 100 times a minute When this happensmdashthat is when another pacemaker takes over the role of the SA nodemdashthat pacemaker is called a usurping pacemaker because it usurps the rightful role of the SA node In the event that other pacemakers of the heart start to compete with the SA node the fastest pacemaker always wins

Conductivity

Conductivity is the cardiac cellrsquos ability to transmit an electrical impulse to an adjacent cell Typically cardiac conduction is antegrade that

is it moves forward Electrical impulses actually travel out in all different directions but refracto-riness prevents those cells that were just stimu-lated from depolarization This keeps conduction moving in a single direction through the heart Normal antegrade conduction in a healthy heart goes from SA node to atria to AV node over the bundle of His and down to the ventricles Some individuals have the ability to conduct retro-grade retrograde conduction is abnormal but not uncommon

autonomic nervous system

The heart does not need direct stimulation from the nervous system to generate electricity and to beat but that is not to say that the nervous system does not control the heart rate The autonomic nervous system indirectly affects cardiac conduction by regulating how fast the heart will beat and how hard the heart muscle will squeeze (contractility) The autonomic ner-vous system communicates with the heart through a network of baroreceptors which respond to chemical signals

The autonomic nervous system of the body like certain other physiological systems exists as two systems in balance The autonomic nervous system consists of a sympathetic system and a parasympathetic system They both work together and in a healthy individual one may overdrive the other during specific times The sympathetic ner-vous system is mainly associated with epinephrine (adrenaline) and it has the ability to speed the heart rate and increase cardiac contractility and can increase the speed by which electrical impulses can cross the AV node The sympathetic nervous system takes over when we are stressed frightened (fight-or-flight response) or exerting ourselves

On the other hand the autonomic nervous system also has a parasympathetic system that is associated with acetylcholine This system can decrease the heart rate decrease cardiac contrac-tility and cause impulses crossing the AV node to be slowed The parasympathetic system might take over when we are sleeping or at rest

18 The nuts and bolts of implantable device therapy pacemakers

the nuts and bolts of the cardiac conduction system

bull The heartrsquos natural pacemaker is the sinoatrial (SA) node which generates an electrical impulse spontaneously about 60ndash100 times a minute in a healthy individual

bull The normal conduction path is from the SA node over the atria to the atrioventricular (AV) node and then via the bundle of His down the right and left bundle branches and into the Purkinje fibers

bull The AV node has the ability to slow down the electrical impulse This serves two main purposes it allows for atrial kick and it protects the ventricles from beating too rapidly

bull In the healthy heart the right and left atria contract simultaneously and then the right and left ventricles contract simultaneously

bull Depolarization and repolarization of myocar-dial cells are driven by positively and negatively charged ions which cross the cell membrane A resting cardiac cell is more negative on the inside than the surrounding media but when it depolarizes it becomes more positive on the inside relative to the surrounding media Following depolarization the ions move and the cell returns to being more negative on the inside than the outside in a process known as repolarization

bull Depolarization causes the cardiac cell to contract

bull The main ions involved in depolarization and repolarization are sodium potassium and calcium ions

bull Although the heartrsquos natural pacemaker is the SA node the AV junction and the Purkinje fibers (ventricles) can also assume (usurp) this role if the SA node cannot generate electricity properly The AV junction and Purkinje fibers are not as reliable as a healthy AV node and typically generate electricity more slowly (the AV junction about 40ndash60 times a minute and the Purkinje fibers 20ndash40 times a minute)

bull However sometimes the AV junction or Purkinje fibers may generate electricity very

quickly this is abnormal When they do they may take over the heart When pacemakers of the heart compete the fastest one drives the heart

bull Automaticity is an inherent property of certain cardiac conductive cells to depolarize spontaneously Not all cardiac conduction cells possess automaticity but the SA node AV junction and Purkinje fibers possess automaticity

bull Refractoriness is the natural and expected behavior of cardiac conductive cells that makes them unresponsive to electrical impulses immediately after they have depolarized The natural refractory period is very brief

bull The faster the heart rate the shorter the refractory period the slower the heart rate the longer the refractory period

bull Refractoriness can also occur as a dysfunction bull Conductivity is the cardiac cellrsquos ability to

transmit an electrical impulse to adjacent cells Refractoriness means that cells that just depolarized will not depolarize again This keeps conduction moving forward (antegrade conduction)

bull Antegrade conduction is normal retrograde conduction is abnormal but is not rare

bull The autonomic nervous system regulates the heart rate by controlling how fast the heart beats and how vigorously it contracts

bull The autonomic nervous system consists of a sympathetic and parasympathetic nervous system which work together but at certain times one or the other is dominant The sympathetic nervous system makes the heart beat faster and more vigorously it takes over during times of stress exertion or fight or flight The parasympathetic ner-vous system makes the heart beat slowly and less vigorously it takes over during periods of rest

bull The neurotransmitters associated with these two systems are epinephrine or adrenaline (sympathetic nervous system) and acetylcho-line (parasympathetic nervous system)

Chapter 2 Cardiac conduction system 19

test your knowledge

Please write in the landmarks of the cardiac conduction system Select from the following (each is used only once)

AV node Bachmanrsquos bundle Bundle of His

Internodal tracts Left bundle branch Purkinje fibers

Right bundle branch Ventricles

1 Which of the following is not considered one of the heartrsquos intrinsic pacemakersA AV junctionB SA nodeC Purkinje fibersD Aorta

2 Depolarization and repolarization involve the movement of charged ions across cardiac cell membranes What types of ions are primarily involvedA Calcium potassium and sodiumB Acetylcholine and epinephrineC Magnesium and folateD All of the above

3 Refractoriness refers toA The ability of cardiac conduction cells to

contract

B The inability of cardiac conduction cells to respond to an electrical stimulus

C The unique permeability of the cardiac cellsD Abnormal cardiac conduction

4 When a cardiac cell depolarizes the interior of the cell becomes what with respect to the surrounding mediaA More positiveB More negativeC More acidicD Hotter

5 Fill in the blank In the healthy heart the faster the heart rate the ___ the refractory periodA More vigorousB Less reliableC LongerD Shorter

20 The nuts and bolts of implantable device therapy pacemakers

6 Which specific part of the nervous system governs the fight-or-flight response and is associated with epinephrine (adrenaline) (Pick the most exact answer)A Autonomic nervous systemB Parasympathetic nervous systemC Sympathetic nervous systemD Central nervous system

7 In normal antegrade conduction the electrical impulse travels from the SA node over the atria to the AV node and to what structure before reaching the ventriclesA The intermodal tractsB The Purkinje fibersC The bundle of HisD The ventricular apex

8 Which of the following is true about retrograde cardiac conductionA Everyone has itB It is perfectly normal and can be helpful in

stressful situations

C It is abnormal but not uncommonD It is impossible to diagnose definitively

except in an autopsy

9 What chemical is associated with the parasympathetic nervous systemA AcetylcholineB PotassiumC EpinephrineD Dopamine

10 The stimulation of the myocardial cells of which of the following structures is most closely associated with ventricular depolarizationA SA nodeB AV junctionC Bundle of HisD Purkinje fibers

8 The nuts and bolts of implantable device therapy pacemakers

lodged in the trabeculae Trabeculae are a fibrous network that could be described as similar to the pores of a sponge (see Figure 17)

On the left side of the heart the blood flow pattern is similar Blood flows into the left atrium over the open mitral valve and into the left atrium At the end of ventricular diastole as the ventricle is distended and stretched atrial kick delivers more blood into left ventricle The volume of blood cre-ates pressure The mitral valve closes As the left ventricle contracts the growing pressure over-comes the resistance on the aortic valve and it opens allowing blood to flow out into the aorta and beyond

While any valve can become diseased or dys-functional valvular disease more commonly affects the mitral and the aortic valves (left-sided valves) than the right-sided valves (tricuspid and pul-monary) These left valves are more likely to suffer damage or disease because of the high-pressure environment in which they function exposing them to more potential damage Other damage can occur when plaque builds up in the coronary arteries and the aorta The aorta can become

diseased typically in the case of an aortic aneurysm or dissection Carotid arteries in the neck are also frequently a site for atherosclerotic disease

the right atrial appendage

The right atrial appendage (RAA) is an area near the right atrium which is a preferred site for attach-ing active-fixation atrial leads (see Figure 18) The RAA serves no obvious purpose and may be sacri-ficed in certain heart surgeries such as a coronary artery bypass graft (CABG) procedure CABG or bypass surgery involves stopping the heart and running the blood through a machine to reoxygen-ate it during the course of the procedure This is done by attaching a large-diameter hose to the RAA when surgery concludes and this hose is removed it can damage the RAA to the point that it is completely or partially surgically removed In patients who have had a CABG procedure the RAA may not be available for right atrial lead placement For such patients the right atrial lead is often placed against the lateral wall of the right atrium It is sometimes possible to fixate a right

Figure 17 Trabeculae form a fibrous network within the ventricles Passive-fixation ventricular leads are held in place by lodging their tines or fins in the trabeculae

Chapter 1 Cardiovascular anatomy and physiology 9

atrial lead in the remnants of the RAA if a portion of it is preserved

arteries and veins

Any discussion about the heart necessarily involves the vasculature which is why we commonly refer to our circulatory system as the cardiovascular

system As most of us remember from anatomy classes arteries carry blood away from the heart while veins transport blood back to the heart (The exceptions are the PV and arteries described earlier) Arteries carry oxygen-rich blood while veins carry deoxygenated blood What is of importance to device clinicians is that arteries are high-pressure vessels while veins are low-pressure vessels Arteries tend to be more muscular in structure and muscular contractions of the arteries help to move blood outward into the body Veins tend to be less muscular than arteries but unlike arteries they contain a system of tiny interior valves The purpose of these little valves is to maintain a unidirectional flow of blood (see Figure 19)

Some of the most important veins in the body are the SVC and its counterpart the IVC The SVC takes blood from the upper part of the body and delivers it into the right atrium with the IVC doing the same for blood from the lower part of the body The coronary veins networked around the outside of the heart carry blood from the heart tissue and deliver it back to the right atrium they are closely linked to the coronary arteries which carry reoxygenated blood from the left ventricle back to the heart muscle The CS is the site where

Right atrial appendage

Figure 18 The right atrial appendage serves no obvious hemodynamic purpose but is often the site of fixation of atrial leads

Artery Vein

Figure 19 Veins and arteries have different purposes different anatomical structure and different pressures Overall veins are low-pressure systems that move blood by a series of little valves that keep blood flowing in the same direction Arteries are high-pressure systems and are more muscular so that they force blood forward by squeezing

10 The nuts and bolts of implantable device therapy pacemakers

the great middle and small cardiac veins all drain the CS takes this blood and delivers it back to the right atrium

The PV carry reoxygenated blood from the lungs and deliver it to the left atrium so that it can be pumped back out to the body by the left ventricle

The most important artery in the body is by far the aorta The aorta is connected to the left ven-tricle and it is the first and main conduit that takes reoxygenated blood from the left ventricle and sends it out to the body As this reoxygenated blood makes its way out into the body its first two stops are the head (brain) and the heart muscle Inadequate oxygenated blood to the brain can very quickly provoke symptoms This is the reason that depressed cardiac output is associated with symptoms like dizziness light-headedness feeling woozy and fatigue Severe symptoms might include syncope

The coronary arteries arise from the root of the aorta just above the cusps of the aortic valve Coronary arteries receive most blood during dias-tole so blood is more or less pouring back into these vessels The coronary artery system is a networkmdashalmost a meshmdashof middle-sized to very small vessels that cover the exterior surface of the heart (see Figure 110)

Coronary arteries get their name from the Latin word for crown (such as in our word coronation) because this network of vessels sits like a crown atop the heart and encircles it The heart gets blood from outside in as oxygen-rich blood in these coro-nary arteries is delivered to the exterior of the heart muscle Coronary arteries are not within the heart they are epicardial structures The two main coro-nary arteries are the right coronary artery (RCA) and the left coronary artery (LCA) The RCA and LCA come off the aorta and are located in the AV groove an exterior structure below the atria and above the ventricles The RCA runs from the aorta in the right AV groove while the LCA comes off the aorta and quickly branches to the left main The left main is a very short artery (about the size of a thumbnail) which branches quickly into the left anterior descending artery (LAD) and the circum-flex artery The LAD travels down the front of the

left side of the heart The circumflex artery can be found in the left AV groove

The RCA bifurcates to the posterior descending artery The right atrium and right ventricle get their supply of oxygen-rich blood from the RCA Much of the cardiac conduction system is right sided that is it commences with the sinoatrial (SA) node in the high right atrium and travels down to the AV node on the right side of the septum These important electrical structures get their oxygen-rich blood supply from the RCA The electrical system travels down the ventricles to the inferior wall of the left ventricle (located at the bottom of the heart but belonging to the left ventricle) the inferior wall is also fed oxygen-rich blood by the RCA An occlusion of the RCA such as might occur in a myocardial infarction (MI) may result in death of the heart muscle it feeds which includes the right atrium the right ventricle and the inferior wall of the left ventricle and it might affect the conduction system including the SA node and the AV node An inferior wall MI is typically the result of a clogged RCA

RCA

LCA

Aorta

Figure 110 The coronary arteries of the heart branch out to surround the entire heart muscle The two most prominent coronary arteries are the right coronary artery (RCA) and the left coronary artery (LCA)

Chapter 1 Cardiovascular anatomy and physiology 11

For an MI the more distal the occlusion (that is further from the aorta) the less potential damage the heart attack will have In other words the worst occlusions are proximal Whether or not a patient survives an MI comes down to the numbers The loss of over 40 of the left ventricular muscle mass results in death However that 40 is cumulative For example a person can survive an MI that costs him 30 of left ventricular muscle mass but if that heart attack is followed by another that kills 15 of the left ventricular muscle mass the second (milder) heart attack will prove fatal This is the reason many people survive a first heart attack but die of a second or third attack even though their fatal MI may be relatively mild

The LCA branches quickly to the left main which branches almost at once to the LAD The LAD runs on the outside of the heart along the front of the heart along the septum The left main supplies the left atrium and the majority of the left ventricle in other words the major pumping por-tion of the heart An occlusion in the left main can be particularly disastrous because it is so far upstream and affects the heartrsquos left ventricular muscle mass No wonder they call the left main the widow maker

The coronary arteries do not run in straight lines they twist and turn and sometimes make very sharp bends People with coronary artery disease (CAD) typically have plaque buildup in the areas where the coronary arteries bend at sharp angles These are areas where the blood flow creates a lot of turbulence and plaque and other substances can collect and build up It is typically to find blockage and occlusions at these places

The LAD supplies the anterior wall of the left ventricle and the septum with oxygen-rich blood Since part of the heartrsquos conduction system runs through the septum (the right and left bundle branches) an LAD occlusion can damage the heartrsquos ability to conduct electricity properly

The circumflex artery runs along the AV groove laterally and thus supplies oxygen-rich blood to the lateral wall of the left ventricle along with the left atrium The circumflex runs along the AV groove to the left heading toward the back of the heart where it meets with the RCA which

travels along the AV groove to the left toward the back of the heart Both the circumflex and the RCA are responsible for providing oxygenated blood to the posterior wall of the ventricle But which is the more important provider That depends on crux and whether the heart is right or left dominant

The crux refers to an imaginary line drawn exactly down the middle of the ventricular poste-rior wall without any regard to anatomical landmarks If the RCA crosses the crux then the heart is said to be right dominant and the RCA is the greater supplier of oxygenated blood to the ven-tricular posterior wall On the other hand if the circumflex crosses the crux then the heart is said to be left dominant and the circumflex is the more significant provider Roughly 60 of the population is right dominant

While this information about the vasculature is important it is even more important for clini-cians to recognize that there is tremendous interpatient variability in terms of venous anatomy While we can describe major structures and typical venous formations anomalies are very common These anomalies may be minor variations or they can be very pronounced such as missing veins and arteries Clinicians who deal with the cardiovascular system must be prepared for large and small differences in vascular anatomy The cardiovascular system is incredibly resilient and can adapt to some anomalies by building up collateral circulation For example a person with a missing or incompetent vessel may over time build up a network of smaller collateral vessels that compensates by doing the same job It is not unusual to see vessels grow larger or branch out to compensate Sometimes when an artery is blocked collateral circulation will build up and compensate by delivering oxygen-rich blood to the heart muscle

Every artery in the body has a matching or corresponding vein usually located in close proximity This applies to the coronary system as well Note that there are great differences in coro-nary venous anatomy among patients which becomes important when implanting left ventric-ular leads for CRT devices (see Figure 111)

12 The nuts and bolts of implantable device therapy pacemakers

the nuts and bolts of cardiovascular anatomy and physiology

bull The heart consists of four chambers which may be considered the upper chambers (right atrium and left atrium) and lower chambers (right ven-tricle and left ventricle) or may be thought of as the right heart (right atrium and right ventricle) and left heart (left atrium and left ventricle)

bull Deoxygenated blood enters the right atrium flows to the right ventricle and is pumped out via the pulmonary valve to the lungs The blood is reoxygenated in the lungs and then enters the left atrium and flows to the left atrium and out of the aortic valve into the aorta and the rest of the body

bull The heart has two outflow tracts a right ven-tricular outflow tract (RVOT) and a left ven-tricular outflow tract (LVOT)

bull The largest chamber of the heart is the left ven-tricle making up about 23 of the cardiac mass The left ventricle is muscular and massive because it must pump against approximately 10 times the pressure against which the right ven-tricle pumps (120 vs 12 mmHg)

bull The two cardiac valves most susceptible to dysfunction and disease are the mitral and aortic valve because they function in a high-pressure environment

bull The chordae tendineae connect the leaflets of the valves to the papillary muscles and look like little parachutes These help the valves open and close properly

bull In general arteries transport blood away from the heart are muscular and contract to help move the blood on its way and have thicker walls than veins Veins have thinner less muscular walls and carry oxygen-depleted blood back to the heart using tiny interior valves to keep the blood flow moving forward Arteries transport oxygenated blood and can be considered high-pressure systems while veins transported deoxygenated blood in a low-pressure system

bull Cardiac contraction is systole cardiac relaxation is diastole Atrial systole occurs at the very end of ventricular diastole in something nicknamed atrial kick In a healthy individual about 20ndash30 of cardiac output comes from atrial kick

bull The heart is a muscle that is fed with oxygenated blood through a network of coronary arteries Coronary arteries get oxygen-rich blood from the aorta and they perfuse the heart muscle during diastole as blood drains back

bull Although arteries and veins generally take blood away from or to the heart respectively

Rightventricle

Leftventricle

Leftatrium

Aorta

Rightatrium

Rightcoronaryartery

Leftcoronaryartery

Circumflexartery

Superiorvena cava

Inferiorvena cava

Arterial system

Rightventricle

Leftventricle

Greatcardiac vein

Smallcardiac vein

Middlecardiac vein

Leftatrium

Aorta

Rightatrium

Superiorvena cava

Inferiorvena cava

Venous system

Coronarysinus

Figure 111 The coronary artery system is matched by a coronary venous system

Chapter 1 Cardiovascular anatomy and physiology 13

test your knowledge

Fill in these anatomical landmarks of the heart They include the four chambers the four valves and the bodyrsquos main artery

the pulmonary vessels are the exception The pulmonary artery transports deoxygenated blood while the pulmonary veins carry oxygen-ated blood This is because of their special loca-tion and the fact that their work is taking blood from the right side to the left side of the heart

bull The main coronary arteries are the right coro-nary artery and the left coronary artery The left coronary artery branches quickly to a short vessel known as the left main (the widow maker) and the circumflex artery

bull A myocardial infarction can kill the heart muscle The clogged arteries determine the loc ation of the affected muscle A right coro-nary artery occlusion will affect the right atrium and ventricle and the inferior wall of the left ventricle while a left coronary artery occlusion will affect the left atrium and most of the left ventricle By far the most dangerous occlusion is in the left main because it feeds the majority of the heart muscle with oxygen-rich blood

answer the following questions

1 What are the names of the two atrioventricular valves of the heartA Atrial and ventricularB Tricuspid and mitralC Pulmonary and pulmonicD Aortic and superior vena cava

2 Which chambers of the heart contain trabeculaeA AtriaB VentriclesC Right-sided chambers onlyD Left-sided chambers only

14 The nuts and bolts of implantable device therapy pacemakers

3 Why can trabeculae be important to device cliniciansA They are good places to secure a passive-fixa-

tion leadB They help regulate blood pressureC They prevent atherosclerosisD All of the above

4 Which of the following is not true about the aortaA It is an arteryB It consists of an ascending portion a descend-

ing portion and an archC It carries only oxygenated bloodD It is the largest vein in the body

5 From which artery does blood enter the right atriumA Superior vena cavaB Inferior vena cavaC Coronary sinusD All of the above

6 What is atrial kickA Ventricular pressure on the tricuspid valve

forcing it shutB Electrical impulses originating in the atrium

to cause the heart to beat fasterC Atrial systole that forces the most blood possible

into the already-full and diastolic ventriclesD A type of arrhythmia

7 Why would a person with chronic AF likely have decreased cardiac outputA AF is associated with low blood volumeB AF causes the heart to beat very slowlyC AF may cause the heart to experience fre-

quent pausesD AF reduces or eliminates atrial kickmdashand

atrial kick contributes to cardiac output

8 Which of the following is often a preferred location for affixing a cardiac pacing leadA The right ventricular outflow tract (RVOT)B The left ventricular outflow tract (LVOT)C The excised right atrial appendageD The tricuspid valve

9 What connects the valve leaflets to the papillary musclesA TrabeculaeB ArteriesC Chordae tendineaeD Tiny valves

10 Which coronary artery is nicknamed the widow maker because an occlusion in this vessel will deprive the majority of the heartrsquos muscle mass of oxygen-rich bloodA Left mainB CircumflexC Right coronary arteryD Inferior vena cava

15

The Nuts and Bolts of Implantable Device Therapy Pacemakers First Edition Tom Kenny copy 2015 John Wiley amp Sons Ltd Published 2015 by John Wiley amp Sons Ltd

Chapter 2

Cardiac conduction system

Introduction

The sinoatrial (SA) node is the heartrsquos natural pacemaker In the healthy heart the SA node fires and the electrical stimulus conducts along special pathways through the heart that cause cells in specific areas to depolarize After the stimulus passes the area the cells repolarize Myocardial cell depolarization and repolarization govern the heartrsquos contraction and relaxation sequencemdashin other words the heart rhythm Rhythm disorders occur when this healthy conduction system is in some way disrupted or impaired

the conduction pathways

When the SA node fires and sends out electrical energy the stimulus travels from the SA node in the high right atrium out over the atria via path-ways known as internodal tracts Most of these internodal tracts are located in the right atrium the

main tract over to the left atrium is called Bachmannrsquos bundle It is because of Bachmannrsquos bundle linking the right atrium to the left atrium that the atria can contract simultaneously that is as these two upper chambers contract as one coherent unit After traversing the atria the impulse comes down through conduction tracts to the atrioventricular (AV) node

The AV node is an area of highly specialized tissue located below the atria and above the ventri-cles on the right side of the septum Anatomically the AV node rests in the AV groove The tissue in the healthy AV node has the ability to change the speed of the electrical impulse specifically it slows the stimulus down The slowdown serves two important purposes First slowing the impulse allows the atria time to contract and provide the very valuable atrial kick to ventricular filling Second the slowed impulse protects the ventricles from being forced to beat too quickly that is to beat before they have been adequately filled with blood

Once the impulse crosses the AV node it travels down into the ventricles and accelerates its speed The impulse travels through the bundle of His (pronounced hiss) to the bundle branches (right and left) and then out to the network of Purkinje fibers This allows both ventricles to depolarize and contract simultaneously (Figure 21)

polarization depolarization and repolarization

Cardiac conduction is driven by the ability of myo-cardial cells to be depolarized and repolarized Depolarization occurs at the cellular level and is

