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Electrocardiograms (ECGs)Exam questions frequently ask candidates to explain electrical activity (and blood pressure changes) during the cardiac cycle.

This Factsheet:• Summarises the electrical activity of the cardiac cycle• Describes how electrocardiogram (ECG) traces can be used to identify heart defects• Illustrates common exam questions on this topic

What happens during a heart beat?Fig. 1 shows the pathway of electrical activity through the heart .

sino-atrialnode (SAN/pacemaker) inwall of rightatrium

artri-ventricular node(AVN) in theatrioventricular septum

intraventricular septum

bundle of His

Purkine fibres

RV

RALV

LA

LA = Left AtriumLV = Left Ventricle

RV = Right VentricleRA = Right Atrium

aorta 1. wave of excitation that starts eachheart beat is sent from the SAN

2. spreads over atrial walls causingcontraction