2

Learning objectives

bullensp Describe the electrical conduction system of the healthy heart

bullensp List the main anatomical landmarks of normal cardiac conduction

bullensp Name the anatomical location of the heartrsquos natural pacemaker

bullensp Explain depolarization and repolarization in electrical terms and how it relates to the cardiac contraction

bullensp Define the term automaticity and how it relates to cardiac conduction

16 The nuts and bolts of implantable device therapy pacemakers

in turn driven by electrolytes or positively and negatively charged ions (potassium sodium and calcium mainly) which are able to move in and out of the cardiac cells A myocardial cell may be said to be polarized when in its resting state that is there is no electrical activity going on In the rest-ing state the interior of the myocardial cell is somewhat more negatively charged that the outside This state is called the resting state or sometimes phase 0 of the action potential

An electrical stimulus changes the resting state abruptly The electrical energy allows for the movement of ions to the point that the interior of the myocardial cell is now more positive than the outside In other words the charge reverses and the cell is said to be depolarized When this happens the cell contracts when this happens to a large area of the myocardium the heart muscle contracts Cardiac contraction occurs as a result of cellular depolarization

When the electrical stimulus moves past the myocardial cell the ions move in reverse and the interior of the cell assumes its previous negative charge with respect to the outside This process is

called repolarization and it brings the myocar-dial cell back to the resting state When this happens the cell relaxes When a large area of the myocardium repolarizes the heart resumes its previous shape (loses its contraction)

Intrinsic pacemakers

The healthy heartrsquos natural pacemaker is the SA node but the SA node is not the only area of the heart that can generate an electrical pulse The SA node serves as the heartrsquos pacemaker because it generates these pulses faster than other areas In the healthy heart the SA node can fire at around 60ndash100 times a minute corresponding to a healthy heart rate The AV junction can also fire but it is much slower at around 40ndash60 times a minute Even the Purkinje fibers (the ventri-cles) can act as a pacemaker but they fire at only 20ndash40 times a minute Thus in a healthy individual the SA node outpaces these other potential pacemakers The AV junction and ven-tricles are sometimes called backup pacemakers because they take over the heart rate in the event

Sinoatrial (SA) node

Internodal tracts

Right bundle branch

Left bundle branch

Purkinje fibers

Bachmannrsquos bundle

Bundle of His

Atrioventricular (AV) node

Figure 21 The conduction pathways in the healthy heart

Chapter 2 Cardiac conduction system 17

that the SA node is not able to pace the heart While these backup pacemakers may help keep the heart beating in the event of a dysfunctional SA node they are slow and not as reliable as the SA node

The healthy heart is able to pace itself because of a property called automaticity Automaticity is an inherent property of individual cardiac conductive cells that allows them to depolarize spontaneously In other words some cells in the heart (such as the cells of the SA node) are able to generate an electrical stimulus on their own

refractoriness

Refractoriness refers to the temporary inability of a cardiac cell to respond to an electrical stimulus Refractoriness can be the normal appropriate and expected behavior of cardiac cells For example immediately after electrical energy has passed through cardiac cells they go through a phase where they are unresponsive to electrical energy no matter how much voltage is delivered This normal refractoriness is called the physiological refractory period and it is very brief (it is mea-sured in thousandths of a second) Of course the exact duration of a refractory period is based in part on the heart rate the faster the heart rate the shorter the refractory period and vice versa

Refractoriness may also occur when certain cells become dysfunctional that is cardiac cells may lose their ability to conduct properly Furthermore the heartrsquos inherent pacemakers may change their rate for instance the AV junction may speed up and start to deliver impulses at 80 or 100 times a minute When this happensmdashthat is when another pacemaker takes over the role of the SA nodemdashthat pacemaker is called a usurping pacemaker because it usurps the rightful role of the SA node In the event that other pacemakers of the heart start to compete with the SA node the fastest pacemaker always wins

Conductivity

Conductivity is the cardiac cellrsquos ability to transmit an electrical impulse to an adjacent cell Typically cardiac conduction is antegrade that

is it moves forward Electrical impulses actually travel out in all different directions but refracto-riness prevents those cells that were just stimu-lated from depolarization This keeps conduction moving in a single direction through the heart Normal antegrade conduction in a healthy heart goes from SA node to atria to AV node over the bundle of His and down to the ventricles Some individuals have the ability to conduct retro-grade retrograde conduction is abnormal but not uncommon

autonomic nervous system

The heart does not need direct stimulation from the nervous system to generate electricity and to beat but that is not to say that the nervous system does not control the heart rate The autonomic nervous system indirectly affects cardiac conduction by regulating how fast the heart will beat and how hard the heart muscle will squeeze (contractility) The autonomic ner-vous system communicates with the heart through a network of baroreceptors which respond to chemical signals

The autonomic nervous system of the body like certain other physiological systems exists as two systems in balance The autonomic nervous system consists of a sympathetic system and a parasympathetic system They both work together and in a healthy individual one may overdrive the other during specific times The sympathetic ner-vous system is mainly associated with epinephrine (adrenaline) and it has the ability to speed the heart rate and increase cardiac contractility and can increase the speed by which electrical impulses can cross the AV node The sympathetic nervous system takes over when we are stressed frightened (fight-or-flight response) or exerting ourselves

On the other hand the autonomic nervous system also has a parasympathetic system that is associated with acetylcholine This system can decrease the heart rate decrease cardiac contrac-tility and cause impulses crossing the AV node to be slowed The parasympathetic system might take over when we are sleeping or at rest

18 The nuts and bolts of implantable device therapy pacemakers

the nuts and bolts of the cardiac conduction system

bull The heartrsquos natural pacemaker is the sinoatrial (SA) node which generates an electrical impulse spontaneously about 60ndash100 times a minute in a healthy individual

bull The normal conduction path is from the SA node over the atria to the atrioventricular (AV) node and then via the bundle of His down the right and left bundle branches and into the Purkinje fibers

bull The AV node has the ability to slow down the electrical impulse This serves two main purposes it allows for atrial kick and it protects the ventricles from beating too rapidly

bull In the healthy heart the right and left atria contract simultaneously and then the right and left ventricles contract simultaneously

bull Depolarization and repolarization of myocar-dial cells are driven by positively and negatively charged ions which cross the cell membrane A resting cardiac cell is more negative on the inside than the surrounding media but when it depolarizes it becomes more positive on the inside relative to the surrounding media Following depolarization the ions move and the cell returns to being more negative on the inside than the outside in a process known as repolarization

bull Depolarization causes the cardiac cell to contract

bull The main ions involved in depolarization and repolarization are sodium potassium and calcium ions

bull Although the heartrsquos natural pacemaker is the SA node the AV junction and the Purkinje fibers (ventricles) can also assume (usurp) this role if the SA node cannot generate electricity properly The AV junction and Purkinje fibers are not as reliable as a healthy AV node and typically generate electricity more slowly (the AV junction about 40ndash60 times a minute and the Purkinje fibers 20ndash40 times a minute)

bull However sometimes the AV junction or Purkinje fibers may generate electricity very

quickly this is abnormal When they do they may take over the heart When pacemakers of the heart compete the fastest one drives the heart

bull Automaticity is an inherent property of certain cardiac conductive cells to depolarize spontaneously Not all cardiac conduction cells possess automaticity but the SA node AV junction and Purkinje fibers possess automaticity

bull Refractoriness is the natural and expected behavior of cardiac conductive cells that makes them unresponsive to electrical impulses immediately after they have depolarized The natural refractory period is very brief

bull The faster the heart rate the shorter the refractory period the slower the heart rate the longer the refractory period

bull Refractoriness can also occur as a dysfunction bull Conductivity is the cardiac cellrsquos ability to

transmit an electrical impulse to adjacent cells Refractoriness means that cells that just depolarized will not depolarize again This keeps conduction moving forward (antegrade conduction)

bull Antegrade conduction is normal retrograde conduction is abnormal but is not rare

bull The autonomic nervous system regulates the heart rate by controlling how fast the heart beats and how vigorously it contracts

bull The autonomic nervous system consists of a sympathetic and parasympathetic nervous system which work together but at certain times one or the other is dominant The sympathetic nervous system makes the heart beat faster and more vigorously it takes over during times of stress exertion or fight or flight The parasympathetic ner-vous system makes the heart beat slowly and less vigorously it takes over during periods of rest

bull The neurotransmitters associated with these two systems are epinephrine or adrenaline (sympathetic nervous system) and acetylcho-line (parasympathetic nervous system)

Chapter 2 Cardiac conduction system 19

test your knowledge

Please write in the landmarks of the cardiac conduction system Select from the following (each is used only once)

AV node Bachmanrsquos bundle Bundle of His

Internodal tracts Left bundle branch Purkinje fibers

Right bundle branch Ventricles

1 Which of the following is not considered one of the heartrsquos intrinsic pacemakersA AV junctionB SA nodeC Purkinje fibersD Aorta

2 Depolarization and repolarization involve the movement of charged ions across cardiac cell membranes What types of ions are primarily involvedA Calcium potassium and sodiumB Acetylcholine and epinephrineC Magnesium and folateD All of the above

3 Refractoriness refers toA The ability of cardiac conduction cells to

contract

B The inability of cardiac conduction cells to respond to an electrical stimulus

C The unique permeability of the cardiac cellsD Abnormal cardiac conduction

4 When a cardiac cell depolarizes the interior of the cell becomes what with respect to the surrounding mediaA More positiveB More negativeC More acidicD Hotter

5 Fill in the blank In the healthy heart the faster the heart rate the ___ the refractory periodA More vigorousB Less reliableC LongerD Shorter

20 The nuts and bolts of implantable device therapy pacemakers

6 Which specific part of the nervous system governs the fight-or-flight response and is associated with epinephrine (adrenaline) (Pick the most exact answer)A Autonomic nervous systemB Parasympathetic nervous systemC Sympathetic nervous systemD Central nervous system

7 In normal antegrade conduction the electrical impulse travels from the SA node over the atria to the AV node and to what structure before reaching the ventriclesA The intermodal tractsB The Purkinje fibersC The bundle of HisD The ventricular apex

8 Which of the following is true about retrograde cardiac conductionA Everyone has itB It is perfectly normal and can be helpful in

stressful situations

C It is abnormal but not uncommonD It is impossible to diagnose definitively

except in an autopsy

9 What chemical is associated with the parasympathetic nervous systemA AcetylcholineB PotassiumC EpinephrineD Dopamine

10 The stimulation of the myocardial cells of which of the following structures is most closely associated with ventricular depolarizationA SA nodeB AV junctionC Bundle of HisD Purkinje fibers

Chapter 1 Cardiovascular anatomy and physiology 9

atrial lead in the remnants of the RAA if a portion of it is preserved

arteries and veins

Any discussion about the heart necessarily involves the vasculature which is why we commonly refer to our circulatory system as the cardiovascular

system As most of us remember from anatomy classes arteries carry blood away from the heart while veins transport blood back to the heart (The exceptions are the PV and arteries described earlier) Arteries carry oxygen-rich blood while veins carry deoxygenated blood What is of importance to device clinicians is that arteries are high-pressure vessels while veins are low-pressure vessels Arteries tend to be more muscular in structure and muscular contractions of the arteries help to move blood outward into the body Veins tend to be less muscular than arteries but unlike arteries they contain a system of tiny interior valves The purpose of these little valves is to maintain a unidirectional flow of blood (see Figure 19)

Some of the most important veins in the body are the SVC and its counterpart the IVC The SVC takes blood from the upper part of the body and delivers it into the right atrium with the IVC doing the same for blood from the lower part of the body The coronary veins networked around the outside of the heart carry blood from the heart tissue and deliver it back to the right atrium they are closely linked to the coronary arteries which carry reoxygenated blood from the left ventricle back to the heart muscle The CS is the site where

Right atrial appendage

Figure 18 The right atrial appendage serves no obvious hemodynamic purpose but is often the site of fixation of atrial leads

Artery Vein

Figure 19 Veins and arteries have different purposes different anatomical structure and different pressures Overall veins are low-pressure systems that move blood by a series of little valves that keep blood flowing in the same direction Arteries are high-pressure systems and are more muscular so that they force blood forward by squeezing

10 The nuts and bolts of implantable device therapy pacemakers

the great middle and small cardiac veins all drain the CS takes this blood and delivers it back to the right atrium

The PV carry reoxygenated blood from the lungs and deliver it to the left atrium so that it can be pumped back out to the body by the left ventricle

The most important artery in the body is by far the aorta The aorta is connected to the left ven-tricle and it is the first and main conduit that takes reoxygenated blood from the left ventricle and sends it out to the body As this reoxygenated blood makes its way out into the body its first two stops are the head (brain) and the heart muscle Inadequate oxygenated blood to the brain can very quickly provoke symptoms This is the reason that depressed cardiac output is associated with symptoms like dizziness light-headedness feeling woozy and fatigue Severe symptoms might include syncope

The coronary arteries arise from the root of the aorta just above the cusps of the aortic valve Coronary arteries receive most blood during dias-tole so blood is more or less pouring back into these vessels The coronary artery system is a networkmdashalmost a meshmdashof middle-sized to very small vessels that cover the exterior surface of the heart (see Figure 110)

Coronary arteries get their name from the Latin word for crown (such as in our word coronation) because this network of vessels sits like a crown atop the heart and encircles it The heart gets blood from outside in as oxygen-rich blood in these coro-nary arteries is delivered to the exterior of the heart muscle Coronary arteries are not within the heart they are epicardial structures The two main coro-nary arteries are the right coronary artery (RCA) and the left coronary artery (LCA) The RCA and LCA come off the aorta and are located in the AV groove an exterior structure below the atria and above the ventricles The RCA runs from the aorta in the right AV groove while the LCA comes off the aorta and quickly branches to the left main The left main is a very short artery (about the size of a thumbnail) which branches quickly into the left anterior descending artery (LAD) and the circum-flex artery The LAD travels down the front of the

left side of the heart The circumflex artery can be found in the left AV groove

The RCA bifurcates to the posterior descending artery The right atrium and right ventricle get their supply of oxygen-rich blood from the RCA Much of the cardiac conduction system is right sided that is it commences with the sinoatrial (SA) node in the high right atrium and travels down to the AV node on the right side of the septum These important electrical structures get their oxygen-rich blood supply from the RCA The electrical system travels down the ventricles to the inferior wall of the left ventricle (located at the bottom of the heart but belonging to the left ventricle) the inferior wall is also fed oxygen-rich blood by the RCA An occlusion of the RCA such as might occur in a myocardial infarction (MI) may result in death of the heart muscle it feeds which includes the right atrium the right ventricle and the inferior wall of the left ventricle and it might affect the conduction system including the SA node and the AV node An inferior wall MI is typically the result of a clogged RCA

RCA

LCA

Aorta

Figure 110 The coronary arteries of the heart branch out to surround the entire heart muscle The two most prominent coronary arteries are the right coronary artery (RCA) and the left coronary artery (LCA)

Chapter 1 Cardiovascular anatomy and physiology 11

For an MI the more distal the occlusion (that is further from the aorta) the less potential damage the heart attack will have In other words the worst occlusions are proximal Whether or not a patient survives an MI comes down to the numbers The loss of over 40 of the left ventricular muscle mass results in death However that 40 is cumulative For example a person can survive an MI that costs him 30 of left ventricular muscle mass but if that heart attack is followed by another that kills 15 of the left ventricular muscle mass the second (milder) heart attack will prove fatal This is the reason many people survive a first heart attack but die of a second or third attack even though their fatal MI may be relatively mild

The LCA branches quickly to the left main which branches almost at once to the LAD The LAD runs on the outside of the heart along the front of the heart along the septum The left main supplies the left atrium and the majority of the left ventricle in other words the major pumping por-tion of the heart An occlusion in the left main can be particularly disastrous because it is so far upstream and affects the heartrsquos left ventricular muscle mass No wonder they call the left main the widow maker

The coronary arteries do not run in straight lines they twist and turn and sometimes make very sharp bends People with coronary artery disease (CAD) typically have plaque buildup in the areas where the coronary arteries bend at sharp angles These are areas where the blood flow creates a lot of turbulence and plaque and other substances can collect and build up It is typically to find blockage and occlusions at these places

The LAD supplies the anterior wall of the left ventricle and the septum with oxygen-rich blood Since part of the heartrsquos conduction system runs through the septum (the right and left bundle branches) an LAD occlusion can damage the heartrsquos ability to conduct electricity properly

The circumflex artery runs along the AV groove laterally and thus supplies oxygen-rich blood to the lateral wall of the left ventricle along with the left atrium The circumflex runs along the AV groove to the left heading toward the back of the heart where it meets with the RCA which

travels along the AV groove to the left toward the back of the heart Both the circumflex and the RCA are responsible for providing oxygenated blood to the posterior wall of the ventricle But which is the more important provider That depends on crux and whether the heart is right or left dominant

The crux refers to an imaginary line drawn exactly down the middle of the ventricular poste-rior wall without any regard to anatomical landmarks If the RCA crosses the crux then the heart is said to be right dominant and the RCA is the greater supplier of oxygenated blood to the ven-tricular posterior wall On the other hand if the circumflex crosses the crux then the heart is said to be left dominant and the circumflex is the more significant provider Roughly 60 of the population is right dominant

While this information about the vasculature is important it is even more important for clini-cians to recognize that there is tremendous interpatient variability in terms of venous anatomy While we can describe major structures and typical venous formations anomalies are very common These anomalies may be minor variations or they can be very pronounced such as missing veins and arteries Clinicians who deal with the cardiovascular system must be prepared for large and small differences in vascular anatomy The cardiovascular system is incredibly resilient and can adapt to some anomalies by building up collateral circulation For example a person with a missing or incompetent vessel may over time build up a network of smaller collateral vessels that compensates by doing the same job It is not unusual to see vessels grow larger or branch out to compensate Sometimes when an artery is blocked collateral circulation will build up and compensate by delivering oxygen-rich blood to the heart muscle

Every artery in the body has a matching or corresponding vein usually located in close proximity This applies to the coronary system as well Note that there are great differences in coro-nary venous anatomy among patients which becomes important when implanting left ventric-ular leads for CRT devices (see Figure 111)

12 The nuts and bolts of implantable device therapy pacemakers

the nuts and bolts of cardiovascular anatomy and physiology

bull The heart consists of four chambers which may be considered the upper chambers (right atrium and left atrium) and lower chambers (right ven-tricle and left ventricle) or may be thought of as the right heart (right atrium and right ventricle) and left heart (left atrium and left ventricle)

bull Deoxygenated blood enters the right atrium flows to the right ventricle and is pumped out via the pulmonary valve to the lungs The blood is reoxygenated in the lungs and then enters the left atrium and flows to the left atrium and out of the aortic valve into the aorta and the rest of the body

bull The heart has two outflow tracts a right ven-tricular outflow tract (RVOT) and a left ven-tricular outflow tract (LVOT)

bull The largest chamber of the heart is the left ven-tricle making up about 23 of the cardiac mass The left ventricle is muscular and massive because it must pump against approximately 10 times the pressure against which the right ven-tricle pumps (120 vs 12 mmHg)

bull The two cardiac valves most susceptible to dysfunction and disease are the mitral and aortic valve because they function in a high-pressure environment

bull The chordae tendineae connect the leaflets of the valves to the papillary muscles and look like little parachutes These help the valves open and close properly

bull In general arteries transport blood away from the heart are muscular and contract to help move the blood on its way and have thicker walls than veins Veins have thinner less muscular walls and carry oxygen-depleted blood back to the heart using tiny interior valves to keep the blood flow moving forward Arteries transport oxygenated blood and can be considered high-pressure systems while veins transported deoxygenated blood in a low-pressure system

bull Cardiac contraction is systole cardiac relaxation is diastole Atrial systole occurs at the very end of ventricular diastole in something nicknamed atrial kick In a healthy individual about 20ndash30 of cardiac output comes from atrial kick

bull The heart is a muscle that is fed with oxygenated blood through a network of coronary arteries Coronary arteries get oxygen-rich blood from the aorta and they perfuse the heart muscle during diastole as blood drains back

bull Although arteries and veins generally take blood away from or to the heart respectively

Rightventricle

Leftventricle

Leftatrium

Aorta

Rightatrium

Rightcoronaryartery

Leftcoronaryartery

Circumflexartery

Superiorvena cava

Inferiorvena cava

Arterial system

Rightventricle

Leftventricle

Greatcardiac vein

Smallcardiac vein

Middlecardiac vein

Leftatrium

Aorta

Rightatrium

Superiorvena cava

Inferiorvena cava

Venous system

Coronarysinus

Figure 111 The coronary artery system is matched by a coronary venous system

Chapter 1 Cardiovascular anatomy and physiology 13

test your knowledge

Fill in these anatomical landmarks of the heart They include the four chambers the four valves and the bodyrsquos main artery

the pulmonary vessels are the exception The pulmonary artery transports deoxygenated blood while the pulmonary veins carry oxygen-ated blood This is because of their special loca-tion and the fact that their work is taking blood from the right side to the left side of the heart

bull The main coronary arteries are the right coro-nary artery and the left coronary artery The left coronary artery branches quickly to a short vessel known as the left main (the widow maker) and the circumflex artery

bull A myocardial infarction can kill the heart muscle The clogged arteries determine the loc ation of the affected muscle A right coro-nary artery occlusion will affect the right atrium and ventricle and the inferior wall of the left ventricle while a left coronary artery occlusion will affect the left atrium and most of the left ventricle By far the most dangerous occlusion is in the left main because it feeds the majority of the heart muscle with oxygen-rich blood

answer the following questions

1 What are the names of the two atrioventricular valves of the heartA Atrial and ventricularB Tricuspid and mitralC Pulmonary and pulmonicD Aortic and superior vena cava

2 Which chambers of the heart contain trabeculaeA AtriaB VentriclesC Right-sided chambers onlyD Left-sided chambers only

14 The nuts and bolts of implantable device therapy pacemakers

3 Why can trabeculae be important to device cliniciansA They are good places to secure a passive-fixa-

tion leadB They help regulate blood pressureC They prevent atherosclerosisD All of the above

4 Which of the following is not true about the aortaA It is an arteryB It consists of an ascending portion a descend-

ing portion and an archC It carries only oxygenated bloodD It is the largest vein in the body

5 From which artery does blood enter the right atriumA Superior vena cavaB Inferior vena cavaC Coronary sinusD All of the above

6 What is atrial kickA Ventricular pressure on the tricuspid valve

forcing it shutB Electrical impulses originating in the atrium

to cause the heart to beat fasterC Atrial systole that forces the most blood possible

into the already-full and diastolic ventriclesD A type of arrhythmia

7 Why would a person with chronic AF likely have decreased cardiac outputA AF is associated with low blood volumeB AF causes the heart to beat very slowlyC AF may cause the heart to experience fre-

quent pausesD AF reduces or eliminates atrial kickmdashand

atrial kick contributes to cardiac output

8 Which of the following is often a preferred location for affixing a cardiac pacing leadA The right ventricular outflow tract (RVOT)B The left ventricular outflow tract (LVOT)C The excised right atrial appendageD The tricuspid valve

9 What connects the valve leaflets to the papillary musclesA TrabeculaeB ArteriesC Chordae tendineaeD Tiny valves

10 Which coronary artery is nicknamed the widow maker because an occlusion in this vessel will deprive the majority of the heartrsquos muscle mass of oxygen-rich bloodA Left mainB CircumflexC Right coronary arteryD Inferior vena cava

15

The Nuts and Bolts of Implantable Device Therapy Pacemakers First Edition Tom Kenny copy 2015 John Wiley amp Sons Ltd Published 2015 by John Wiley amp Sons Ltd

Chapter 2

Cardiac conduction system

Introduction

The sinoatrial (SA) node is the heartrsquos natural pacemaker In the healthy heart the SA node fires and the electrical stimulus conducts along special pathways through the heart that cause cells in specific areas to depolarize After the stimulus passes the area the cells repolarize Myocardial cell depolarization and repolarization govern the heartrsquos contraction and relaxation sequencemdashin other words the heart rhythm Rhythm disorders occur when this healthy conduction system is in some way disrupted or impaired

the conduction pathways

When the SA node fires and sends out electrical energy the stimulus travels from the SA node in the high right atrium out over the atria via path-ways known as internodal tracts Most of these internodal tracts are located in the right atrium the

main tract over to the left atrium is called Bachmannrsquos bundle It is because of Bachmannrsquos bundle linking the right atrium to the left atrium that the atria can contract simultaneously that is as these two upper chambers contract as one coherent unit After traversing the atria the impulse comes down through conduction tracts to the atrioventricular (AV) node

The AV node is an area of highly specialized tissue located below the atria and above the ventri-cles on the right side of the septum Anatomically the AV node rests in the AV groove The tissue in the healthy AV node has the ability to change the speed of the electrical impulse specifically it slows the stimulus down The slowdown serves two important purposes First slowing the impulse allows the atria time to contract and provide the very valuable atrial kick to ventricular filling Second the slowed impulse protects the ventricles from being forced to beat too quickly that is to beat before they have been adequately filled with blood

Once the impulse crosses the AV node it travels down into the ventricles and accelerates its speed The impulse travels through the bundle of His (pronounced hiss) to the bundle branches (right and left) and then out to the network of Purkinje fibers This allows both ventricles to depolarize and contract simultaneously (Figure 21)

polarization depolarization and repolarization

Cardiac conduction is driven by the ability of myo-cardial cells to be depolarized and repolarized Depolarization occurs at the cellular level and is

2

Learning objectives

bullensp Describe the electrical conduction system of the healthy heart

bullensp List the main anatomical landmarks of normal cardiac conduction

bullensp Name the anatomical location of the heartrsquos natural pacemaker

bullensp Explain depolarization and repolarization in electrical terms and how it relates to the cardiac contraction

bullensp Define the term automaticity and how it relates to cardiac conduction

16 The nuts and bolts of implantable device therapy pacemakers

in turn driven by electrolytes or positively and negatively charged ions (potassium sodium and calcium mainly) which are able to move in and out of the cardiac cells A myocardial cell may be said to be polarized when in its resting state that is there is no electrical activity going on In the rest-ing state the interior of the myocardial cell is somewhat more negatively charged that the outside This state is called the resting state or sometimes phase 0 of the action potential

An electrical stimulus changes the resting state abruptly The electrical energy allows for the movement of ions to the point that the interior of the myocardial cell is now more positive than the outside In other words the charge reverses and the cell is said to be depolarized When this happens the cell contracts when this happens to a large area of the myocardium the heart muscle contracts Cardiac contraction occurs as a result of cellular depolarization

When the electrical stimulus moves past the myocardial cell the ions move in reverse and the interior of the cell assumes its previous negative charge with respect to the outside This process is

called repolarization and it brings the myocar-dial cell back to the resting state When this happens the cell relaxes When a large area of the myocardium repolarizes the heart resumes its previous shape (loses its contraction)

Intrinsic pacemakers

The healthy heartrsquos natural pacemaker is the SA node but the SA node is not the only area of the heart that can generate an electrical pulse The SA node serves as the heartrsquos pacemaker because it generates these pulses faster than other areas In the healthy heart the SA node can fire at around 60ndash100 times a minute corresponding to a healthy heart rate The AV junction can also fire but it is much slower at around 40ndash60 times a minute Even the Purkinje fibers (the ventri-cles) can act as a pacemaker but they fire at only 20ndash40 times a minute Thus in a healthy individual the SA node outpaces these other potential pacemakers The AV junction and ven-tricles are sometimes called backup pacemakers because they take over the heart rate in the event

Sinoatrial (SA) node

Internodal tracts

Right bundle branch

Left bundle branch

Purkinje fibers

Bachmannrsquos bundle

Bundle of His

Atrioventricular (AV) node

Figure 21 The conduction pathways in the healthy heart

Chapter 2 Cardiac conduction system 17

that the SA node is not able to pace the heart While these backup pacemakers may help keep the heart beating in the event of a dysfunctional SA node they are slow and not as reliable as the SA node

The healthy heart is able to pace itself because of a property called automaticity Automaticity is an inherent property of individual cardiac conductive cells that allows them to depolarize spontaneously In other words some cells in the heart (such as the cells of the SA node) are able to generate an electrical stimulus on their own

refractoriness

Refractoriness refers to the temporary inability of a cardiac cell to respond to an electrical stimulus Refractoriness can be the normal appropriate and expected behavior of cardiac cells For example immediately after electrical energy has passed through cardiac cells they go through a phase where they are unresponsive to electrical energy no matter how much voltage is delivered This normal refractoriness is called the physiological refractory period and it is very brief (it is mea-sured in thousandths of a second) Of course the exact duration of a refractory period is based in part on the heart rate the faster the heart rate the shorter the refractory period and vice versa

Refractoriness may also occur when certain cells become dysfunctional that is cardiac cells may lose their ability to conduct properly Furthermore the heartrsquos inherent pacemakers may change their rate for instance the AV junction may speed up and start to deliver impulses at 80 or 100 times a minute When this happensmdashthat is when another pacemaker takes over the role of the SA nodemdashthat pacemaker is called a usurping pacemaker because it usurps the rightful role of the SA node In the event that other pacemakers of the heart start to compete with the SA node the fastest pacemaker always wins

Conductivity

Conductivity is the cardiac cellrsquos ability to transmit an electrical impulse to an adjacent cell Typically cardiac conduction is antegrade that

is it moves forward Electrical impulses actually travel out in all different directions but refracto-riness prevents those cells that were just stimu-lated from depolarization This keeps conduction moving in a single direction through the heart Normal antegrade conduction in a healthy heart goes from SA node to atria to AV node over the bundle of His and down to the ventricles Some individuals have the ability to conduct retro-grade retrograde conduction is abnormal but not uncommon

autonomic nervous system

The heart does not need direct stimulation from the nervous system to generate electricity and to beat but that is not to say that the nervous system does not control the heart rate The autonomic nervous system indirectly affects cardiac conduction by regulating how fast the heart will beat and how hard the heart muscle will squeeze (contractility) The autonomic ner-vous system communicates with the heart through a network of baroreceptors which respond to chemical signals

The autonomic nervous system of the body like certain other physiological systems exists as two systems in balance The autonomic nervous system consists of a sympathetic system and a parasympathetic system They both work together and in a healthy individual one may overdrive the other during specific times The sympathetic ner-vous system is mainly associated with epinephrine (adrenaline) and it has the ability to speed the heart rate and increase cardiac contractility and can increase the speed by which electrical impulses can cross the AV node The sympathetic nervous system takes over when we are stressed frightened (fight-or-flight response) or exerting ourselves

On the other hand the autonomic nervous system also has a parasympathetic system that is associated with acetylcholine This system can decrease the heart rate decrease cardiac contrac-tility and cause impulses crossing the AV node to be slowed The parasympathetic system might take over when we are sleeping or at rest

18 The nuts and bolts of implantable device therapy pacemakers

the nuts and bolts of the cardiac conduction system

bull The heartrsquos natural pacemaker is the sinoatrial (SA) node which generates an electrical impulse spontaneously about 60ndash100 times a minute in a healthy individual

bull The normal conduction path is from the SA node over the atria to the atrioventricular (AV) node and then via the bundle of His down the right and left bundle branches and into the Purkinje fibers

bull The AV node has the ability to slow down the electrical impulse This serves two main purposes it allows for atrial kick and it protects the ventricles from beating too rapidly

bull In the healthy heart the right and left atria contract simultaneously and then the right and left ventricles contract simultaneously

bull Depolarization and repolarization of myocar-dial cells are driven by positively and negatively charged ions which cross the cell membrane A resting cardiac cell is more negative on the inside than the surrounding media but when it depolarizes it becomes more positive on the inside relative to the surrounding media Following depolarization the ions move and the cell returns to being more negative on the inside than the outside in a process known as repolarization

bull Depolarization causes the cardiac cell to contract

bull The main ions involved in depolarization and repolarization are sodium potassium and calcium ions

bull Although the heartrsquos natural pacemaker is the SA node the AV junction and the Purkinje fibers (ventricles) can also assume (usurp) this role if the SA node cannot generate electricity properly The AV junction and Purkinje fibers are not as reliable as a healthy AV node and typically generate electricity more slowly (the AV junction about 40ndash60 times a minute and the Purkinje fibers 20ndash40 times a minute)

bull However sometimes the AV junction or Purkinje fibers may generate electricity very

quickly this is abnormal When they do they may take over the heart When pacemakers of the heart compete the fastest one drives the heart

bull Automaticity is an inherent property of certain cardiac conductive cells to depolarize spontaneously Not all cardiac conduction cells possess automaticity but the SA node AV junction and Purkinje fibers possess automaticity

bull Refractoriness is the natural and expected behavior of cardiac conductive cells that makes them unresponsive to electrical impulses immediately after they have depolarized The natural refractory period is very brief

bull The faster the heart rate the shorter the refractory period the slower the heart rate the longer the refractory period

bull Refractoriness can also occur as a dysfunction bull Conductivity is the cardiac cellrsquos ability to

transmit an electrical impulse to adjacent cells Refractoriness means that cells that just depolarized will not depolarize again This keeps conduction moving forward (antegrade conduction)

bull Antegrade conduction is normal retrograde conduction is abnormal but is not rare

bull The autonomic nervous system regulates the heart rate by controlling how fast the heart beats and how vigorously it contracts

bull The autonomic nervous system consists of a sympathetic and parasympathetic nervous system which work together but at certain times one or the other is dominant The sympathetic nervous system makes the heart beat faster and more vigorously it takes over during times of stress exertion or fight or flight The parasympathetic ner-vous system makes the heart beat slowly and less vigorously it takes over during periods of rest

bull The neurotransmitters associated with these two systems are epinephrine or adrenaline (sympathetic nervous system) and acetylcho-line (parasympathetic nervous system)

Chapter 2 Cardiac conduction system 19

test your knowledge

Please write in the landmarks of the cardiac conduction system Select from the following (each is used only once)

AV node Bachmanrsquos bundle Bundle of His

Internodal tracts Left bundle branch Purkinje fibers

Right bundle branch Ventricles

1 Which of the following is not considered one of the heartrsquos intrinsic pacemakersA AV junctionB SA nodeC Purkinje fibersD Aorta

2 Depolarization and repolarization involve the movement of charged ions across cardiac cell membranes What types of ions are primarily involvedA Calcium potassium and sodiumB Acetylcholine and epinephrineC Magnesium and folateD All of the above

3 Refractoriness refers toA The ability of cardiac conduction cells to

contract

B The inability of cardiac conduction cells to respond to an electrical stimulus

C The unique permeability of the cardiac cellsD Abnormal cardiac conduction

4 When a cardiac cell depolarizes the interior of the cell becomes what with respect to the surrounding mediaA More positiveB More negativeC More acidicD Hotter

5 Fill in the blank In the healthy heart the faster the heart rate the ___ the refractory periodA More vigorousB Less reliableC LongerD Shorter

20 The nuts and bolts of implantable device therapy pacemakers

6 Which specific part of the nervous system governs the fight-or-flight response and is associated with epinephrine (adrenaline) (Pick the most exact answer)A Autonomic nervous systemB Parasympathetic nervous systemC Sympathetic nervous systemD Central nervous system

7 In normal antegrade conduction the electrical impulse travels from the SA node over the atria to the AV node and to what structure before reaching the ventriclesA The intermodal tractsB The Purkinje fibersC The bundle of HisD The ventricular apex

8 Which of the following is true about retrograde cardiac conductionA Everyone has itB It is perfectly normal and can be helpful in

stressful situations

C It is abnormal but not uncommonD It is impossible to diagnose definitively

except in an autopsy

9 What chemical is associated with the parasympathetic nervous systemA AcetylcholineB PotassiumC EpinephrineD Dopamine

10 The stimulation of the myocardial cells of which of the following structures is most closely associated with ventricular depolarizationA SA nodeB AV junctionC Bundle of HisD Purkinje fibers

10 The nuts and bolts of implantable device therapy pacemakers

the great middle and small cardiac veins all drain the CS takes this blood and delivers it back to the right atrium

The PV carry reoxygenated blood from the lungs and deliver it to the left atrium so that it can be pumped back out to the body by the left ventricle

The most important artery in the body is by far the aorta The aorta is connected to the left ven-tricle and it is the first and main conduit that takes reoxygenated blood from the left ventricle and sends it out to the body As this reoxygenated blood makes its way out into the body its first two stops are the head (brain) and the heart muscle Inadequate oxygenated blood to the brain can very quickly provoke symptoms This is the reason that depressed cardiac output is associated with symptoms like dizziness light-headedness feeling woozy and fatigue Severe symptoms might include syncope

The coronary arteries arise from the root of the aorta just above the cusps of the aortic valve Coronary arteries receive most blood during dias-tole so blood is more or less pouring back into these vessels The coronary artery system is a networkmdashalmost a meshmdashof middle-sized to very small vessels that cover the exterior surface of the heart (see Figure 110)

Coronary arteries get their name from the Latin word for crown (such as in our word coronation) because this network of vessels sits like a crown atop the heart and encircles it The heart gets blood from outside in as oxygen-rich blood in these coro-nary arteries is delivered to the exterior of the heart muscle Coronary arteries are not within the heart they are epicardial structures The two main coro-nary arteries are the right coronary artery (RCA) and the left coronary artery (LCA) The RCA and LCA come off the aorta and are located in the AV groove an exterior structure below the atria and above the ventricles The RCA runs from the aorta in the right AV groove while the LCA comes off the aorta and quickly branches to the left main The left main is a very short artery (about the size of a thumbnail) which branches quickly into the left anterior descending artery (LAD) and the circum-flex artery The LAD travels down the front of the

left side of the heart The circumflex artery can be found in the left AV groove

The RCA bifurcates to the posterior descending artery The right atrium and right ventricle get their supply of oxygen-rich blood from the RCA Much of the cardiac conduction system is right sided that is it commences with the sinoatrial (SA) node in the high right atrium and travels down to the AV node on the right side of the septum These important electrical structures get their oxygen-rich blood supply from the RCA The electrical system travels down the ventricles to the inferior wall of the left ventricle (located at the bottom of the heart but belonging to the left ventricle) the inferior wall is also fed oxygen-rich blood by the RCA An occlusion of the RCA such as might occur in a myocardial infarction (MI) may result in death of the heart muscle it feeds which includes the right atrium the right ventricle and the inferior wall of the left ventricle and it might affect the conduction system including the SA node and the AV node An inferior wall MI is typically the result of a clogged RCA

RCA

LCA

Aorta

Figure 110 The coronary arteries of the heart branch out to surround the entire heart muscle The two most prominent coronary arteries are the right coronary artery (RCA) and the left coronary artery (LCA)

Chapter 1 Cardiovascular anatomy and physiology 11

For an MI the more distal the occlusion (that is further from the aorta) the less potential damage the heart attack will have In other words the worst occlusions are proximal Whether or not a patient survives an MI comes down to the numbers The loss of over 40 of the left ventricular muscle mass results in death However that 40 is cumulative For example a person can survive an MI that costs him 30 of left ventricular muscle mass but if that heart attack is followed by another that kills 15 of the left ventricular muscle mass the second (milder) heart attack will prove fatal This is the reason many people survive a first heart attack but die of a second or third attack even though their fatal MI may be relatively mild

The LCA branches quickly to the left main which branches almost at once to the LAD The LAD runs on the outside of the heart along the front of the heart along the septum The left main supplies the left atrium and the majority of the left ventricle in other words the major pumping por-tion of the heart An occlusion in the left main can be particularly disastrous because it is so far upstream and affects the heartrsquos left ventricular muscle mass No wonder they call the left main the widow maker

The coronary arteries do not run in straight lines they twist and turn and sometimes make very sharp bends People with coronary artery disease (CAD) typically have plaque buildup in the areas where the coronary arteries bend at sharp angles These are areas where the blood flow creates a lot of turbulence and plaque and other substances can collect and build up It is typically to find blockage and occlusions at these places

The LAD supplies the anterior wall of the left ventricle and the septum with oxygen-rich blood Since part of the heartrsquos conduction system runs through the septum (the right and left bundle branches) an LAD occlusion can damage the heartrsquos ability to conduct electricity properly

The circumflex artery runs along the AV groove laterally and thus supplies oxygen-rich blood to the lateral wall of the left ventricle along with the left atrium The circumflex runs along the AV groove to the left heading toward the back of the heart where it meets with the RCA which

travels along the AV groove to the left toward the back of the heart Both the circumflex and the RCA are responsible for providing oxygenated blood to the posterior wall of the ventricle But which is the more important provider That depends on crux and whether the heart is right or left dominant

The crux refers to an imaginary line drawn exactly down the middle of the ventricular poste-rior wall without any regard to anatomical landmarks If the RCA crosses the crux then the heart is said to be right dominant and the RCA is the greater supplier of oxygenated blood to the ven-tricular posterior wall On the other hand if the circumflex crosses the crux then the heart is said to be left dominant and the circumflex is the more significant provider Roughly 60 of the population is right dominant

While this information about the vasculature is important it is even more important for clini-cians to recognize that there is tremendous interpatient variability in terms of venous anatomy While we can describe major structures and typical venous formations anomalies are very common These anomalies may be minor variations or they can be very pronounced such as missing veins and arteries Clinicians who deal with the cardiovascular system must be prepared for large and small differences in vascular anatomy The cardiovascular system is incredibly resilient and can adapt to some anomalies by building up collateral circulation For example a person with a missing or incompetent vessel may over time build up a network of smaller collateral vessels that compensates by doing the same job It is not unusual to see vessels grow larger or branch out to compensate Sometimes when an artery is blocked collateral circulation will build up and compensate by delivering oxygen-rich blood to the heart muscle

Every artery in the body has a matching or corresponding vein usually located in close proximity This applies to the coronary system as well Note that there are great differences in coro-nary venous anatomy among patients which becomes important when implanting left ventric-ular leads for CRT devices (see Figure 111)

12 The nuts and bolts of implantable device therapy pacemakers

the nuts and bolts of cardiovascular anatomy and physiology

bull The heart consists of four chambers which may be considered the upper chambers (right atrium and left atrium) and lower chambers (right ven-tricle and left ventricle) or may be thought of as the right heart (right atrium and right ventricle) and left heart (left atrium and left ventricle)

bull Deoxygenated blood enters the right atrium flows to the right ventricle and is pumped out via the pulmonary valve to the lungs The blood is reoxygenated in the lungs and then enters the left atrium and flows to the left atrium and out of the aortic valve into the aorta and the rest of the body

bull The heart has two outflow tracts a right ven-tricular outflow tract (RVOT) and a left ven-tricular outflow tract (LVOT)

bull The largest chamber of the heart is the left ven-tricle making up about 23 of the cardiac mass The left ventricle is muscular and massive because it must pump against approximately 10 times the pressure against which the right ven-tricle pumps (120 vs 12 mmHg)

bull The two cardiac valves most susceptible to dysfunction and disease are the mitral and aortic valve because they function in a high-pressure environment

bull The chordae tendineae connect the leaflets of the valves to the papillary muscles and look like little parachutes These help the valves open and close properly

bull In general arteries transport blood away from the heart are muscular and contract to help move the blood on its way and have thicker walls than veins Veins have thinner less muscular walls and carry oxygen-depleted blood back to the heart using tiny interior valves to keep the blood flow moving forward Arteries transport oxygenated blood and can be considered high-pressure systems while veins transported deoxygenated blood in a low-pressure system

bull Cardiac contraction is systole cardiac relaxation is diastole Atrial systole occurs at the very end of ventricular diastole in something nicknamed atrial kick In a healthy individual about 20ndash30 of cardiac output comes from atrial kick

bull The heart is a muscle that is fed with oxygenated blood through a network of coronary arteries Coronary arteries get oxygen-rich blood from the aorta and they perfuse the heart muscle during diastole as blood drains back

bull Although arteries and veins generally take blood away from or to the heart respectively

Rightventricle

Leftventricle

Leftatrium

Aorta

Rightatrium

Rightcoronaryartery

Leftcoronaryartery

Circumflexartery

Superiorvena cava

Inferiorvena cava

Arterial system

Rightventricle

Leftventricle

Greatcardiac vein

Smallcardiac vein

Middlecardiac vein

Leftatrium

Aorta

Rightatrium

Superiorvena cava

Inferiorvena cava

Venous system

Coronarysinus

Figure 111 The coronary artery system is matched by a coronary venous system

Chapter 1 Cardiovascular anatomy and physiology 13

test your knowledge

Fill in these anatomical landmarks of the heart They include the four chambers the four valves and the bodyrsquos main artery

the pulmonary vessels are the exception The pulmonary artery transports deoxygenated blood while the pulmonary veins carry oxygen-ated blood This is because of their special loca-tion and the fact that their work is taking blood from the right side to the left side of the heart

bull The main coronary arteries are the right coro-nary artery and the left coronary artery The left coronary artery branches quickly to a short vessel known as the left main (the widow maker) and the circumflex artery

bull A myocardial infarction can kill the heart muscle The clogged arteries determine the loc ation of the affected muscle A right coro-nary artery occlusion will affect the right atrium and ventricle and the inferior wall of the left ventricle while a left coronary artery occlusion will affect the left atrium and most of the left ventricle By far the most dangerous occlusion is in the left main because it feeds the majority of the heart muscle with oxygen-rich blood

answer the following questions

1 What are the names of the two atrioventricular valves of the heartA Atrial and ventricularB Tricuspid and mitralC Pulmonary and pulmonicD Aortic and superior vena cava

2 Which chambers of the heart contain trabeculaeA AtriaB VentriclesC Right-sided chambers onlyD Left-sided chambers only

14 The nuts and bolts of implantable device therapy pacemakers

3 Why can trabeculae be important to device cliniciansA They are good places to secure a passive-fixa-

tion leadB They help regulate blood pressureC They prevent atherosclerosisD All of the above

4 Which of the following is not true about the aortaA It is an arteryB It consists of an ascending portion a descend-

ing portion and an archC It carries only oxygenated bloodD It is the largest vein in the body

5 From which artery does blood enter the right atriumA Superior vena cavaB Inferior vena cavaC Coronary sinusD All of the above

6 What is atrial kickA Ventricular pressure on the tricuspid valve

forcing it shutB Electrical impulses originating in the atrium

to cause the heart to beat fasterC Atrial systole that forces the most blood possible

into the already-full and diastolic ventriclesD A type of arrhythmia

7 Why would a person with chronic AF likely have decreased cardiac outputA AF is associated with low blood volumeB AF causes the heart to beat very slowlyC AF may cause the heart to experience fre-

quent pausesD AF reduces or eliminates atrial kickmdashand

atrial kick contributes to cardiac output

8 Which of the following is often a preferred location for affixing a cardiac pacing leadA The right ventricular outflow tract (RVOT)B The left ventricular outflow tract (LVOT)C The excised right atrial appendageD The tricuspid valve

9 What connects the valve leaflets to the papillary musclesA TrabeculaeB ArteriesC Chordae tendineaeD Tiny valves

10 Which coronary artery is nicknamed the widow maker because an occlusion in this vessel will deprive the majority of the heartrsquos muscle mass of oxygen-rich bloodA Left mainB CircumflexC Right coronary arteryD Inferior vena cava

15

The Nuts and Bolts of Implantable Device Therapy Pacemakers First Edition Tom Kenny copy 2015 John Wiley amp Sons Ltd Published 2015 by John Wiley amp Sons Ltd

Chapter 2

Cardiac conduction system

Introduction

The sinoatrial (SA) node is the heartrsquos natural pacemaker In the healthy heart the SA node fires and the electrical stimulus conducts along special pathways through the heart that cause cells in specific areas to depolarize After the stimulus passes the area the cells repolarize Myocardial cell depolarization and repolarization govern the heartrsquos contraction and relaxation sequencemdashin other words the heart rhythm Rhythm disorders occur when this healthy conduction system is in some way disrupted or impaired

the conduction pathways

When the SA node fires and sends out electrical energy the stimulus travels from the SA node in the high right atrium out over the atria via path-ways known as internodal tracts Most of these internodal tracts are located in the right atrium the

main tract over to the left atrium is called Bachmannrsquos bundle It is because of Bachmannrsquos bundle linking the right atrium to the left atrium that the atria can contract simultaneously that is as these two upper chambers contract as one coherent unit After traversing the atria the impulse comes down through conduction tracts to the atrioventricular (AV) node

The AV node is an area of highly specialized tissue located below the atria and above the ventri-cles on the right side of the septum Anatomically the AV node rests in the AV groove The tissue in the healthy AV node has the ability to change the speed of the electrical impulse specifically it slows the stimulus down The slowdown serves two important purposes First slowing the impulse allows the atria time to contract and provide the very valuable atrial kick to ventricular filling Second the slowed impulse protects the ventricles from being forced to beat too quickly that is to beat before they have been adequately filled with blood

Once the impulse crosses the AV node it travels down into the ventricles and accelerates its speed The impulse travels through the bundle of His (pronounced hiss) to the bundle branches (right and left) and then out to the network of Purkinje fibers This allows both ventricles to depolarize and contract simultaneously (Figure 21)

polarization depolarization and repolarization

Cardiac conduction is driven by the ability of myo-cardial cells to be depolarized and repolarized Depolarization occurs at the cellular level and is

2

Learning objectives

bullensp Describe the electrical conduction system of the healthy heart

bullensp List the main anatomical landmarks of normal cardiac conduction

bullensp Name the anatomical location of the heartrsquos natural pacemaker

bullensp Explain depolarization and repolarization in electrical terms and how it relates to the cardiac contraction

bullensp Define the term automaticity and how it relates to cardiac conduction

16 The nuts and bolts of implantable device therapy pacemakers

in turn driven by electrolytes or positively and negatively charged ions (potassium sodium and calcium mainly) which are able to move in and out of the cardiac cells A myocardial cell may be said to be polarized when in its resting state that is there is no electrical activity going on In the rest-ing state the interior of the myocardial cell is somewhat more negatively charged that the outside This state is called the resting state or sometimes phase 0 of the action potential

An electrical stimulus changes the resting state abruptly The electrical energy allows for the movement of ions to the point that the interior of the myocardial cell is now more positive than the outside In other words the charge reverses and the cell is said to be depolarized When this happens the cell contracts when this happens to a large area of the myocardium the heart muscle contracts Cardiac contraction occurs as a result of cellular depolarization

When the electrical stimulus moves past the myocardial cell the ions move in reverse and the interior of the cell assumes its previous negative charge with respect to the outside This process is

called repolarization and it brings the myocar-dial cell back to the resting state When this happens the cell relaxes When a large area of the myocardium repolarizes the heart resumes its previous shape (loses its contraction)

Intrinsic pacemakers

The healthy heartrsquos natural pacemaker is the SA node but the SA node is not the only area of the heart that can generate an electrical pulse The SA node serves as the heartrsquos pacemaker because it generates these pulses faster than other areas In the healthy heart the SA node can fire at around 60ndash100 times a minute corresponding to a healthy heart rate The AV junction can also fire but it is much slower at around 40ndash60 times a minute Even the Purkinje fibers (the ventri-cles) can act as a pacemaker but they fire at only 20ndash40 times a minute Thus in a healthy individual the SA node outpaces these other potential pacemakers The AV junction and ven-tricles are sometimes called backup pacemakers because they take over the heart rate in the event

Sinoatrial (SA) node

Internodal tracts

Right bundle branch

Left bundle branch

Purkinje fibers

Bachmannrsquos bundle

Bundle of His

Atrioventricular (AV) node

Figure 21 The conduction pathways in the healthy heart

Chapter 2 Cardiac conduction system 17

that the SA node is not able to pace the heart While these backup pacemakers may help keep the heart beating in the event of a dysfunctional SA node they are slow and not as reliable as the SA node

The healthy heart is able to pace itself because of a property called automaticity Automaticity is an inherent property of individual cardiac conductive cells that allows them to depolarize spontaneously In other words some cells in the heart (such as the cells of the SA node) are able to generate an electrical stimulus on their own

refractoriness

Refractoriness refers to the temporary inability of a cardiac cell to respond to an electrical stimulus Refractoriness can be the normal appropriate and expected behavior of cardiac cells For example immediately after electrical energy has passed through cardiac cells they go through a phase where they are unresponsive to electrical energy no matter how much voltage is delivered This normal refractoriness is called the physiological refractory period and it is very brief (it is mea-sured in thousandths of a second) Of course the exact duration of a refractory period is based in part on the heart rate the faster the heart rate the shorter the refractory period and vice versa

Refractoriness may also occur when certain cells become dysfunctional that is cardiac cells may lose their ability to conduct properly Furthermore the heartrsquos inherent pacemakers may change their rate for instance the AV junction may speed up and start to deliver impulses at 80 or 100 times a minute When this happensmdashthat is when another pacemaker takes over the role of the SA nodemdashthat pacemaker is called a usurping pacemaker because it usurps the rightful role of the SA node In the event that other pacemakers of the heart start to compete with the SA node the fastest pacemaker always wins

Conductivity

Conductivity is the cardiac cellrsquos ability to transmit an electrical impulse to an adjacent cell Typically cardiac conduction is antegrade that

is it moves forward Electrical impulses actually travel out in all different directions but refracto-riness prevents those cells that were just stimu-lated from depolarization This keeps conduction moving in a single direction through the heart Normal antegrade conduction in a healthy heart goes from SA node to atria to AV node over the bundle of His and down to the ventricles Some individuals have the ability to conduct retro-grade retrograde conduction is abnormal but not uncommon

autonomic nervous system

The heart does not need direct stimulation from the nervous system to generate electricity and to beat but that is not to say that the nervous system does not control the heart rate The autonomic nervous system indirectly affects cardiac conduction by regulating how fast the heart will beat and how hard the heart muscle will squeeze (contractility) The autonomic ner-vous system communicates with the heart through a network of baroreceptors which respond to chemical signals

The autonomic nervous system of the body like certain other physiological systems exists as two systems in balance The autonomic nervous system consists of a sympathetic system and a parasympathetic system They both work together and in a healthy individual one may overdrive the other during specific times The sympathetic ner-vous system is mainly associated with epinephrine (adrenaline) and it has the ability to speed the heart rate and increase cardiac contractility and can increase the speed by which electrical impulses can cross the AV node The sympathetic nervous system takes over when we are stressed frightened (fight-or-flight response) or exerting ourselves

On the other hand the autonomic nervous system also has a parasympathetic system that is associated with acetylcholine This system can decrease the heart rate decrease cardiac contrac-tility and cause impulses crossing the AV node to be slowed The parasympathetic system might take over when we are sleeping or at rest

18 The nuts and bolts of implantable device therapy pacemakers

the nuts and bolts of the cardiac conduction system

bull The heartrsquos natural pacemaker is the sinoatrial (SA) node which generates an electrical impulse spontaneously about 60ndash100 times a minute in a healthy individual

bull The normal conduction path is from the SA node over the atria to the atrioventricular (AV) node and then via the bundle of His down the right and left bundle branches and into the Purkinje fibers

bull The AV node has the ability to slow down the electrical impulse This serves two main purposes it allows for atrial kick and it protects the ventricles from beating too rapidly

bull In the healthy heart the right and left atria contract simultaneously and then the right and left ventricles contract simultaneously

bull Depolarization and repolarization of myocar-dial cells are driven by positively and negatively charged ions which cross the cell membrane A resting cardiac cell is more negative on the inside than the surrounding media but when it depolarizes it becomes more positive on the inside relative to the surrounding media Following depolarization the ions move and the cell returns to being more negative on the inside than the outside in a process known as repolarization

bull Depolarization causes the cardiac cell to contract

bull The main ions involved in depolarization and repolarization are sodium potassium and calcium ions

bull Although the heartrsquos natural pacemaker is the SA node the AV junction and the Purkinje fibers (ventricles) can also assume (usurp) this role if the SA node cannot generate electricity properly The AV junction and Purkinje fibers are not as reliable as a healthy AV node and typically generate electricity more slowly (the AV junction about 40ndash60 times a minute and the Purkinje fibers 20ndash40 times a minute)

bull However sometimes the AV junction or Purkinje fibers may generate electricity very

quickly this is abnormal When they do they may take over the heart When pacemakers of the heart compete the fastest one drives the heart

bull Automaticity is an inherent property of certain cardiac conductive cells to depolarize spontaneously Not all cardiac conduction cells possess automaticity but the SA node AV junction and Purkinje fibers possess automaticity

bull Refractoriness is the natural and expected behavior of cardiac conductive cells that makes them unresponsive to electrical impulses immediately after they have depolarized The natural refractory period is very brief

bull The faster the heart rate the shorter the refractory period the slower the heart rate the longer the refractory period

bull Refractoriness can also occur as a dysfunction bull Conductivity is the cardiac cellrsquos ability to

transmit an electrical impulse to adjacent cells Refractoriness means that cells that just depolarized will not depolarize again This keeps conduction moving forward (antegrade conduction)

bull Antegrade conduction is normal retrograde conduction is abnormal but is not rare

bull The autonomic nervous system regulates the heart rate by controlling how fast the heart beats and how vigorously it contracts

bull The autonomic nervous system consists of a sympathetic and parasympathetic nervous system which work together but at certain times one or the other is dominant The sympathetic nervous system makes the heart beat faster and more vigorously it takes over during times of stress exertion or fight or flight The parasympathetic ner-vous system makes the heart beat slowly and less vigorously it takes over during periods of rest

bull The neurotransmitters associated with these two systems are epinephrine or adrenaline (sympathetic nervous system) and acetylcho-line (parasympathetic nervous system)

Chapter 2 Cardiac conduction system 19

test your knowledge

Please write in the landmarks of the cardiac conduction system Select from the following (each is used only once)

AV node Bachmanrsquos bundle Bundle of His

Internodal tracts Left bundle branch Purkinje fibers

Right bundle branch Ventricles

1 Which of the following is not considered one of the heartrsquos intrinsic pacemakersA AV junctionB SA nodeC Purkinje fibersD Aorta

2 Depolarization and repolarization involve the movement of charged ions across cardiac cell membranes What types of ions are primarily involvedA Calcium potassium and sodiumB Acetylcholine and epinephrineC Magnesium and folateD All of the above

3 Refractoriness refers toA The ability of cardiac conduction cells to

contract

B The inability of cardiac conduction cells to respond to an electrical stimulus

C The unique permeability of the cardiac cellsD Abnormal cardiac conduction

4 When a cardiac cell depolarizes the interior of the cell becomes what with respect to the surrounding mediaA More positiveB More negativeC More acidicD Hotter

5 Fill in the blank In the healthy heart the faster the heart rate the ___ the refractory periodA More vigorousB Less reliableC LongerD Shorter

20 The nuts and bolts of implantable device therapy pacemakers

6 Which specific part of the nervous system governs the fight-or-flight response and is associated with epinephrine (adrenaline) (Pick the most exact answer)A Autonomic nervous systemB Parasympathetic nervous systemC Sympathetic nervous systemD Central nervous system

7 In normal antegrade conduction the electrical impulse travels from the SA node over the atria to the AV node and to what structure before reaching the ventriclesA The intermodal tractsB The Purkinje fibersC The bundle of HisD The ventricular apex

8 Which of the following is true about retrograde cardiac conductionA Everyone has itB It is perfectly normal and can be helpful in

stressful situations

C It is abnormal but not uncommonD It is impossible to diagnose definitively

except in an autopsy

9 What chemical is associated with the parasympathetic nervous systemA AcetylcholineB PotassiumC EpinephrineD Dopamine

10 The stimulation of the myocardial cells of which of the following structures is most closely associated with ventricular depolarizationA SA nodeB AV junctionC Bundle of HisD Purkinje fibers

Chapter 1 Cardiovascular anatomy and physiology 11

For an MI the more distal the occlusion (that is further from the aorta) the less potential damage the heart attack will have In other words the worst occlusions are proximal Whether or not a patient survives an MI comes down to the numbers The loss of over 40 of the left ventricular muscle mass results in death However that 40 is cumulative For example a person can survive an MI that costs him 30 of left ventricular muscle mass but if that heart attack is followed by another that kills 15 of the left ventricular muscle mass the second (milder) heart attack will prove fatal This is the reason many people survive a first heart attack but die of a second or third attack even though their fatal MI may be relatively mild

The LCA branches quickly to the left main which branches almost at once to the LAD The LAD runs on the outside of the heart along the front of the heart along the septum The left main supplies the left atrium and the majority of the left ventricle in other words the major pumping por-tion of the heart An occlusion in the left main can be particularly disastrous because it is so far upstream and affects the heartrsquos left ventricular muscle mass No wonder they call the left main the widow maker

The coronary arteries do not run in straight lines they twist and turn and sometimes make very sharp bends People with coronary artery disease (CAD) typically have plaque buildup in the areas where the coronary arteries bend at sharp angles These are areas where the blood flow creates a lot of turbulence and plaque and other substances can collect and build up It is typically to find blockage and occlusions at these places

The LAD supplies the anterior wall of the left ventricle and the septum with oxygen-rich blood Since part of the heartrsquos conduction system runs through the septum (the right and left bundle branches) an LAD occlusion can damage the heartrsquos ability to conduct electricity properly

The circumflex artery runs along the AV groove laterally and thus supplies oxygen-rich blood to the lateral wall of the left ventricle along with the left atrium The circumflex runs along the AV groove to the left heading toward the back of the heart where it meets with the RCA which

travels along the AV groove to the left toward the back of the heart Both the circumflex and the RCA are responsible for providing oxygenated blood to the posterior wall of the ventricle But which is the more important provider That depends on crux and whether the heart is right or left dominant

The crux refers to an imaginary line drawn exactly down the middle of the ventricular poste-rior wall without any regard to anatomical landmarks If the RCA crosses the crux then the heart is said to be right dominant and the RCA is the greater supplier of oxygenated blood to the ven-tricular posterior wall On the other hand if the circumflex crosses the crux then the heart is said to be left dominant and the circumflex is the more significant provider Roughly 60 of the population is right dominant

While this information about the vasculature is important it is even more important for clini-cians to recognize that there is tremendous interpatient variability in terms of venous anatomy While we can describe major structures and typical venous formations anomalies are very common These anomalies may be minor variations or they can be very pronounced such as missing veins and arteries Clinicians who deal with the cardiovascular system must be prepared for large and small differences in vascular anatomy The cardiovascular system is incredibly resilient and can adapt to some anomalies by building up collateral circulation For example a person with a missing or incompetent vessel may over time build up a network of smaller collateral vessels that compensates by doing the same job It is not unusual to see vessels grow larger or branch out to compensate Sometimes when an artery is blocked collateral circulation will build up and compensate by delivering oxygen-rich blood to the heart muscle

Every artery in the body has a matching or corresponding vein usually located in close proximity This applies to the coronary system as well Note that there are great differences in coro-nary venous anatomy among patients which becomes important when implanting left ventric-ular leads for CRT devices (see Figure 111)

12 The nuts and bolts of implantable device therapy pacemakers

the nuts and bolts of cardiovascular anatomy and physiology

bull The heart consists of four chambers which may be considered the upper chambers (right atrium and left atrium) and lower chambers (right ven-tricle and left ventricle) or may be thought of as the right heart (right atrium and right ventricle) and left heart (left atrium and left ventricle)

bull Deoxygenated blood enters the right atrium flows to the right ventricle and is pumped out via the pulmonary valve to the lungs The blood is reoxygenated in the lungs and then enters the left atrium and flows to the left atrium and out of the aortic valve into the aorta and the rest of the body

bull The heart has two outflow tracts a right ven-tricular outflow tract (RVOT) and a left ven-tricular outflow tract (LVOT)

bull The largest chamber of the heart is the left ven-tricle making up about 23 of the cardiac mass The left ventricle is muscular and massive because it must pump against approximately 10 times the pressure against which the right ven-tricle pumps (120 vs 12 mmHg)

bull The two cardiac valves most susceptible to dysfunction and disease are the mitral and aortic valve because they function in a high-pressure environment

bull The chordae tendineae connect the leaflets of the valves to the papillary muscles and look like little parachutes These help the valves open and close properly

bull In general arteries transport blood away from the heart are muscular and contract to help move the blood on its way and have thicker walls than veins Veins have thinner less muscular walls and carry oxygen-depleted blood back to the heart using tiny interior valves to keep the blood flow moving forward Arteries transport oxygenated blood and can be considered high-pressure systems while veins transported deoxygenated blood in a low-pressure system

bull Cardiac contraction is systole cardiac relaxation is diastole Atrial systole occurs at the very end of ventricular diastole in something nicknamed atrial kick In a healthy individual about 20ndash30 of cardiac output comes from atrial kick

bull The heart is a muscle that is fed with oxygenated blood through a network of coronary arteries Coronary arteries get oxygen-rich blood from the aorta and they perfuse the heart muscle during diastole as blood drains back

bull Although arteries and veins generally take blood away from or to the heart respectively

Rightventricle

Leftventricle

Leftatrium

Aorta

Rightatrium

Rightcoronaryartery

Leftcoronaryartery

Circumflexartery

Superiorvena cava

Inferiorvena cava

Arterial system

Rightventricle

Leftventricle

Greatcardiac vein

Smallcardiac vein

Middlecardiac vein

Leftatrium

Aorta

Rightatrium

Superiorvena cava

Inferiorvena cava

Venous system

Coronarysinus

Figure 111 The coronary artery system is matched by a coronary venous system

Chapter 1 Cardiovascular anatomy and physiology 13

test your knowledge

Fill in these anatomical landmarks of the heart They include the four chambers the four valves and the bodyrsquos main artery

the pulmonary vessels are the exception The pulmonary artery transports deoxygenated blood while the pulmonary veins carry oxygen-ated blood This is because of their special loca-tion and the fact that their work is taking blood from the right side to the left side of the heart

bull The main coronary arteries are the right coro-nary artery and the left coronary artery The left coronary artery branches quickly to a short vessel known as the left main (the widow maker) and the circumflex artery

bull A myocardial infarction can kill the heart muscle The clogged arteries determine the loc ation of the affected muscle A right coro-nary artery occlusion will affect the right atrium and ventricle and the inferior wall of the left ventricle while a left coronary artery occlusion will affect the left atrium and most of the left ventricle By far the most dangerous occlusion is in the left main because it feeds the majority of the heart muscle with oxygen-rich blood

answer the following questions

1 What are the names of the two atrioventricular valves of the heartA Atrial and ventricularB Tricuspid and mitralC Pulmonary and pulmonicD Aortic and superior vena cava

2 Which chambers of the heart contain trabeculaeA AtriaB VentriclesC Right-sided chambers onlyD Left-sided chambers only

14 The nuts and bolts of implantable device therapy pacemakers

3 Why can trabeculae be important to device cliniciansA They are good places to secure a passive-fixa-

tion leadB They help regulate blood pressureC They prevent atherosclerosisD All of the above

4 Which of the following is not true about the aortaA It is an arteryB It consists of an ascending portion a descend-

ing portion and an archC It carries only oxygenated bloodD It is the largest vein in the body

5 From which artery does blood enter the right atriumA Superior vena cavaB Inferior vena cavaC Coronary sinusD All of the above

6 What is atrial kickA Ventricular pressure on the tricuspid valve

forcing it shutB Electrical impulses originating in the atrium

to cause the heart to beat fasterC Atrial systole that forces the most blood possible

into the already-full and diastolic ventriclesD A type of arrhythmia

7 Why would a person with chronic AF likely have decreased cardiac outputA AF is associated with low blood volumeB AF causes the heart to beat very slowlyC AF may cause the heart to experience fre-

quent pausesD AF reduces or eliminates atrial kickmdashand

atrial kick contributes to cardiac output

8 Which of the following is often a preferred location for affixing a cardiac pacing leadA The right ventricular outflow tract (RVOT)B The left ventricular outflow tract (LVOT)C The excised right atrial appendageD The tricuspid valve

9 What connects the valve leaflets to the papillary musclesA TrabeculaeB ArteriesC Chordae tendineaeD Tiny valves

10 Which coronary artery is nicknamed the widow maker because an occlusion in this vessel will deprive the majority of the heartrsquos muscle mass of oxygen-rich bloodA Left mainB CircumflexC Right coronary arteryD Inferior vena cava

15

The Nuts and Bolts of Implantable Device Therapy Pacemakers First Edition Tom Kenny copy 2015 John Wiley amp Sons Ltd Published 2015 by John Wiley amp Sons Ltd

Chapter 2

Cardiac conduction system

Introduction

The sinoatrial (SA) node is the heartrsquos natural pacemaker In the healthy heart the SA node fires and the electrical stimulus conducts along special pathways through the heart that cause cells in specific areas to depolarize After the stimulus passes the area the cells repolarize Myocardial cell depolarization and repolarization govern the heartrsquos contraction and relaxation sequencemdashin other words the heart rhythm Rhythm disorders occur when this healthy conduction system is in some way disrupted or impaired

the conduction pathways

When the SA node fires and sends out electrical energy the stimulus travels from the SA node in the high right atrium out over the atria via path-ways known as internodal tracts Most of these internodal tracts are located in the right atrium the

main tract over to the left atrium is called Bachmannrsquos bundle It is because of Bachmannrsquos bundle linking the right atrium to the left atrium that the atria can contract simultaneously that is as these two upper chambers contract as one coherent unit After traversing the atria the impulse comes down through conduction tracts to the atrioventricular (AV) node

The AV node is an area of highly specialized tissue located below the atria and above the ventri-cles on the right side of the septum Anatomically the AV node rests in the AV groove The tissue in the healthy AV node has the ability to change the speed of the electrical impulse specifically it slows the stimulus down The slowdown serves two important purposes First slowing the impulse allows the atria time to contract and provide the very valuable atrial kick to ventricular filling Second the slowed impulse protects the ventricles from being forced to beat too quickly that is to beat before they have been adequately filled with blood

Once the impulse crosses the AV node it travels down into the ventricles and accelerates its speed The impulse travels through the bundle of His (pronounced hiss) to the bundle branches (right and left) and then out to the network of Purkinje fibers This allows both ventricles to depolarize and contract simultaneously (Figure 21)

polarization depolarization and repolarization

Cardiac conduction is driven by the ability of myo-cardial cells to be depolarized and repolarized Depolarization occurs at the cellular level and is

2

Learning objectives

bullensp Describe the electrical conduction system of the healthy heart

bullensp List the main anatomical landmarks of normal cardiac conduction

bullensp Name the anatomical location of the heartrsquos natural pacemaker

bullensp Explain depolarization and repolarization in electrical terms and how it relates to the cardiac contraction

bullensp Define the term automaticity and how it relates to cardiac conduction

16 The nuts and bolts of implantable device therapy pacemakers

in turn driven by electrolytes or positively and negatively charged ions (potassium sodium and calcium mainly) which are able to move in and out of the cardiac cells A myocardial cell may be said to be polarized when in its resting state that is there is no electrical activity going on In the rest-ing state the interior of the myocardial cell is somewhat more negatively charged that the outside This state is called the resting state or sometimes phase 0 of the action potential

An electrical stimulus changes the resting state abruptly The electrical energy allows for the movement of ions to the point that the interior of the myocardial cell is now more positive than the outside In other words the charge reverses and the cell is said to be depolarized When this happens the cell contracts when this happens to a large area of the myocardium the heart muscle contracts Cardiac contraction occurs as a result of cellular depolarization

When the electrical stimulus moves past the myocardial cell the ions move in reverse and the interior of the cell assumes its previous negative charge with respect to the outside This process is

called repolarization and it brings the myocar-dial cell back to the resting state When this happens the cell relaxes When a large area of the myocardium repolarizes the heart resumes its previous shape (loses its contraction)

Intrinsic pacemakers

The healthy heartrsquos natural pacemaker is the SA node but the SA node is not the only area of the heart that can generate an electrical pulse The SA node serves as the heartrsquos pacemaker because it generates these pulses faster than other areas In the healthy heart the SA node can fire at around 60ndash100 times a minute corresponding to a healthy heart rate The AV junction can also fire but it is much slower at around 40ndash60 times a minute Even the Purkinje fibers (the ventri-cles) can act as a pacemaker but they fire at only 20ndash40 times a minute Thus in a healthy individual the SA node outpaces these other potential pacemakers The AV junction and ven-tricles are sometimes called backup pacemakers because they take over the heart rate in the event

Sinoatrial (SA) node

Internodal tracts

Right bundle branch

Left bundle branch

Purkinje fibers

Bachmannrsquos bundle

Bundle of His

Atrioventricular (AV) node

Figure 21 The conduction pathways in the healthy heart

Chapter 2 Cardiac conduction system 17

that the SA node is not able to pace the heart While these backup pacemakers may help keep the heart beating in the event of a dysfunctional SA node they are slow and not as reliable as the SA node

The healthy heart is able to pace itself because of a property called automaticity Automaticity is an inherent property of individual cardiac conductive cells that allows them to depolarize spontaneously In other words some cells in the heart (such as the cells of the SA node) are able to generate an electrical stimulus on their own

refractoriness

Refractoriness refers to the temporary inability of a cardiac cell to respond to an electrical stimulus Refractoriness can be the normal appropriate and expected behavior of cardiac cells For example immediately after electrical energy has passed through cardiac cells they go through a phase where they are unresponsive to electrical energy no matter how much voltage is delivered This normal refractoriness is called the physiological refractory period and it is very brief (it is mea-sured in thousandths of a second) Of course the exact duration of a refractory period is based in part on the heart rate the faster the heart rate the shorter the refractory period and vice versa

Refractoriness may also occur when certain cells become dysfunctional that is cardiac cells may lose their ability to conduct properly Furthermore the heartrsquos inherent pacemakers may change their rate for instance the AV junction may speed up and start to deliver impulses at 80 or 100 times a minute When this happensmdashthat is when another pacemaker takes over the role of the SA nodemdashthat pacemaker is called a usurping pacemaker because it usurps the rightful role of the SA node In the event that other pacemakers of the heart start to compete with the SA node the fastest pacemaker always wins

Conductivity

Conductivity is the cardiac cellrsquos ability to transmit an electrical impulse to an adjacent cell Typically cardiac conduction is antegrade that

is it moves forward Electrical impulses actually travel out in all different directions but refracto-riness prevents those cells that were just stimu-lated from depolarization This keeps conduction moving in a single direction through the heart Normal antegrade conduction in a healthy heart goes from SA node to atria to AV node over the bundle of His and down to the ventricles Some individuals have the ability to conduct retro-grade retrograde conduction is abnormal but not uncommon

autonomic nervous system

The heart does not need direct stimulation from the nervous system to generate electricity and to beat but that is not to say that the nervous system does not control the heart rate The autonomic nervous system indirectly affects cardiac conduction by regulating how fast the heart will beat and how hard the heart muscle will squeeze (contractility) The autonomic ner-vous system communicates with the heart through a network of baroreceptors which respond to chemical signals

The autonomic nervous system of the body like certain other physiological systems exists as two systems in balance The autonomic nervous system consists of a sympathetic system and a parasympathetic system They both work together and in a healthy individual one may overdrive the other during specific times The sympathetic ner-vous system is mainly associated with epinephrine (adrenaline) and it has the ability to speed the heart rate and increase cardiac contractility and can increase the speed by which electrical impulses can cross the AV node The sympathetic nervous system takes over when we are stressed frightened (fight-or-flight response) or exerting ourselves

On the other hand the autonomic nervous system also has a parasympathetic system that is associated with acetylcholine This system can decrease the heart rate decrease cardiac contrac-tility and cause impulses crossing the AV node to be slowed The parasympathetic system might take over when we are sleeping or at rest

18 The nuts and bolts of implantable device therapy pacemakers

the nuts and bolts of the cardiac conduction system

bull The heartrsquos natural pacemaker is the sinoatrial (SA) node which generates an electrical impulse spontaneously about 60ndash100 times a minute in a healthy individual

bull The normal conduction path is from the SA node over the atria to the atrioventricular (AV) node and then via the bundle of His down the right and left bundle branches and into the Purkinje fibers

bull The AV node has the ability to slow down the electrical impulse This serves two main purposes it allows for atrial kick and it protects the ventricles from beating too rapidly

bull In the healthy heart the right and left atria contract simultaneously and then the right and left ventricles contract simultaneously

bull Depolarization and repolarization of myocar-dial cells are driven by positively and negatively charged ions which cross the cell membrane A resting cardiac cell is more negative on the inside than the surrounding media but when it depolarizes it becomes more positive on the inside relative to the surrounding media Following depolarization the ions move and the cell returns to being more negative on the inside than the outside in a process known as repolarization

bull Depolarization causes the cardiac cell to contract

bull The main ions involved in depolarization and repolarization are sodium potassium and calcium ions

bull Although the heartrsquos natural pacemaker is the SA node the AV junction and the Purkinje fibers (ventricles) can also assume (usurp) this role if the SA node cannot generate electricity properly The AV junction and Purkinje fibers are not as reliable as a healthy AV node and typically generate electricity more slowly (the AV junction about 40ndash60 times a minute and the Purkinje fibers 20ndash40 times a minute)

bull However sometimes the AV junction or Purkinje fibers may generate electricity very

quickly this is abnormal When they do they may take over the heart When pacemakers of the heart compete the fastest one drives the heart

bull Automaticity is an inherent property of certain cardiac conductive cells to depolarize spontaneously Not all cardiac conduction cells possess automaticity but the SA node AV junction and Purkinje fibers possess automaticity

bull Refractoriness is the natural and expected behavior of cardiac conductive cells that makes them unresponsive to electrical impulses immediately after they have depolarized The natural refractory period is very brief

bull The faster the heart rate the shorter the refractory period the slower the heart rate the longer the refractory period

bull Refractoriness can also occur as a dysfunction bull Conductivity is the cardiac cellrsquos ability to

transmit an electrical impulse to adjacent cells Refractoriness means that cells that just depolarized will not depolarize again This keeps conduction moving forward (antegrade conduction)

bull Antegrade conduction is normal retrograde conduction is abnormal but is not rare

bull The autonomic nervous system regulates the heart rate by controlling how fast the heart beats and how vigorously it contracts

bull The autonomic nervous system consists of a sympathetic and parasympathetic nervous system which work together but at certain times one or the other is dominant The sympathetic nervous system makes the heart beat faster and more vigorously it takes over during times of stress exertion or fight or flight The parasympathetic ner-vous system makes the heart beat slowly and less vigorously it takes over during periods of rest

bull The neurotransmitters associated with these two systems are epinephrine or adrenaline (sympathetic nervous system) and acetylcho-line (parasympathetic nervous system)

Chapter 2 Cardiac conduction system 19

test your knowledge

Please write in the landmarks of the cardiac conduction system Select from the following (each is used only once)

AV node Bachmanrsquos bundle Bundle of His

Internodal tracts Left bundle branch Purkinje fibers

Right bundle branch Ventricles

1 Which of the following is not considered one of the heartrsquos intrinsic pacemakersA AV junctionB SA nodeC Purkinje fibersD Aorta

2 Depolarization and repolarization involve the movement of charged ions across cardiac cell membranes What types of ions are primarily involvedA Calcium potassium and sodiumB Acetylcholine and epinephrineC Magnesium and folateD All of the above

3 Refractoriness refers toA The ability of cardiac conduction cells to

contract

B The inability of cardiac conduction cells to respond to an electrical stimulus

C The unique permeability of the cardiac cellsD Abnormal cardiac conduction

4 When a cardiac cell depolarizes the interior of the cell becomes what with respect to the surrounding mediaA More positiveB More negativeC More acidicD Hotter

5 Fill in the blank In the healthy heart the faster the heart rate the ___ the refractory periodA More vigorousB Less reliableC LongerD Shorter

20 The nuts and bolts of implantable device therapy pacemakers

6 Which specific part of the nervous system governs the fight-or-flight response and is associated with epinephrine (adrenaline) (Pick the most exact answer)A Autonomic nervous systemB Parasympathetic nervous systemC Sympathetic nervous systemD Central nervous system

7 In normal antegrade conduction the electrical impulse travels from the SA node over the atria to the AV node and to what structure before reaching the ventriclesA The intermodal tractsB The Purkinje fibersC The bundle of HisD The ventricular apex

8 Which of the following is true about retrograde cardiac conductionA Everyone has itB It is perfectly normal and can be helpful in

stressful situations

C It is abnormal but not uncommonD It is impossible to diagnose definitively

except in an autopsy

9 What chemical is associated with the parasympathetic nervous systemA AcetylcholineB PotassiumC EpinephrineD Dopamine

10 The stimulation of the myocardial cells of which of the following structures is most closely associated with ventricular depolarizationA SA nodeB AV junctionC Bundle of HisD Purkinje fibers

12 The nuts and bolts of implantable device therapy pacemakers

the nuts and bolts of cardiovascular anatomy and physiology

bull The heart consists of four chambers which may be considered the upper chambers (right atrium and left atrium) and lower chambers (right ven-tricle and left ventricle) or may be thought of as the right heart (right atrium and right ventricle) and left heart (left atrium and left ventricle)

bull Deoxygenated blood enters the right atrium flows to the right ventricle and is pumped out via the pulmonary valve to the lungs The blood is reoxygenated in the lungs and then enters the left atrium and flows to the left atrium and out of the aortic valve into the aorta and the rest of the body

bull The heart has two outflow tracts a right ven-tricular outflow tract (RVOT) and a left ven-tricular outflow tract (LVOT)

bull The largest chamber of the heart is the left ven-tricle making up about 23 of the cardiac mass The left ventricle is muscular and massive because it must pump against approximately 10 times the pressure against which the right ven-tricle pumps (120 vs 12 mmHg)

bull The two cardiac valves most susceptible to dysfunction and disease are the mitral and aortic valve because they function in a high-pressure environment

bull The chordae tendineae connect the leaflets of the valves to the papillary muscles and look like little parachutes These help the valves open and close properly

bull In general arteries transport blood away from the heart are muscular and contract to help move the blood on its way and have thicker walls than veins Veins have thinner less muscular walls and carry oxygen-depleted blood back to the heart using tiny interior valves to keep the blood flow moving forward Arteries transport oxygenated blood and can be considered high-pressure systems while veins transported deoxygenated blood in a low-pressure system

bull Cardiac contraction is systole cardiac relaxation is diastole Atrial systole occurs at the very end of ventricular diastole in something nicknamed atrial kick In a healthy individual about 20ndash30 of cardiac output comes from atrial kick

bull The heart is a muscle that is fed with oxygenated blood through a network of coronary arteries Coronary arteries get oxygen-rich blood from the aorta and they perfuse the heart muscle during diastole as blood drains back

bull Although arteries and veins generally take blood away from or to the heart respectively

Rightventricle

Leftventricle

Leftatrium

Aorta

Rightatrium

Rightcoronaryartery

Leftcoronaryartery

Circumflexartery

Superiorvena cava

Inferiorvena cava

Arterial system

Rightventricle

Leftventricle

Greatcardiac vein

Smallcardiac vein

Middlecardiac vein

Leftatrium

Aorta

Rightatrium

Superiorvena cava

Inferiorvena cava

Venous system

Coronarysinus

Figure 111 The coronary artery system is matched by a coronary venous system

Chapter 1 Cardiovascular anatomy and physiology 13

test your knowledge

Fill in these anatomical landmarks of the heart They include the four chambers the four valves and the bodyrsquos main artery

the pulmonary vessels are the exception The pulmonary artery transports deoxygenated blood while the pulmonary veins carry oxygen-ated blood This is because of their special loca-tion and the fact that their work is taking blood from the right side to the left side of the heart

bull The main coronary arteries are the right coro-nary artery and the left coronary artery The left coronary artery branches quickly to a short vessel known as the left main (the widow maker) and the circumflex artery

bull A myocardial infarction can kill the heart muscle The clogged arteries determine the loc ation of the affected muscle A right coro-nary artery occlusion will affect the right atrium and ventricle and the inferior wall of the left ventricle while a left coronary artery occlusion will affect the left atrium and most of the left ventricle By far the most dangerous occlusion is in the left main because it feeds the majority of the heart muscle with oxygen-rich blood

answer the following questions

1 What are the names of the two atrioventricular valves of the heartA Atrial and ventricularB Tricuspid and mitralC Pulmonary and pulmonicD Aortic and superior vena cava

2 Which chambers of the heart contain trabeculaeA AtriaB VentriclesC Right-sided chambers onlyD Left-sided chambers only

14 The nuts and bolts of implantable device therapy pacemakers

3 Why can trabeculae be important to device cliniciansA They are good places to secure a passive-fixa-

tion leadB They help regulate blood pressureC They prevent atherosclerosisD All of the above

4 Which of the following is not true about the aortaA It is an arteryB It consists of an ascending portion a descend-

ing portion and an archC It carries only oxygenated bloodD It is the largest vein in the body

5 From which artery does blood enter the right atriumA Superior vena cavaB Inferior vena cavaC Coronary sinusD All of the above

6 What is atrial kickA Ventricular pressure on the tricuspid valve

forcing it shutB Electrical impulses originating in the atrium

to cause the heart to beat fasterC Atrial systole that forces the most blood possible

into the already-full and diastolic ventriclesD A type of arrhythmia

7 Why would a person with chronic AF likely have decreased cardiac outputA AF is associated with low blood volumeB AF causes the heart to beat very slowlyC AF may cause the heart to experience fre-

quent pausesD AF reduces or eliminates atrial kickmdashand

atrial kick contributes to cardiac output

8 Which of the following is often a preferred location for affixing a cardiac pacing leadA The right ventricular outflow tract (RVOT)B The left ventricular outflow tract (LVOT)C The excised right atrial appendageD The tricuspid valve

9 What connects the valve leaflets to the papillary musclesA TrabeculaeB ArteriesC Chordae tendineaeD Tiny valves

10 Which coronary artery is nicknamed the widow maker because an occlusion in this vessel will deprive the majority of the heartrsquos muscle mass of oxygen-rich bloodA Left mainB CircumflexC Right coronary arteryD Inferior vena cava

15

The Nuts and Bolts of Implantable Device Therapy Pacemakers First Edition Tom Kenny copy 2015 John Wiley amp Sons Ltd Published 2015 by John Wiley amp Sons Ltd

Chapter 2

Cardiac conduction system

Introduction

The sinoatrial (SA) node is the heartrsquos natural pacemaker In the healthy heart the SA node fires and the electrical stimulus conducts along special pathways through the heart that cause cells in specific areas to depolarize After the stimulus passes the area the cells repolarize Myocardial cell depolarization and repolarization govern the heartrsquos contraction and relaxation sequencemdashin other words the heart rhythm Rhythm disorders occur when this healthy conduction system is in some way disrupted or impaired

the conduction pathways

When the SA node fires and sends out electrical energy the stimulus travels from the SA node in the high right atrium out over the atria via path-ways known as internodal tracts Most of these internodal tracts are located in the right atrium the

main tract over to the left atrium is called Bachmannrsquos bundle It is because of Bachmannrsquos bundle linking the right atrium to the left atrium that the atria can contract simultaneously that is as these two upper chambers contract as one coherent unit After traversing the atria the impulse comes down through conduction tracts to the atrioventricular (AV) node

The AV node is an area of highly specialized tissue located below the atria and above the ventri-cles on the right side of the septum Anatomically the AV node rests in the AV groove The tissue in the healthy AV node has the ability to change the speed of the electrical impulse specifically it slows the stimulus down The slowdown serves two important purposes First slowing the impulse allows the atria time to contract and provide the very valuable atrial kick to ventricular filling Second the slowed impulse protects the ventricles from being forced to beat too quickly that is to beat before they have been adequately filled with blood

Once the impulse crosses the AV node it travels down into the ventricles and accelerates its speed The impulse travels through the bundle of His (pronounced hiss) to the bundle branches (right and left) and then out to the network of Purkinje fibers This allows both ventricles to depolarize and contract simultaneously (Figure 21)

polarization depolarization and repolarization

Cardiac conduction is driven by the ability of myo-cardial cells to be depolarized and repolarized Depolarization occurs at the cellular level and is

2

Learning objectives

bullensp Describe the electrical conduction system of the healthy heart

bullensp List the main anatomical landmarks of normal cardiac conduction

bullensp Name the anatomical location of the heartrsquos natural pacemaker

bullensp Explain depolarization and repolarization in electrical terms and how it relates to the cardiac contraction

bullensp Define the term automaticity and how it relates to cardiac conduction

16 The nuts and bolts of implantable device therapy pacemakers

in turn driven by electrolytes or positively and negatively charged ions (potassium sodium and calcium mainly) which are able to move in and out of the cardiac cells A myocardial cell may be said to be polarized when in its resting state that is there is no electrical activity going on In the rest-ing state the interior of the myocardial cell is somewhat more negatively charged that the outside This state is called the resting state or sometimes phase 0 of the action potential

An electrical stimulus changes the resting state abruptly The electrical energy allows for the movement of ions to the point that the interior of the myocardial cell is now more positive than the outside In other words the charge reverses and the cell is said to be depolarized When this happens the cell contracts when this happens to a large area of the myocardium the heart muscle contracts Cardiac contraction occurs as a result of cellular depolarization

When the electrical stimulus moves past the myocardial cell the ions move in reverse and the interior of the cell assumes its previous negative charge with respect to the outside This process is

called repolarization and it brings the myocar-dial cell back to the resting state When this happens the cell relaxes When a large area of the myocardium repolarizes the heart resumes its previous shape (loses its contraction)

Intrinsic pacemakers

The healthy heartrsquos natural pacemaker is the SA node but the SA node is not the only area of the heart that can generate an electrical pulse The SA node serves as the heartrsquos pacemaker because it generates these pulses faster than other areas In the healthy heart the SA node can fire at around 60ndash100 times a minute corresponding to a healthy heart rate The AV junction can also fire but it is much slower at around 40ndash60 times a minute Even the Purkinje fibers (the ventri-cles) can act as a pacemaker but they fire at only 20ndash40 times a minute Thus in a healthy individual the SA node outpaces these other potential pacemakers The AV junction and ven-tricles are sometimes called backup pacemakers because they take over the heart rate in the event

Sinoatrial (SA) node

Internodal tracts

Right bundle branch

Left bundle branch

Purkinje fibers

Bachmannrsquos bundle

Bundle of His

Atrioventricular (AV) node

Figure 21 The conduction pathways in the healthy heart

Chapter 2 Cardiac conduction system 17

that the SA node is not able to pace the heart While these backup pacemakers may help keep the heart beating in the event of a dysfunctional SA node they are slow and not as reliable as the SA node

The healthy heart is able to pace itself because of a property called automaticity Automaticity is an inherent property of individual cardiac conductive cells that allows them to depolarize spontaneously In other words some cells in the heart (such as the cells of the SA node) are able to generate an electrical stimulus on their own

refractoriness

Refractoriness refers to the temporary inability of a cardiac cell to respond to an electrical stimulus Refractoriness can be the normal appropriate and expected behavior of cardiac cells For example immediately after electrical energy has passed through cardiac cells they go through a phase where they are unresponsive to electrical energy no matter how much voltage is delivered This normal refractoriness is called the physiological refractory period and it is very brief (it is mea-sured in thousandths of a second) Of course the exact duration of a refractory period is based in part on the heart rate the faster the heart rate the shorter the refractory period and vice versa

Refractoriness may also occur when certain cells become dysfunctional that is cardiac cells may lose their ability to conduct properly Furthermore the heartrsquos inherent pacemakers may change their rate for instance the AV junction may speed up and start to deliver impulses at 80 or 100 times a minute When this happensmdashthat is when another pacemaker takes over the role of the SA nodemdashthat pacemaker is called a usurping pacemaker because it usurps the rightful role of the SA node In the event that other pacemakers of the heart start to compete with the SA node the fastest pacemaker always wins

Conductivity

Conductivity is the cardiac cellrsquos ability to transmit an electrical impulse to an adjacent cell Typically cardiac conduction is antegrade that

is it moves forward Electrical impulses actually travel out in all different directions but refracto-riness prevents those cells that were just stimu-lated from depolarization This keeps conduction moving in a single direction through the heart Normal antegrade conduction in a healthy heart goes from SA node to atria to AV node over the bundle of His and down to the ventricles Some individuals have the ability to conduct retro-grade retrograde conduction is abnormal but not uncommon

autonomic nervous system

The heart does not need direct stimulation from the nervous system to generate electricity and to beat but that is not to say that the nervous system does not control the heart rate The autonomic nervous system indirectly affects cardiac conduction by regulating how fast the heart will beat and how hard the heart muscle will squeeze (contractility) The autonomic ner-vous system communicates with the heart through a network of baroreceptors which respond to chemical signals

The autonomic nervous system of the body like certain other physiological systems exists as two systems in balance The autonomic nervous system consists of a sympathetic system and a parasympathetic system They both work together and in a healthy individual one may overdrive the other during specific times The sympathetic ner-vous system is mainly associated with epinephrine (adrenaline) and it has the ability to speed the heart rate and increase cardiac contractility and can increase the speed by which electrical impulses can cross the AV node The sympathetic nervous system takes over when we are stressed frightened (fight-or-flight response) or exerting ourselves

On the other hand the autonomic nervous system also has a parasympathetic system that is associated with acetylcholine This system can decrease the heart rate decrease cardiac contrac-tility and cause impulses crossing the AV node to be slowed The parasympathetic system might take over when we are sleeping or at rest

18 The nuts and bolts of implantable device therapy pacemakers

the nuts and bolts of the cardiac conduction system

bull The heartrsquos natural pacemaker is the sinoatrial (SA) node which generates an electrical impulse spontaneously about 60ndash100 times a minute in a healthy individual

bull The normal conduction path is from the SA node over the atria to the atrioventricular (AV) node and then via the bundle of His down the right and left bundle branches and into the Purkinje fibers

bull The AV node has the ability to slow down the electrical impulse This serves two main purposes it allows for atrial kick and it protects the ventricles from beating too rapidly

bull In the healthy heart the right and left atria contract simultaneously and then the right and left ventricles contract simultaneously

bull Depolarization and repolarization of myocar-dial cells are driven by positively and negatively charged ions which cross the cell membrane A resting cardiac cell is more negative on the inside than the surrounding media but when it depolarizes it becomes more positive on the inside relative to the surrounding media Following depolarization the ions move and the cell returns to being more negative on the inside than the outside in a process known as repolarization

bull Depolarization causes the cardiac cell to contract

bull The main ions involved in depolarization and repolarization are sodium potassium and calcium ions

bull Although the heartrsquos natural pacemaker is the SA node the AV junction and the Purkinje fibers (ventricles) can also assume (usurp) this role if the SA node cannot generate electricity properly The AV junction and Purkinje fibers are not as reliable as a healthy AV node and typically generate electricity more slowly (the AV junction about 40ndash60 times a minute and the Purkinje fibers 20ndash40 times a minute)

bull However sometimes the AV junction or Purkinje fibers may generate electricity very

quickly this is abnormal When they do they may take over the heart When pacemakers of the heart compete the fastest one drives the heart

bull Automaticity is an inherent property of certain cardiac conductive cells to depolarize spontaneously Not all cardiac conduction cells possess automaticity but the SA node AV junction and Purkinje fibers possess automaticity

bull Refractoriness is the natural and expected behavior of cardiac conductive cells that makes them unresponsive to electrical impulses immediately after they have depolarized The natural refractory period is very brief

bull The faster the heart rate the shorter the refractory period the slower the heart rate the longer the refractory period

bull Refractoriness can also occur as a dysfunction bull Conductivity is the cardiac cellrsquos ability to

transmit an electrical impulse to adjacent cells Refractoriness means that cells that just depolarized will not depolarize again This keeps conduction moving forward (antegrade conduction)

bull Antegrade conduction is normal retrograde conduction is abnormal but is not rare

bull The autonomic nervous system regulates the heart rate by controlling how fast the heart beats and how vigorously it contracts

bull The autonomic nervous system consists of a sympathetic and parasympathetic nervous system which work together but at certain times one or the other is dominant The sympathetic nervous system makes the heart beat faster and more vigorously it takes over during times of stress exertion or fight or flight The parasympathetic ner-vous system makes the heart beat slowly and less vigorously it takes over during periods of rest

bull The neurotransmitters associated with these two systems are epinephrine or adrenaline (sympathetic nervous system) and acetylcho-line (parasympathetic nervous system)

Chapter 2 Cardiac conduction system 19

test your knowledge

Please write in the landmarks of the cardiac conduction system Select from the following (each is used only once)

AV node Bachmanrsquos bundle Bundle of His

Internodal tracts Left bundle branch Purkinje fibers

Right bundle branch Ventricles

1 Which of the following is not considered one of the heartrsquos intrinsic pacemakersA AV junctionB SA nodeC Purkinje fibersD Aorta

2 Depolarization and repolarization involve the movement of charged ions across cardiac cell membranes What types of ions are primarily involvedA Calcium potassium and sodiumB Acetylcholine and epinephrineC Magnesium and folateD All of the above

3 Refractoriness refers toA The ability of cardiac conduction cells to

contract

B The inability of cardiac conduction cells to respond to an electrical stimulus

C The unique permeability of the cardiac cellsD Abnormal cardiac conduction

4 When a cardiac cell depolarizes the interior of the cell becomes what with respect to the surrounding mediaA More positiveB More negativeC More acidicD Hotter

5 Fill in the blank In the healthy heart the faster the heart rate the ___ the refractory periodA More vigorousB Less reliableC LongerD Shorter

20 The nuts and bolts of implantable device therapy pacemakers

6 Which specific part of the nervous system governs the fight-or-flight response and is associated with epinephrine (adrenaline) (Pick the most exact answer)A Autonomic nervous systemB Parasympathetic nervous systemC Sympathetic nervous systemD Central nervous system

7 In normal antegrade conduction the electrical impulse travels from the SA node over the atria to the AV node and to what structure before reaching the ventriclesA The intermodal tractsB The Purkinje fibersC The bundle of HisD The ventricular apex

8 Which of the following is true about retrograde cardiac conductionA Everyone has itB It is perfectly normal and can be helpful in

stressful situations

C It is abnormal but not uncommonD It is impossible to diagnose definitively

except in an autopsy

9 What chemical is associated with the parasympathetic nervous systemA AcetylcholineB PotassiumC EpinephrineD Dopamine

10 The stimulation of the myocardial cells of which of the following structures is most closely associated with ventricular depolarizationA SA nodeB AV junctionC Bundle of HisD Purkinje fibers

Chapter 1 Cardiovascular anatomy and physiology 13

test your knowledge

Fill in these anatomical landmarks of the heart They include the four chambers the four valves and the bodyrsquos main artery

the pulmonary vessels are the exception The pulmonary artery transports deoxygenated blood while the pulmonary veins carry oxygen-ated blood This is because of their special loca-tion and the fact that their work is taking blood from the right side to the left side of the heart

bull The main coronary arteries are the right coro-nary artery and the left coronary artery The left coronary artery branches quickly to a short vessel known as the left main (the widow maker) and the circumflex artery

bull A myocardial infarction can kill the heart muscle The clogged arteries determine the loc ation of the affected muscle A right coro-nary artery occlusion will affect the right atrium and ventricle and the inferior wall of the left ventricle while a left coronary artery occlusion will affect the left atrium and most of the left ventricle By far the most dangerous occlusion is in the left main because it feeds the majority of the heart muscle with oxygen-rich blood

answer the following questions

1 What are the names of the two atrioventricular valves of the heartA Atrial and ventricularB Tricuspid and mitralC Pulmonary and pulmonicD Aortic and superior vena cava

2 Which chambers of the heart contain trabeculaeA AtriaB VentriclesC Right-sided chambers onlyD Left-sided chambers only

14 The nuts and bolts of implantable device therapy pacemakers

3 Why can trabeculae be important to device cliniciansA They are good places to secure a passive-fixa-

tion leadB They help regulate blood pressureC They prevent atherosclerosisD All of the above

4 Which of the following is not true about the aortaA It is an arteryB It consists of an ascending portion a descend-

ing portion and an archC It carries only oxygenated bloodD It is the largest vein in the body

5 From which artery does blood enter the right atriumA Superior vena cavaB Inferior vena cavaC Coronary sinusD All of the above

6 What is atrial kickA Ventricular pressure on the tricuspid valve

forcing it shutB Electrical impulses originating in the atrium

to cause the heart to beat fasterC Atrial systole that forces the most blood possible

into the already-full and diastolic ventriclesD A type of arrhythmia

7 Why would a person with chronic AF likely have decreased cardiac outputA AF is associated with low blood volumeB AF causes the heart to beat very slowlyC AF may cause the heart to experience fre-

quent pausesD AF reduces or eliminates atrial kickmdashand

atrial kick contributes to cardiac output

8 Which of the following is often a preferred location for affixing a cardiac pacing leadA The right ventricular outflow tract (RVOT)B The left ventricular outflow tract (LVOT)C The excised right atrial appendageD The tricuspid valve

9 What connects the valve leaflets to the papillary musclesA TrabeculaeB ArteriesC Chordae tendineaeD Tiny valves

10 Which coronary artery is nicknamed the widow maker because an occlusion in this vessel will deprive the majority of the heartrsquos muscle mass of oxygen-rich bloodA Left mainB CircumflexC Right coronary arteryD Inferior vena cava

15

The Nuts and Bolts of Implantable Device Therapy Pacemakers First Edition Tom Kenny copy 2015 John Wiley amp Sons Ltd Published 2015 by John Wiley amp Sons Ltd

Chapter 2

Cardiac conduction system

Introduction

The sinoatrial (SA) node is the heartrsquos natural pacemaker In the healthy heart the SA node fires and the electrical stimulus conducts along special pathways through the heart that cause cells in specific areas to depolarize After the stimulus passes the area the cells repolarize Myocardial cell depolarization and repolarization govern the heartrsquos contraction and relaxation sequencemdashin other words the heart rhythm Rhythm disorders occur when this healthy conduction system is in some way disrupted or impaired

the conduction pathways

When the SA node fires and sends out electrical energy the stimulus travels from the SA node in the high right atrium out over the atria via path-ways known as internodal tracts Most of these internodal tracts are located in the right atrium the

main tract over to the left atrium is called Bachmannrsquos bundle It is because of Bachmannrsquos bundle linking the right atrium to the left atrium that the atria can contract simultaneously that is as these two upper chambers contract as one coherent unit After traversing the atria the impulse comes down through conduction tracts to the atrioventricular (AV) node

The AV node is an area of highly specialized tissue located below the atria and above the ventri-cles on the right side of the septum Anatomically the AV node rests in the AV groove The tissue in the healthy AV node has the ability to change the speed of the electrical impulse specifically it slows the stimulus down The slowdown serves two important purposes First slowing the impulse allows the atria time to contract and provide the very valuable atrial kick to ventricular filling Second the slowed impulse protects the ventricles from being forced to beat too quickly that is to beat before they have been adequately filled with blood

Once the impulse crosses the AV node it travels down into the ventricles and accelerates its speed The impulse travels through the bundle of His (pronounced hiss) to the bundle branches (right and left) and then out to the network of Purkinje fibers This allows both ventricles to depolarize and contract simultaneously (Figure 21)

polarization depolarization and repolarization

Cardiac conduction is driven by the ability of myo-cardial cells to be depolarized and repolarized Depolarization occurs at the cellular level and is

2

Learning objectives

bullensp Describe the electrical conduction system of the healthy heart

bullensp List the main anatomical landmarks of normal cardiac conduction

bullensp Name the anatomical location of the heartrsquos natural pacemaker

bullensp Explain depolarization and repolarization in electrical terms and how it relates to the cardiac contraction

bullensp Define the term automaticity and how it relates to cardiac conduction

16 The nuts and bolts of implantable device therapy pacemakers

in turn driven by electrolytes or positively and negatively charged ions (potassium sodium and calcium mainly) which are able to move in and out of the cardiac cells A myocardial cell may be said to be polarized when in its resting state that is there is no electrical activity going on In the rest-ing state the interior of the myocardial cell is somewhat more negatively charged that the outside This state is called the resting state or sometimes phase 0 of the action potential

An electrical stimulus changes the resting state abruptly The electrical energy allows for the movement of ions to the point that the interior of the myocardial cell is now more positive than the outside In other words the charge reverses and the cell is said to be depolarized When this happens the cell contracts when this happens to a large area of the myocardium the heart muscle contracts Cardiac contraction occurs as a result of cellular depolarization

When the electrical stimulus moves past the myocardial cell the ions move in reverse and the interior of the cell assumes its previous negative charge with respect to the outside This process is

called repolarization and it brings the myocar-dial cell back to the resting state When this happens the cell relaxes When a large area of the myocardium repolarizes the heart resumes its previous shape (loses its contraction)

Intrinsic pacemakers

The healthy heartrsquos natural pacemaker is the SA node but the SA node is not the only area of the heart that can generate an electrical pulse The SA node serves as the heartrsquos pacemaker because it generates these pulses faster than other areas In the healthy heart the SA node can fire at around 60ndash100 times a minute corresponding to a healthy heart rate The AV junction can also fire but it is much slower at around 40ndash60 times a minute Even the Purkinje fibers (the ventri-cles) can act as a pacemaker but they fire at only 20ndash40 times a minute Thus in a healthy individual the SA node outpaces these other potential pacemakers The AV junction and ven-tricles are sometimes called backup pacemakers because they take over the heart rate in the event

Sinoatrial (SA) node

Internodal tracts

Right bundle branch

Left bundle branch

Purkinje fibers

Bachmannrsquos bundle

Bundle of His

Atrioventricular (AV) node

Figure 21 The conduction pathways in the healthy heart

Chapter 2 Cardiac conduction system 17

that the SA node is not able to pace the heart While these backup pacemakers may help keep the heart beating in the event of a dysfunctional SA node they are slow and not as reliable as the SA node

The healthy heart is able to pace itself because of a property called automaticity Automaticity is an inherent property of individual cardiac conductive cells that allows them to depolarize spontaneously In other words some cells in the heart (such as the cells of the SA node) are able to generate an electrical stimulus on their own

refractoriness

Refractoriness refers to the temporary inability of a cardiac cell to respond to an electrical stimulus Refractoriness can be the normal appropriate and expected behavior of cardiac cells For example immediately after electrical energy has passed through cardiac cells they go through a phase where they are unresponsive to electrical energy no matter how much voltage is delivered This normal refractoriness is called the physiological refractory period and it is very brief (it is mea-sured in thousandths of a second) Of course the exact duration of a refractory period is based in part on the heart rate the faster the heart rate the shorter the refractory period and vice versa

Refractoriness may also occur when certain cells become dysfunctional that is cardiac cells may lose their ability to conduct properly Furthermore the heartrsquos inherent pacemakers may change their rate for instance the AV junction may speed up and start to deliver impulses at 80 or 100 times a minute When this happensmdashthat is when another pacemaker takes over the role of the SA nodemdashthat pacemaker is called a usurping pacemaker because it usurps the rightful role of the SA node In the event that other pacemakers of the heart start to compete with the SA node the fastest pacemaker always wins

Conductivity

Conductivity is the cardiac cellrsquos ability to transmit an electrical impulse to an adjacent cell Typically cardiac conduction is antegrade that

is it moves forward Electrical impulses actually travel out in all different directions but refracto-riness prevents those cells that were just stimu-lated from depolarization This keeps conduction moving in a single direction through the heart Normal antegrade conduction in a healthy heart goes from SA node to atria to AV node over the bundle of His and down to the ventricles Some individuals have the ability to conduct retro-grade retrograde conduction is abnormal but not uncommon

autonomic nervous system

The heart does not need direct stimulation from the nervous system to generate electricity and to beat but that is not to say that the nervous system does not control the heart rate The autonomic nervous system indirectly affects cardiac conduction by regulating how fast the heart will beat and how hard the heart muscle will squeeze (contractility) The autonomic ner-vous system communicates with the heart through a network of baroreceptors which respond to chemical signals

The autonomic nervous system of the body like certain other physiological systems exists as two systems in balance The autonomic nervous system consists of a sympathetic system and a parasympathetic system They both work together and in a healthy individual one may overdrive the other during specific times The sympathetic ner-vous system is mainly associated with epinephrine (adrenaline) and it has the ability to speed the heart rate and increase cardiac contractility and can increase the speed by which electrical impulses can cross the AV node The sympathetic nervous system takes over when we are stressed frightened (fight-or-flight response) or exerting ourselves

On the other hand the autonomic nervous system also has a parasympathetic system that is associated with acetylcholine This system can decrease the heart rate decrease cardiac contrac-tility and cause impulses crossing the AV node to be slowed The parasympathetic system might take over when we are sleeping or at rest

18 The nuts and bolts of implantable device therapy pacemakers

the nuts and bolts of the cardiac conduction system

bull The heartrsquos natural pacemaker is the sinoatrial (SA) node which generates an electrical impulse spontaneously about 60ndash100 times a minute in a healthy individual

bull The normal conduction path is from the SA node over the atria to the atrioventricular (AV) node and then via the bundle of His down the right and left bundle branches and into the Purkinje fibers

bull The AV node has the ability to slow down the electrical impulse This serves two main purposes it allows for atrial kick and it protects the ventricles from beating too rapidly

bull In the healthy heart the right and left atria contract simultaneously and then the right and left ventricles contract simultaneously

bull Depolarization and repolarization of myocar-dial cells are driven by positively and negatively charged ions which cross the cell membrane A resting cardiac cell is more negative on the inside than the surrounding media but when it depolarizes it becomes more positive on the inside relative to the surrounding media Following depolarization the ions move and the cell returns to being more negative on the inside than the outside in a process known as repolarization

bull Depolarization causes the cardiac cell to contract

bull The main ions involved in depolarization and repolarization are sodium potassium and calcium ions

bull Although the heartrsquos natural pacemaker is the SA node the AV junction and the Purkinje fibers (ventricles) can also assume (usurp) this role if the SA node cannot generate electricity properly The AV junction and Purkinje fibers are not as reliable as a healthy AV node and typically generate electricity more slowly (the AV junction about 40ndash60 times a minute and the Purkinje fibers 20ndash40 times a minute)

bull However sometimes the AV junction or Purkinje fibers may generate electricity very

quickly this is abnormal When they do they may take over the heart When pacemakers of the heart compete the fastest one drives the heart

bull Automaticity is an inherent property of certain cardiac conductive cells to depolarize spontaneously Not all cardiac conduction cells possess automaticity but the SA node AV junction and Purkinje fibers possess automaticity

bull Refractoriness is the natural and expected behavior of cardiac conductive cells that makes them unresponsive to electrical impulses immediately after they have depolarized The natural refractory period is very brief

bull The faster the heart rate the shorter the refractory period the slower the heart rate the longer the refractory period

bull Refractoriness can also occur as a dysfunction bull Conductivity is the cardiac cellrsquos ability to

transmit an electrical impulse to adjacent cells Refractoriness means that cells that just depolarized will not depolarize again This keeps conduction moving forward (antegrade conduction)

bull Antegrade conduction is normal retrograde conduction is abnormal but is not rare

bull The autonomic nervous system regulates the heart rate by controlling how fast the heart beats and how vigorously it contracts

bull The autonomic nervous system consists of a sympathetic and parasympathetic nervous system which work together but at certain times one or the other is dominant The sympathetic nervous system makes the heart beat faster and more vigorously it takes over during times of stress exertion or fight or flight The parasympathetic ner-vous system makes the heart beat slowly and less vigorously it takes over during periods of rest

bull The neurotransmitters associated with these two systems are epinephrine or adrenaline (sympathetic nervous system) and acetylcho-line (parasympathetic nervous system)

Chapter 2 Cardiac conduction system 19

test your knowledge

Please write in the landmarks of the cardiac conduction system Select from the following (each is used only once)

AV node Bachmanrsquos bundle Bundle of His

Internodal tracts Left bundle branch Purkinje fibers

Right bundle branch Ventricles

1 Which of the following is not considered one of the heartrsquos intrinsic pacemakersA AV junctionB SA nodeC Purkinje fibersD Aorta

2 Depolarization and repolarization involve the movement of charged ions across cardiac cell membranes What types of ions are primarily involvedA Calcium potassium and sodiumB Acetylcholine and epinephrineC Magnesium and folateD All of the above

3 Refractoriness refers toA The ability of cardiac conduction cells to

contract

B The inability of cardiac conduction cells to respond to an electrical stimulus

C The unique permeability of the cardiac cellsD Abnormal cardiac conduction

4 When a cardiac cell depolarizes the interior of the cell becomes what with respect to the surrounding mediaA More positiveB More negativeC More acidicD Hotter

5 Fill in the blank In the healthy heart the faster the heart rate the ___ the refractory periodA More vigorousB Less reliableC LongerD Shorter

20 The nuts and bolts of implantable device therapy pacemakers

6 Which specific part of the nervous system governs the fight-or-flight response and is associated with epinephrine (adrenaline) (Pick the most exact answer)A Autonomic nervous systemB Parasympathetic nervous systemC Sympathetic nervous systemD Central nervous system

7 In normal antegrade conduction the electrical impulse travels from the SA node over the atria to the AV node and to what structure before reaching the ventriclesA The intermodal tractsB The Purkinje fibersC The bundle of HisD The ventricular apex

8 Which of the following is true about retrograde cardiac conductionA Everyone has itB It is perfectly normal and can be helpful in

stressful situations

C It is abnormal but not uncommonD It is impossible to diagnose definitively

except in an autopsy

9 What chemical is associated with the parasympathetic nervous systemA AcetylcholineB PotassiumC EpinephrineD Dopamine

10 The stimulation of the myocardial cells of which of the following structures is most closely associated with ventricular depolarizationA SA nodeB AV junctionC Bundle of HisD Purkinje fibers

14 The nuts and bolts of implantable device therapy pacemakers

3 Why can trabeculae be important to device cliniciansA They are good places to secure a passive-fixa-

tion leadB They help regulate blood pressureC They prevent atherosclerosisD All of the above

4 Which of the following is not true about the aortaA It is an arteryB It consists of an ascending portion a descend-

ing portion and an archC It carries only oxygenated bloodD It is the largest vein in the body

5 From which artery does blood enter the right atriumA Superior vena cavaB Inferior vena cavaC Coronary sinusD All of the above

6 What is atrial kickA Ventricular pressure on the tricuspid valve

forcing it shutB Electrical impulses originating in the atrium

to cause the heart to beat fasterC Atrial systole that forces the most blood possible

into the already-full and diastolic ventriclesD A type of arrhythmia

7 Why would a person with chronic AF likely have decreased cardiac outputA AF is associated with low blood volumeB AF causes the heart to beat very slowlyC AF may cause the heart to experience fre-

quent pausesD AF reduces or eliminates atrial kickmdashand

atrial kick contributes to cardiac output

8 Which of the following is often a preferred location for affixing a cardiac pacing leadA The right ventricular outflow tract (RVOT)B The left ventricular outflow tract (LVOT)C The excised right atrial appendageD The tricuspid valve

9 What connects the valve leaflets to the papillary musclesA TrabeculaeB ArteriesC Chordae tendineaeD Tiny valves

10 Which coronary artery is nicknamed the widow maker because an occlusion in this vessel will deprive the majority of the heartrsquos muscle mass of oxygen-rich bloodA Left mainB CircumflexC Right coronary arteryD Inferior vena cava

15

The Nuts and Bolts of Implantable Device Therapy Pacemakers First Edition Tom Kenny copy 2015 John Wiley amp Sons Ltd Published 2015 by John Wiley amp Sons Ltd

Chapter 2

Cardiac conduction system

Introduction

The sinoatrial (SA) node is the heartrsquos natural pacemaker In the healthy heart the SA node fires and the electrical stimulus conducts along special pathways through the heart that cause cells in specific areas to depolarize After the stimulus passes the area the cells repolarize Myocardial cell depolarization and repolarization govern the heartrsquos contraction and relaxation sequencemdashin other words the heart rhythm Rhythm disorders occur when this healthy conduction system is in some way disrupted or impaired

the conduction pathways

When the SA node fires and sends out electrical energy the stimulus travels from the SA node in the high right atrium out over the atria via path-ways known as internodal tracts Most of these internodal tracts are located in the right atrium the

main tract over to the left atrium is called Bachmannrsquos bundle It is because of Bachmannrsquos bundle linking the right atrium to the left atrium that the atria can contract simultaneously that is as these two upper chambers contract as one coherent unit After traversing the atria the impulse comes down through conduction tracts to the atrioventricular (AV) node

The AV node is an area of highly specialized tissue located below the atria and above the ventri-cles on the right side of the septum Anatomically the AV node rests in the AV groove The tissue in the healthy AV node has the ability to change the speed of the electrical impulse specifically it slows the stimulus down The slowdown serves two important purposes First slowing the impulse allows the atria time to contract and provide the very valuable atrial kick to ventricular filling Second the slowed impulse protects the ventricles from being forced to beat too quickly that is to beat before they have been adequately filled with blood

Once the impulse crosses the AV node it travels down into the ventricles and accelerates its speed The impulse travels through the bundle of His (pronounced hiss) to the bundle branches (right and left) and then out to the network of Purkinje fibers This allows both ventricles to depolarize and contract simultaneously (Figure 21)

polarization depolarization and repolarization

Cardiac conduction is driven by the ability of myo-cardial cells to be depolarized and repolarized Depolarization occurs at the cellular level and is

2

Learning objectives

bullensp Describe the electrical conduction system of the healthy heart

bullensp List the main anatomical landmarks of normal cardiac conduction

bullensp Name the anatomical location of the heartrsquos natural pacemaker

bullensp Explain depolarization and repolarization in electrical terms and how it relates to the cardiac contraction

bullensp Define the term automaticity and how it relates to cardiac conduction

16 The nuts and bolts of implantable device therapy pacemakers

in turn driven by electrolytes or positively and negatively charged ions (potassium sodium and calcium mainly) which are able to move in and out of the cardiac cells A myocardial cell may be said to be polarized when in its resting state that is there is no electrical activity going on In the rest-ing state the interior of the myocardial cell is somewhat more negatively charged that the outside This state is called the resting state or sometimes phase 0 of the action potential

An electrical stimulus changes the resting state abruptly The electrical energy allows for the movement of ions to the point that the interior of the myocardial cell is now more positive than the outside In other words the charge reverses and the cell is said to be depolarized When this happens the cell contracts when this happens to a large area of the myocardium the heart muscle contracts Cardiac contraction occurs as a result of cellular depolarization

When the electrical stimulus moves past the myocardial cell the ions move in reverse and the interior of the cell assumes its previous negative charge with respect to the outside This process is

called repolarization and it brings the myocar-dial cell back to the resting state When this happens the cell relaxes When a large area of the myocardium repolarizes the heart resumes its previous shape (loses its contraction)

Intrinsic pacemakers

The healthy heartrsquos natural pacemaker is the SA node but the SA node is not the only area of the heart that can generate an electrical pulse The SA node serves as the heartrsquos pacemaker because it generates these pulses faster than other areas In the healthy heart the SA node can fire at around 60ndash100 times a minute corresponding to a healthy heart rate The AV junction can also fire but it is much slower at around 40ndash60 times a minute Even the Purkinje fibers (the ventri-cles) can act as a pacemaker but they fire at only 20ndash40 times a minute Thus in a healthy individual the SA node outpaces these other potential pacemakers The AV junction and ven-tricles are sometimes called backup pacemakers because they take over the heart rate in the event

Sinoatrial (SA) node

Internodal tracts

Right bundle branch

Left bundle branch

Purkinje fibers

Bachmannrsquos bundle

Bundle of His

Atrioventricular (AV) node

Figure 21 The conduction pathways in the healthy heart

Chapter 2 Cardiac conduction system 17

that the SA node is not able to pace the heart While these backup pacemakers may help keep the heart beating in the event of a dysfunctional SA node they are slow and not as reliable as the SA node

The healthy heart is able to pace itself because of a property called automaticity Automaticity is an inherent property of individual cardiac conductive cells that allows them to depolarize spontaneously In other words some cells in the heart (such as the cells of the SA node) are able to generate an electrical stimulus on their own

refractoriness

Refractoriness refers to the temporary inability of a cardiac cell to respond to an electrical stimulus Refractoriness can be the normal appropriate and expected behavior of cardiac cells For example immediately after electrical energy has passed through cardiac cells they go through a phase where they are unresponsive to electrical energy no matter how much voltage is delivered This normal refractoriness is called the physiological refractory period and it is very brief (it is mea-sured in thousandths of a second) Of course the exact duration of a refractory period is based in part on the heart rate the faster the heart rate the shorter the refractory period and vice versa

Refractoriness may also occur when certain cells become dysfunctional that is cardiac cells may lose their ability to conduct properly Furthermore the heartrsquos inherent pacemakers may change their rate for instance the AV junction may speed up and start to deliver impulses at 80 or 100 times a minute When this happensmdashthat is when another pacemaker takes over the role of the SA nodemdashthat pacemaker is called a usurping pacemaker because it usurps the rightful role of the SA node In the event that other pacemakers of the heart start to compete with the SA node the fastest pacemaker always wins

Conductivity

Conductivity is the cardiac cellrsquos ability to transmit an electrical impulse to an adjacent cell Typically cardiac conduction is antegrade that

is it moves forward Electrical impulses actually travel out in all different directions but refracto-riness prevents those cells that were just stimu-lated from depolarization This keeps conduction moving in a single direction through the heart Normal antegrade conduction in a healthy heart goes from SA node to atria to AV node over the bundle of His and down to the ventricles Some individuals have the ability to conduct retro-grade retrograde conduction is abnormal but not uncommon

autonomic nervous system

The heart does not need direct stimulation from the nervous system to generate electricity and to beat but that is not to say that the nervous system does not control the heart rate The autonomic nervous system indirectly affects cardiac conduction by regulating how fast the heart will beat and how hard the heart muscle will squeeze (contractility) The autonomic ner-vous system communicates with the heart through a network of baroreceptors which respond to chemical signals

The autonomic nervous system of the body like certain other physiological systems exists as two systems in balance The autonomic nervous system consists of a sympathetic system and a parasympathetic system They both work together and in a healthy individual one may overdrive the other during specific times The sympathetic ner-vous system is mainly associated with epinephrine (adrenaline) and it has the ability to speed the heart rate and increase cardiac contractility and can increase the speed by which electrical impulses can cross the AV node The sympathetic nervous system takes over when we are stressed frightened (fight-or-flight response) or exerting ourselves

On the other hand the autonomic nervous system also has a parasympathetic system that is associated with acetylcholine This system can decrease the heart rate decrease cardiac contrac-tility and cause impulses crossing the AV node to be slowed The parasympathetic system might take over when we are sleeping or at rest

18 The nuts and bolts of implantable device therapy pacemakers

the nuts and bolts of the cardiac conduction system

bull The heartrsquos natural pacemaker is the sinoatrial (SA) node which generates an electrical impulse spontaneously about 60ndash100 times a minute in a healthy individual

bull The normal conduction path is from the SA node over the atria to the atrioventricular (AV) node and then via the bundle of His down the right and left bundle branches and into the Purkinje fibers

bull The AV node has the ability to slow down the electrical impulse This serves two main purposes it allows for atrial kick and it protects the ventricles from beating too rapidly

bull In the healthy heart the right and left atria contract simultaneously and then the right and left ventricles contract simultaneously

bull Depolarization and repolarization of myocar-dial cells are driven by positively and negatively charged ions which cross the cell membrane A resting cardiac cell is more negative on the inside than the surrounding media but when it depolarizes it becomes more positive on the inside relative to the surrounding media Following depolarization the ions move and the cell returns to being more negative on the inside than the outside in a process known as repolarization

bull Depolarization causes the cardiac cell to contract

bull The main ions involved in depolarization and repolarization are sodium potassium and calcium ions

bull Although the heartrsquos natural pacemaker is the SA node the AV junction and the Purkinje fibers (ventricles) can also assume (usurp) this role if the SA node cannot generate electricity properly The AV junction and Purkinje fibers are not as reliable as a healthy AV node and typically generate electricity more slowly (the AV junction about 40ndash60 times a minute and the Purkinje fibers 20ndash40 times a minute)

bull However sometimes the AV junction or Purkinje fibers may generate electricity very

quickly this is abnormal When they do they may take over the heart When pacemakers of the heart compete the fastest one drives the heart

bull Automaticity is an inherent property of certain cardiac conductive cells to depolarize spontaneously Not all cardiac conduction cells possess automaticity but the SA node AV junction and Purkinje fibers possess automaticity

bull Refractoriness is the natural and expected behavior of cardiac conductive cells that makes them unresponsive to electrical impulses immediately after they have depolarized The natural refractory period is very brief

bull The faster the heart rate the shorter the refractory period the slower the heart rate the longer the refractory period

bull Refractoriness can also occur as a dysfunction bull Conductivity is the cardiac cellrsquos ability to

transmit an electrical impulse to adjacent cells Refractoriness means that cells that just depolarized will not depolarize again This keeps conduction moving forward (antegrade conduction)

bull Antegrade conduction is normal retrograde conduction is abnormal but is not rare

bull The autonomic nervous system regulates the heart rate by controlling how fast the heart beats and how vigorously it contracts

bull The autonomic nervous system consists of a sympathetic and parasympathetic nervous system which work together but at certain times one or the other is dominant The sympathetic nervous system makes the heart beat faster and more vigorously it takes over during times of stress exertion or fight or flight The parasympathetic ner-vous system makes the heart beat slowly and less vigorously it takes over during periods of rest

bull The neurotransmitters associated with these two systems are epinephrine or adrenaline (sympathetic nervous system) and acetylcho-line (parasympathetic nervous system)

Chapter 2 Cardiac conduction system 19

test your knowledge

Please write in the landmarks of the cardiac conduction system Select from the following (each is used only once)

AV node Bachmanrsquos bundle Bundle of His

Internodal tracts Left bundle branch Purkinje fibers

Right bundle branch Ventricles

1 Which of the following is not considered one of the heartrsquos intrinsic pacemakersA AV junctionB SA nodeC Purkinje fibersD Aorta

2 Depolarization and repolarization involve the movement of charged ions across cardiac cell membranes What types of ions are primarily involvedA Calcium potassium and sodiumB Acetylcholine and epinephrineC Magnesium and folateD All of the above

3 Refractoriness refers toA The ability of cardiac conduction cells to

contract

B The inability of cardiac conduction cells to respond to an electrical stimulus

C The unique permeability of the cardiac cellsD Abnormal cardiac conduction

4 When a cardiac cell depolarizes the interior of the cell becomes what with respect to the surrounding mediaA More positiveB More negativeC More acidicD Hotter

5 Fill in the blank In the healthy heart the faster the heart rate the ___ the refractory periodA More vigorousB Less reliableC LongerD Shorter

20 The nuts and bolts of implantable device therapy pacemakers

6 Which specific part of the nervous system governs the fight-or-flight response and is associated with epinephrine (adrenaline) (Pick the most exact answer)A Autonomic nervous systemB Parasympathetic nervous systemC Sympathetic nervous systemD Central nervous system

7 In normal antegrade conduction the electrical impulse travels from the SA node over the atria to the AV node and to what structure before reaching the ventriclesA The intermodal tractsB The Purkinje fibersC The bundle of HisD The ventricular apex

8 Which of the following is true about retrograde cardiac conductionA Everyone has itB It is perfectly normal and can be helpful in

stressful situations

C It is abnormal but not uncommonD It is impossible to diagnose definitively

except in an autopsy

9 What chemical is associated with the parasympathetic nervous systemA AcetylcholineB PotassiumC EpinephrineD Dopamine

10 The stimulation of the myocardial cells of which of the following structures is most closely associated with ventricular depolarizationA SA nodeB AV junctionC Bundle of HisD Purkinje fibers

15

The Nuts and Bolts of Implantable Device Therapy Pacemakers First Edition Tom Kenny copy 2015 John Wiley amp Sons Ltd Published 2015 by John Wiley amp Sons Ltd

Chapter 2

Cardiac conduction system

Introduction

The sinoatrial (SA) node is the heartrsquos natural pacemaker In the healthy heart the SA node fires and the electrical stimulus conducts along special pathways through the heart that cause cells in specific areas to depolarize After the stimulus passes the area the cells repolarize Myocardial cell depolarization and repolarization govern the heartrsquos contraction and relaxation sequencemdashin other words the heart rhythm Rhythm disorders occur when this healthy conduction system is in some way disrupted or impaired

the conduction pathways

When the SA node fires and sends out electrical energy the stimulus travels from the SA node in the high right atrium out over the atria via path-ways known as internodal tracts Most of these internodal tracts are located in the right atrium the

main tract over to the left atrium is called Bachmannrsquos bundle It is because of Bachmannrsquos bundle linking the right atrium to the left atrium that the atria can contract simultaneously that is as these two upper chambers contract as one coherent unit After traversing the atria the impulse comes down through conduction tracts to the atrioventricular (AV) node

The AV node is an area of highly specialized tissue located below the atria and above the ventri-cles on the right side of the septum Anatomically the AV node rests in the AV groove The tissue in the healthy AV node has the ability to change the speed of the electrical impulse specifically it slows the stimulus down The slowdown serves two important purposes First slowing the impulse allows the atria time to contract and provide the very valuable atrial kick to ventricular filling Second the slowed impulse protects the ventricles from being forced to beat too quickly that is to beat before they have been adequately filled with blood

Once the impulse crosses the AV node it travels down into the ventricles and accelerates its speed The impulse travels through the bundle of His (pronounced hiss) to the bundle branches (right and left) and then out to the network of Purkinje fibers This allows both ventricles to depolarize and contract simultaneously (Figure 21)

polarization depolarization and repolarization

Cardiac conduction is driven by the ability of myo-cardial cells to be depolarized and repolarized Depolarization occurs at the cellular level and is

2

Learning objectives

bullensp Describe the electrical conduction system of the healthy heart

bullensp List the main anatomical landmarks of normal cardiac conduction

bullensp Name the anatomical location of the heartrsquos natural pacemaker

bullensp Explain depolarization and repolarization in electrical terms and how it relates to the cardiac contraction

bullensp Define the term automaticity and how it relates to cardiac conduction

16 The nuts and bolts of implantable device therapy pacemakers

in turn driven by electrolytes or positively and negatively charged ions (potassium sodium and calcium mainly) which are able to move in and out of the cardiac cells A myocardial cell may be said to be polarized when in its resting state that is there is no electrical activity going on In the rest-ing state the interior of the myocardial cell is somewhat more negatively charged that the outside This state is called the resting state or sometimes phase 0 of the action potential

An electrical stimulus changes the resting state abruptly The electrical energy allows for the movement of ions to the point that the interior of the myocardial cell is now more positive than the outside In other words the charge reverses and the cell is said to be depolarized When this happens the cell contracts when this happens to a large area of the myocardium the heart muscle contracts Cardiac contraction occurs as a result of cellular depolarization

When the electrical stimulus moves past the myocardial cell the ions move in reverse and the interior of the cell assumes its previous negative charge with respect to the outside This process is

called repolarization and it brings the myocar-dial cell back to the resting state When this happens the cell relaxes When a large area of the myocardium repolarizes the heart resumes its previous shape (loses its contraction)

Intrinsic pacemakers

The healthy heartrsquos natural pacemaker is the SA node but the SA node is not the only area of the heart that can generate an electrical pulse The SA node serves as the heartrsquos pacemaker because it generates these pulses faster than other areas In the healthy heart the SA node can fire at around 60ndash100 times a minute corresponding to a healthy heart rate The AV junction can also fire but it is much slower at around 40ndash60 times a minute Even the Purkinje fibers (the ventri-cles) can act as a pacemaker but they fire at only 20ndash40 times a minute Thus in a healthy individual the SA node outpaces these other potential pacemakers The AV junction and ven-tricles are sometimes called backup pacemakers because they take over the heart rate in the event

Sinoatrial (SA) node

Internodal tracts

Right bundle branch

Left bundle branch

Purkinje fibers

Bachmannrsquos bundle

Bundle of His

Atrioventricular (AV) node

Figure 21 The conduction pathways in the healthy heart

Chapter 2 Cardiac conduction system 17

that the SA node is not able to pace the heart While these backup pacemakers may help keep the heart beating in the event of a dysfunctional SA node they are slow and not as reliable as the SA node

The healthy heart is able to pace itself because of a property called automaticity Automaticity is an inherent property of individual cardiac conductive cells that allows them to depolarize spontaneously In other words some cells in the heart (such as the cells of the SA node) are able to generate an electrical stimulus on their own

refractoriness

Refractoriness refers to the temporary inability of a cardiac cell to respond to an electrical stimulus Refractoriness can be the normal appropriate and expected behavior of cardiac cells For example immediately after electrical energy has passed through cardiac cells they go through a phase where they are unresponsive to electrical energy no matter how much voltage is delivered This normal refractoriness is called the physiological refractory period and it is very brief (it is mea-sured in thousandths of a second) Of course the exact duration of a refractory period is based in part on the heart rate the faster the heart rate the shorter the refractory period and vice versa

Refractoriness may also occur when certain cells become dysfunctional that is cardiac cells may lose their ability to conduct properly Furthermore the heartrsquos inherent pacemakers may change their rate for instance the AV junction may speed up and start to deliver impulses at 80 or 100 times a minute When this happensmdashthat is when another pacemaker takes over the role of the SA nodemdashthat pacemaker is called a usurping pacemaker because it usurps the rightful role of the SA node In the event that other pacemakers of the heart start to compete with the SA node the fastest pacemaker always wins

Conductivity

Conductivity is the cardiac cellrsquos ability to transmit an electrical impulse to an adjacent cell Typically cardiac conduction is antegrade that

is it moves forward Electrical impulses actually travel out in all different directions but refracto-riness prevents those cells that were just stimu-lated from depolarization This keeps conduction moving in a single direction through the heart Normal antegrade conduction in a healthy heart goes from SA node to atria to AV node over the bundle of His and down to the ventricles Some individuals have the ability to conduct retro-grade retrograde conduction is abnormal but not uncommon

autonomic nervous system

The heart does not need direct stimulation from the nervous system to generate electricity and to beat but that is not to say that the nervous system does not control the heart rate The autonomic nervous system indirectly affects cardiac conduction by regulating how fast the heart will beat and how hard the heart muscle will squeeze (contractility) The autonomic ner-vous system communicates with the heart through a network of baroreceptors which respond to chemical signals

The autonomic nervous system of the body like certain other physiological systems exists as two systems in balance The autonomic nervous system consists of a sympathetic system and a parasympathetic system They both work together and in a healthy individual one may overdrive the other during specific times The sympathetic ner-vous system is mainly associated with epinephrine (adrenaline) and it has the ability to speed the heart rate and increase cardiac contractility and can increase the speed by which electrical impulses can cross the AV node The sympathetic nervous system takes over when we are stressed frightened (fight-or-flight response) or exerting ourselves

On the other hand the autonomic nervous system also has a parasympathetic system that is associated with acetylcholine This system can decrease the heart rate decrease cardiac contrac-tility and cause impulses crossing the AV node to be slowed The parasympathetic system might take over when we are sleeping or at rest

18 The nuts and bolts of implantable device therapy pacemakers

the nuts and bolts of the cardiac conduction system

bull The heartrsquos natural pacemaker is the sinoatrial (SA) node which generates an electrical impulse spontaneously about 60ndash100 times a minute in a healthy individual

bull The normal conduction path is from the SA node over the atria to the atrioventricular (AV) node and then via the bundle of His down the right and left bundle branches and into the Purkinje fibers

bull The AV node has the ability to slow down the electrical impulse This serves two main purposes it allows for atrial kick and it protects the ventricles from beating too rapidly

bull In the healthy heart the right and left atria contract simultaneously and then the right and left ventricles contract simultaneously

bull Depolarization and repolarization of myocar-dial cells are driven by positively and negatively charged ions which cross the cell membrane A resting cardiac cell is more negative on the inside than the surrounding media but when it depolarizes it becomes more positive on the inside relative to the surrounding media Following depolarization the ions move and the cell returns to being more negative on the inside than the outside in a process known as repolarization

bull Depolarization causes the cardiac cell to contract

bull The main ions involved in depolarization and repolarization are sodium potassium and calcium ions

bull Although the heartrsquos natural pacemaker is the SA node the AV junction and the Purkinje fibers (ventricles) can also assume (usurp) this role if the SA node cannot generate electricity properly The AV junction and Purkinje fibers are not as reliable as a healthy AV node and typically generate electricity more slowly (the AV junction about 40ndash60 times a minute and the Purkinje fibers 20ndash40 times a minute)

bull However sometimes the AV junction or Purkinje fibers may generate electricity very

quickly this is abnormal When they do they may take over the heart When pacemakers of the heart compete the fastest one drives the heart

bull Automaticity is an inherent property of certain cardiac conductive cells to depolarize spontaneously Not all cardiac conduction cells possess automaticity but the SA node AV junction and Purkinje fibers possess automaticity

bull Refractoriness is the natural and expected behavior of cardiac conductive cells that makes them unresponsive to electrical impulses immediately after they have depolarized The natural refractory period is very brief

bull The faster the heart rate the shorter the refractory period the slower the heart rate the longer the refractory period

bull Refractoriness can also occur as a dysfunction bull Conductivity is the cardiac cellrsquos ability to

transmit an electrical impulse to adjacent cells Refractoriness means that cells that just depolarized will not depolarize again This keeps conduction moving forward (antegrade conduction)

bull Antegrade conduction is normal retrograde conduction is abnormal but is not rare

bull The autonomic nervous system regulates the heart rate by controlling how fast the heart beats and how vigorously it contracts

bull The autonomic nervous system consists of a sympathetic and parasympathetic nervous system which work together but at certain times one or the other is dominant The sympathetic nervous system makes the heart beat faster and more vigorously it takes over during times of stress exertion or fight or flight The parasympathetic ner-vous system makes the heart beat slowly and less vigorously it takes over during periods of rest

bull The neurotransmitters associated with these two systems are epinephrine or adrenaline (sympathetic nervous system) and acetylcho-line (parasympathetic nervous system)

Chapter 2 Cardiac conduction system 19

test your knowledge

Please write in the landmarks of the cardiac conduction system Select from the following (each is used only once)

AV node Bachmanrsquos bundle Bundle of His

Internodal tracts Left bundle branch Purkinje fibers

Right bundle branch Ventricles

1 Which of the following is not considered one of the heartrsquos intrinsic pacemakersA AV junctionB SA nodeC Purkinje fibersD Aorta

2 Depolarization and repolarization involve the movement of charged ions across cardiac cell membranes What types of ions are primarily involvedA Calcium potassium and sodiumB Acetylcholine and epinephrineC Magnesium and folateD All of the above

3 Refractoriness refers toA The ability of cardiac conduction cells to

contract

B The inability of cardiac conduction cells to respond to an electrical stimulus

C The unique permeability of the cardiac cellsD Abnormal cardiac conduction

4 When a cardiac cell depolarizes the interior of the cell becomes what with respect to the surrounding mediaA More positiveB More negativeC More acidicD Hotter

5 Fill in the blank In the healthy heart the faster the heart rate the ___ the refractory periodA More vigorousB Less reliableC LongerD Shorter

20 The nuts and bolts of implantable device therapy pacemakers

6 Which specific part of the nervous system governs the fight-or-flight response and is associated with epinephrine (adrenaline) (Pick the most exact answer)A Autonomic nervous systemB Parasympathetic nervous systemC Sympathetic nervous systemD Central nervous system

7 In normal antegrade conduction the electrical impulse travels from the SA node over the atria to the AV node and to what structure before reaching the ventriclesA The intermodal tractsB The Purkinje fibersC The bundle of HisD The ventricular apex

8 Which of the following is true about retrograde cardiac conductionA Everyone has itB It is perfectly normal and can be helpful in

stressful situations

C It is abnormal but not uncommonD It is impossible to diagnose definitively

except in an autopsy

9 What chemical is associated with the parasympathetic nervous systemA AcetylcholineB PotassiumC EpinephrineD Dopamine

10 The stimulation of the myocardial cells of which of the following structures is most closely associated with ventricular depolarizationA SA nodeB AV junctionC Bundle of HisD Purkinje fibers

16 The nuts and bolts of implantable device therapy pacemakers

in turn driven by electrolytes or positively and negatively charged ions (potassium sodium and calcium mainly) which are able to move in and out of the cardiac cells A myocardial cell may be said to be polarized when in its resting state that is there is no electrical activity going on In the rest-ing state the interior of the myocardial cell is somewhat more negatively charged that the outside This state is called the resting state or sometimes phase 0 of the action potential

An electrical stimulus changes the resting state abruptly The electrical energy allows for the movement of ions to the point that the interior of the myocardial cell is now more positive than the outside In other words the charge reverses and the cell is said to be depolarized When this happens the cell contracts when this happens to a large area of the myocardium the heart muscle contracts Cardiac contraction occurs as a result of cellular depolarization

When the electrical stimulus moves past the myocardial cell the ions move in reverse and the interior of the cell assumes its previous negative charge with respect to the outside This process is

called repolarization and it brings the myocar-dial cell back to the resting state When this happens the cell relaxes When a large area of the myocardium repolarizes the heart resumes its previous shape (loses its contraction)

Intrinsic pacemakers

The healthy heartrsquos natural pacemaker is the SA node but the SA node is not the only area of the heart that can generate an electrical pulse The SA node serves as the heartrsquos pacemaker because it generates these pulses faster than other areas In the healthy heart the SA node can fire at around 60ndash100 times a minute corresponding to a healthy heart rate The AV junction can also fire but it is much slower at around 40ndash60 times a minute Even the Purkinje fibers (the ventri-cles) can act as a pacemaker but they fire at only 20ndash40 times a minute Thus in a healthy individual the SA node outpaces these other potential pacemakers The AV junction and ven-tricles are sometimes called backup pacemakers because they take over the heart rate in the event

Sinoatrial (SA) node

Internodal tracts

Right bundle branch

Left bundle branch

Purkinje fibers

Bachmannrsquos bundle

Bundle of His

Atrioventricular (AV) node

Figure 21 The conduction pathways in the healthy heart

Chapter 2 Cardiac conduction system 17

that the SA node is not able to pace the heart While these backup pacemakers may help keep the heart beating in the event of a dysfunctional SA node they are slow and not as reliable as the SA node

The healthy heart is able to pace itself because of a property called automaticity Automaticity is an inherent property of individual cardiac conductive cells that allows them to depolarize spontaneously In other words some cells in the heart (such as the cells of the SA node) are able to generate an electrical stimulus on their own

refractoriness

Refractoriness refers to the temporary inability of a cardiac cell to respond to an electrical stimulus Refractoriness can be the normal appropriate and expected behavior of cardiac cells For example immediately after electrical energy has passed through cardiac cells they go through a phase where they are unresponsive to electrical energy no matter how much voltage is delivered This normal refractoriness is called the physiological refractory period and it is very brief (it is mea-sured in thousandths of a second) Of course the exact duration of a refractory period is based in part on the heart rate the faster the heart rate the shorter the refractory period and vice versa

Refractoriness may also occur when certain cells become dysfunctional that is cardiac cells may lose their ability to conduct properly Furthermore the heartrsquos inherent pacemakers may change their rate for instance the AV junction may speed up and start to deliver impulses at 80 or 100 times a minute When this happensmdashthat is when another pacemaker takes over the role of the SA nodemdashthat pacemaker is called a usurping pacemaker because it usurps the rightful role of the SA node In the event that other pacemakers of the heart start to compete with the SA node the fastest pacemaker always wins

Conductivity

Conductivity is the cardiac cellrsquos ability to transmit an electrical impulse to an adjacent cell Typically cardiac conduction is antegrade that

is it moves forward Electrical impulses actually travel out in all different directions but refracto-riness prevents those cells that were just stimu-lated from depolarization This keeps conduction moving in a single direction through the heart Normal antegrade conduction in a healthy heart goes from SA node to atria to AV node over the bundle of His and down to the ventricles Some individuals have the ability to conduct retro-grade retrograde conduction is abnormal but not uncommon

autonomic nervous system

The heart does not need direct stimulation from the nervous system to generate electricity and to beat but that is not to say that the nervous system does not control the heart rate The autonomic nervous system indirectly affects cardiac conduction by regulating how fast the heart will beat and how hard the heart muscle will squeeze (contractility) The autonomic ner-vous system communicates with the heart through a network of baroreceptors which respond to chemical signals

The autonomic nervous system of the body like certain other physiological systems exists as two systems in balance The autonomic nervous system consists of a sympathetic system and a parasympathetic system They both work together and in a healthy individual one may overdrive the other during specific times The sympathetic ner-vous system is mainly associated with epinephrine (adrenaline) and it has the ability to speed the heart rate and increase cardiac contractility and can increase the speed by which electrical impulses can cross the AV node The sympathetic nervous system takes over when we are stressed frightened (fight-or-flight response) or exerting ourselves

On the other hand the autonomic nervous system also has a parasympathetic system that is associated with acetylcholine This system can decrease the heart rate decrease cardiac contrac-tility and cause impulses crossing the AV node to be slowed The parasympathetic system might take over when we are sleeping or at rest

18 The nuts and bolts of implantable device therapy pacemakers

the nuts and bolts of the cardiac conduction system

bull The heartrsquos natural pacemaker is the sinoatrial (SA) node which generates an electrical impulse spontaneously about 60ndash100 times a minute in a healthy individual

bull The normal conduction path is from the SA node over the atria to the atrioventricular (AV) node and then via the bundle of His down the right and left bundle branches and into the Purkinje fibers

bull The AV node has the ability to slow down the electrical impulse This serves two main purposes it allows for atrial kick and it protects the ventricles from beating too rapidly

bull In the healthy heart the right and left atria contract simultaneously and then the right and left ventricles contract simultaneously

bull Depolarization and repolarization of myocar-dial cells are driven by positively and negatively charged ions which cross the cell membrane A resting cardiac cell is more negative on the inside than the surrounding media but when it depolarizes it becomes more positive on the inside relative to the surrounding media Following depolarization the ions move and the cell returns to being more negative on the inside than the outside in a process known as repolarization

bull Depolarization causes the cardiac cell to contract

bull The main ions involved in depolarization and repolarization are sodium potassium and calcium ions

bull Although the heartrsquos natural pacemaker is the SA node the AV junction and the Purkinje fibers (ventricles) can also assume (usurp) this role if the SA node cannot generate electricity properly The AV junction and Purkinje fibers are not as reliable as a healthy AV node and typically generate electricity more slowly (the AV junction about 40ndash60 times a minute and the Purkinje fibers 20ndash40 times a minute)

bull However sometimes the AV junction or Purkinje fibers may generate electricity very

quickly this is abnormal When they do they may take over the heart When pacemakers of the heart compete the fastest one drives the heart

bull Automaticity is an inherent property of certain cardiac conductive cells to depolarize spontaneously Not all cardiac conduction cells possess automaticity but the SA node AV junction and Purkinje fibers possess automaticity

bull Refractoriness is the natural and expected behavior of cardiac conductive cells that makes them unresponsive to electrical impulses immediately after they have depolarized The natural refractory period is very brief

bull The faster the heart rate the shorter the refractory period the slower the heart rate the longer the refractory period

bull Refractoriness can also occur as a dysfunction bull Conductivity is the cardiac cellrsquos ability to

transmit an electrical impulse to adjacent cells Refractoriness means that cells that just depolarized will not depolarize again This keeps conduction moving forward (antegrade conduction)

bull Antegrade conduction is normal retrograde conduction is abnormal but is not rare

bull The autonomic nervous system regulates the heart rate by controlling how fast the heart beats and how vigorously it contracts

bull The autonomic nervous system consists of a sympathetic and parasympathetic nervous system which work together but at certain times one or the other is dominant The sympathetic nervous system makes the heart beat faster and more vigorously it takes over during times of stress exertion or fight or flight The parasympathetic ner-vous system makes the heart beat slowly and less vigorously it takes over during periods of rest

bull The neurotransmitters associated with these two systems are epinephrine or adrenaline (sympathetic nervous system) and acetylcho-line (parasympathetic nervous system)

Chapter 2 Cardiac conduction system 19

test your knowledge

Please write in the landmarks of the cardiac conduction system Select from the following (each is used only once)

AV node Bachmanrsquos bundle Bundle of His

Internodal tracts Left bundle branch Purkinje fibers

Right bundle branch Ventricles

1 Which of the following is not considered one of the heartrsquos intrinsic pacemakersA AV junctionB SA nodeC Purkinje fibersD Aorta

2 Depolarization and repolarization involve the movement of charged ions across cardiac cell membranes What types of ions are primarily involvedA Calcium potassium and sodiumB Acetylcholine and epinephrineC Magnesium and folateD All of the above

3 Refractoriness refers toA The ability of cardiac conduction cells to

contract

B The inability of cardiac conduction cells to respond to an electrical stimulus

C The unique permeability of the cardiac cellsD Abnormal cardiac conduction

4 When a cardiac cell depolarizes the interior of the cell becomes what with respect to the surrounding mediaA More positiveB More negativeC More acidicD Hotter

5 Fill in the blank In the healthy heart the faster the heart rate the ___ the refractory periodA More vigorousB Less reliableC LongerD Shorter

20 The nuts and bolts of implantable device therapy pacemakers

6 Which specific part of the nervous system governs the fight-or-flight response and is associated with epinephrine (adrenaline) (Pick the most exact answer)A Autonomic nervous systemB Parasympathetic nervous systemC Sympathetic nervous systemD Central nervous system

7 In normal antegrade conduction the electrical impulse travels from the SA node over the atria to the AV node and to what structure before reaching the ventriclesA The intermodal tractsB The Purkinje fibersC The bundle of HisD The ventricular apex

8 Which of the following is true about retrograde cardiac conductionA Everyone has itB It is perfectly normal and can be helpful in

stressful situations

C It is abnormal but not uncommonD It is impossible to diagnose definitively

except in an autopsy

9 What chemical is associated with the parasympathetic nervous systemA AcetylcholineB PotassiumC EpinephrineD Dopamine

10 The stimulation of the myocardial cells of which of the following structures is most closely associated with ventricular depolarizationA SA nodeB AV junctionC Bundle of HisD Purkinje fibers

Chapter 2 Cardiac conduction system 17

that the SA node is not able to pace the heart While these backup pacemakers may help keep the heart beating in the event of a dysfunctional SA node they are slow and not as reliable as the SA node

The healthy heart is able to pace itself because of a property called automaticity Automaticity is an inherent property of individual cardiac conductive cells that allows them to depolarize spontaneously In other words some cells in the heart (such as the cells of the SA node) are able to generate an electrical stimulus on their own

refractoriness

Refractoriness refers to the temporary inability of a cardiac cell to respond to an electrical stimulus Refractoriness can be the normal appropriate and expected behavior of cardiac cells For example immediately after electrical energy has passed through cardiac cells they go through a phase where they are unresponsive to electrical energy no matter how much voltage is delivered This normal refractoriness is called the physiological refractory period and it is very brief (it is mea-sured in thousandths of a second) Of course the exact duration of a refractory period is based in part on the heart rate the faster the heart rate the shorter the refractory period and vice versa

Refractoriness may also occur when certain cells become dysfunctional that is cardiac cells may lose their ability to conduct properly Furthermore the heartrsquos inherent pacemakers may change their rate for instance the AV junction may speed up and start to deliver impulses at 80 or 100 times a minute When this happensmdashthat is when another pacemaker takes over the role of the SA nodemdashthat pacemaker is called a usurping pacemaker because it usurps the rightful role of the SA node In the event that other pacemakers of the heart start to compete with the SA node the fastest pacemaker always wins

Conductivity

Conductivity is the cardiac cellrsquos ability to transmit an electrical impulse to an adjacent cell Typically cardiac conduction is antegrade that

is it moves forward Electrical impulses actually travel out in all different directions but refracto-riness prevents those cells that were just stimu-lated from depolarization This keeps conduction moving in a single direction through the heart Normal antegrade conduction in a healthy heart goes from SA node to atria to AV node over the bundle of His and down to the ventricles Some individuals have the ability to conduct retro-grade retrograde conduction is abnormal but not uncommon

autonomic nervous system

The heart does not need direct stimulation from the nervous system to generate electricity and to beat but that is not to say that the nervous system does not control the heart rate The autonomic nervous system indirectly affects cardiac conduction by regulating how fast the heart will beat and how hard the heart muscle will squeeze (contractility) The autonomic ner-vous system communicates with the heart through a network of baroreceptors which respond to chemical signals

The autonomic nervous system of the body like certain other physiological systems exists as two systems in balance The autonomic nervous system consists of a sympathetic system and a parasympathetic system They both work together and in a healthy individual one may overdrive the other during specific times The sympathetic ner-vous system is mainly associated with epinephrine (adrenaline) and it has the ability to speed the heart rate and increase cardiac contractility and can increase the speed by which electrical impulses can cross the AV node The sympathetic nervous system takes over when we are stressed frightened (fight-or-flight response) or exerting ourselves

On the other hand the autonomic nervous system also has a parasympathetic system that is associated with acetylcholine This system can decrease the heart rate decrease cardiac contrac-tility and cause impulses crossing the AV node to be slowed The parasympathetic system might take over when we are sleeping or at rest

18 The nuts and bolts of implantable device therapy pacemakers

the nuts and bolts of the cardiac conduction system

bull The heartrsquos natural pacemaker is the sinoatrial (SA) node which generates an electrical impulse spontaneously about 60ndash100 times a minute in a healthy individual

bull The normal conduction path is from the SA node over the atria to the atrioventricular (AV) node and then via the bundle of His down the right and left bundle branches and into the Purkinje fibers

bull The AV node has the ability to slow down the electrical impulse This serves two main purposes it allows for atrial kick and it protects the ventricles from beating too rapidly

bull In the healthy heart the right and left atria contract simultaneously and then the right and left ventricles contract simultaneously

bull Depolarization and repolarization of myocar-dial cells are driven by positively and negatively charged ions which cross the cell membrane A resting cardiac cell is more negative on the inside than the surrounding media but when it depolarizes it becomes more positive on the inside relative to the surrounding media Following depolarization the ions move and the cell returns to being more negative on the inside than the outside in a process known as repolarization

bull Depolarization causes the cardiac cell to contract

bull The main ions involved in depolarization and repolarization are sodium potassium and calcium ions

bull Although the heartrsquos natural pacemaker is the SA node the AV junction and the Purkinje fibers (ventricles) can also assume (usurp) this role if the SA node cannot generate electricity properly The AV junction and Purkinje fibers are not as reliable as a healthy AV node and typically generate electricity more slowly (the AV junction about 40ndash60 times a minute and the Purkinje fibers 20ndash40 times a minute)

bull However sometimes the AV junction or Purkinje fibers may generate electricity very

quickly this is abnormal When they do they may take over the heart When pacemakers of the heart compete the fastest one drives the heart

bull Automaticity is an inherent property of certain cardiac conductive cells to depolarize spontaneously Not all cardiac conduction cells possess automaticity but the SA node AV junction and Purkinje fibers possess automaticity

bull Refractoriness is the natural and expected behavior of cardiac conductive cells that makes them unresponsive to electrical impulses immediately after they have depolarized The natural refractory period is very brief

bull The faster the heart rate the shorter the refractory period the slower the heart rate the longer the refractory period

bull Refractoriness can also occur as a dysfunction bull Conductivity is the cardiac cellrsquos ability to

transmit an electrical impulse to adjacent cells Refractoriness means that cells that just depolarized will not depolarize again This keeps conduction moving forward (antegrade conduction)

bull Antegrade conduction is normal retrograde conduction is abnormal but is not rare

bull The autonomic nervous system regulates the heart rate by controlling how fast the heart beats and how vigorously it contracts

bull The autonomic nervous system consists of a sympathetic and parasympathetic nervous system which work together but at certain times one or the other is dominant The sympathetic nervous system makes the heart beat faster and more vigorously it takes over during times of stress exertion or fight or flight The parasympathetic ner-vous system makes the heart beat slowly and less vigorously it takes over during periods of rest

bull The neurotransmitters associated with these two systems are epinephrine or adrenaline (sympathetic nervous system) and acetylcho-line (parasympathetic nervous system)

Chapter 2 Cardiac conduction system 19

test your knowledge

Please write in the landmarks of the cardiac conduction system Select from the following (each is used only once)

AV node Bachmanrsquos bundle Bundle of His

Internodal tracts Left bundle branch Purkinje fibers

Right bundle branch Ventricles

1 Which of the following is not considered one of the heartrsquos intrinsic pacemakersA AV junctionB SA nodeC Purkinje fibersD Aorta

2 Depolarization and repolarization involve the movement of charged ions across cardiac cell membranes What types of ions are primarily involvedA Calcium potassium and sodiumB Acetylcholine and epinephrineC Magnesium and folateD All of the above

3 Refractoriness refers toA The ability of cardiac conduction cells to

contract

B The inability of cardiac conduction cells to respond to an electrical stimulus

C The unique permeability of the cardiac cellsD Abnormal cardiac conduction

4 When a cardiac cell depolarizes the interior of the cell becomes what with respect to the surrounding mediaA More positiveB More negativeC More acidicD Hotter

5 Fill in the blank In the healthy heart the faster the heart rate the ___ the refractory periodA More vigorousB Less reliableC LongerD Shorter

20 The nuts and bolts of implantable device therapy pacemakers

6 Which specific part of the nervous system governs the fight-or-flight response and is associated with epinephrine (adrenaline) (Pick the most exact answer)A Autonomic nervous systemB Parasympathetic nervous systemC Sympathetic nervous systemD Central nervous system

7 In normal antegrade conduction the electrical impulse travels from the SA node over the atria to the AV node and to what structure before reaching the ventriclesA The intermodal tractsB The Purkinje fibersC The bundle of HisD The ventricular apex

8 Which of the following is true about retrograde cardiac conductionA Everyone has itB It is perfectly normal and can be helpful in

stressful situations

C It is abnormal but not uncommonD It is impossible to diagnose definitively

except in an autopsy

9 What chemical is associated with the parasympathetic nervous systemA AcetylcholineB PotassiumC EpinephrineD Dopamine

10 The stimulation of the myocardial cells of which of the following structures is most closely associated with ventricular depolarizationA SA nodeB AV junctionC Bundle of HisD Purkinje fibers

18 The nuts and bolts of implantable device therapy pacemakers

the nuts and bolts of the cardiac conduction system

bull The heartrsquos natural pacemaker is the sinoatrial (SA) node which generates an electrical impulse spontaneously about 60ndash100 times a minute in a healthy individual

bull The normal conduction path is from the SA node over the atria to the atrioventricular (AV) node and then via the bundle of His down the right and left bundle branches and into the Purkinje fibers

bull The AV node has the ability to slow down the electrical impulse This serves two main purposes it allows for atrial kick and it protects the ventricles from beating too rapidly

bull In the healthy heart the right and left atria contract simultaneously and then the right and left ventricles contract simultaneously

bull Depolarization and repolarization of myocar-dial cells are driven by positively and negatively charged ions which cross the cell membrane A resting cardiac cell is more negative on the inside than the surrounding media but when it depolarizes it becomes more positive on the inside relative to the surrounding media Following depolarization the ions move and the cell returns to being more negative on the inside than the outside in a process known as repolarization

bull Depolarization causes the cardiac cell to contract

bull The main ions involved in depolarization and repolarization are sodium potassium and calcium ions

bull Although the heartrsquos natural pacemaker is the SA node the AV junction and the Purkinje fibers (ventricles) can also assume (usurp) this role if the SA node cannot generate electricity properly The AV junction and Purkinje fibers are not as reliable as a healthy AV node and typically generate electricity more slowly (the AV junction about 40ndash60 times a minute and the Purkinje fibers 20ndash40 times a minute)

bull However sometimes the AV junction or Purkinje fibers may generate electricity very

quickly this is abnormal When they do they may take over the heart When pacemakers of the heart compete the fastest one drives the heart

bull Automaticity is an inherent property of certain cardiac conductive cells to depolarize spontaneously Not all cardiac conduction cells possess automaticity but the SA node AV junction and Purkinje fibers possess automaticity

bull Refractoriness is the natural and expected behavior of cardiac conductive cells that makes them unresponsive to electrical impulses immediately after they have depolarized The natural refractory period is very brief

bull The faster the heart rate the shorter the refractory period the slower the heart rate the longer the refractory period

bull Refractoriness can also occur as a dysfunction bull Conductivity is the cardiac cellrsquos ability to

transmit an electrical impulse to adjacent cells Refractoriness means that cells that just depolarized will not depolarize again This keeps conduction moving forward (antegrade conduction)

bull Antegrade conduction is normal retrograde conduction is abnormal but is not rare

bull The autonomic nervous system regulates the heart rate by controlling how fast the heart beats and how vigorously it contracts

bull The autonomic nervous system consists of a sympathetic and parasympathetic nervous system which work together but at certain times one or the other is dominant The sympathetic nervous system makes the heart beat faster and more vigorously it takes over during times of stress exertion or fight or flight The parasympathetic ner-vous system makes the heart beat slowly and less vigorously it takes over during periods of rest

bull The neurotransmitters associated with these two systems are epinephrine or adrenaline (sympathetic nervous system) and acetylcho-line (parasympathetic nervous system)

Chapter 2 Cardiac conduction system 19

test your knowledge

Please write in the landmarks of the cardiac conduction system Select from the following (each is used only once)

AV node Bachmanrsquos bundle Bundle of His

Internodal tracts Left bundle branch Purkinje fibers

Right bundle branch Ventricles

1 Which of the following is not considered one of the heartrsquos intrinsic pacemakersA AV junctionB SA nodeC Purkinje fibersD Aorta

2 Depolarization and repolarization involve the movement of charged ions across cardiac cell membranes What types of ions are primarily involvedA Calcium potassium and sodiumB Acetylcholine and epinephrineC Magnesium and folateD All of the above

3 Refractoriness refers toA The ability of cardiac conduction cells to

contract

B The inability of cardiac conduction cells to respond to an electrical stimulus

C The unique permeability of the cardiac cellsD Abnormal cardiac conduction

4 When a cardiac cell depolarizes the interior of the cell becomes what with respect to the surrounding mediaA More positiveB More negativeC More acidicD Hotter

5 Fill in the blank In the healthy heart the faster the heart rate the ___ the refractory periodA More vigorousB Less reliableC LongerD Shorter

20 The nuts and bolts of implantable device therapy pacemakers

6 Which specific part of the nervous system governs the fight-or-flight response and is associated with epinephrine (adrenaline) (Pick the most exact answer)A Autonomic nervous systemB Parasympathetic nervous systemC Sympathetic nervous systemD Central nervous system

7 In normal antegrade conduction the electrical impulse travels from the SA node over the atria to the AV node and to what structure before reaching the ventriclesA The intermodal tractsB The Purkinje fibersC The bundle of HisD The ventricular apex

8 Which of the following is true about retrograde cardiac conductionA Everyone has itB It is perfectly normal and can be helpful in

stressful situations

C It is abnormal but not uncommonD It is impossible to diagnose definitively

except in an autopsy

9 What chemical is associated with the parasympathetic nervous systemA AcetylcholineB PotassiumC EpinephrineD Dopamine

10 The stimulation of the myocardial cells of which of the following structures is most closely associated with ventricular depolarizationA SA nodeB AV junctionC Bundle of HisD Purkinje fibers

Chapter 2 Cardiac conduction system 19

test your knowledge

Please write in the landmarks of the cardiac conduction system Select from the following (each is used only once)

AV node Bachmanrsquos bundle Bundle of His

Internodal tracts Left bundle branch Purkinje fibers

Right bundle branch Ventricles

1 Which of the following is not considered one of the heartrsquos intrinsic pacemakersA AV junctionB SA nodeC Purkinje fibersD Aorta

2 Depolarization and repolarization involve the movement of charged ions across cardiac cell membranes What types of ions are primarily involvedA Calcium potassium and sodiumB Acetylcholine and epinephrineC Magnesium and folateD All of the above

3 Refractoriness refers toA The ability of cardiac conduction cells to

contract

B The inability of cardiac conduction cells to respond to an electrical stimulus

C The unique permeability of the cardiac cellsD Abnormal cardiac conduction

4 When a cardiac cell depolarizes the interior of the cell becomes what with respect to the surrounding mediaA More positiveB More negativeC More acidicD Hotter

5 Fill in the blank In the healthy heart the faster the heart rate the ___ the refractory periodA More vigorousB Less reliableC LongerD Shorter

20 The nuts and bolts of implantable device therapy pacemakers

6 Which specific part of the nervous system governs the fight-or-flight response and is associated with epinephrine (adrenaline) (Pick the most exact answer)A Autonomic nervous systemB Parasympathetic nervous systemC Sympathetic nervous systemD Central nervous system

7 In normal antegrade conduction the electrical impulse travels from the SA node over the atria to the AV node and to what structure before reaching the ventriclesA The intermodal tractsB The Purkinje fibersC The bundle of HisD The ventricular apex

8 Which of the following is true about retrograde cardiac conductionA Everyone has itB It is perfectly normal and can be helpful in

stressful situations

C It is abnormal but not uncommonD It is impossible to diagnose definitively

except in an autopsy

9 What chemical is associated with the parasympathetic nervous systemA AcetylcholineB PotassiumC EpinephrineD Dopamine

10 The stimulation of the myocardial cells of which of the following structures is most closely associated with ventricular depolarizationA SA nodeB AV junctionC Bundle of HisD Purkinje fibers

20 The nuts and bolts of implantable device therapy pacemakers

6 Which specific part of the nervous system governs the fight-or-flight response and is associated with epinephrine (adrenaline) (Pick the most exact answer)A Autonomic nervous systemB Parasympathetic nervous systemC Sympathetic nervous systemD Central nervous system

7 In normal antegrade conduction the electrical impulse travels from the SA node over the atria to the AV node and to what structure before reaching the ventriclesA The intermodal tractsB The Purkinje fibersC The bundle of HisD The ventricular apex

8 Which of the following is true about retrograde cardiac conductionA Everyone has itB It is perfectly normal and can be helpful in

stressful situations

C It is abnormal but not uncommonD It is impossible to diagnose definitively

except in an autopsy

9 What chemical is associated with the parasympathetic nervous systemA AcetylcholineB PotassiumC EpinephrineD Dopamine

10 The stimulation of the myocardial cells of which of the following structures is most closely associated with ventricular depolarizationA SA nodeB AV junctionC Bundle of HisD Purkinje fibers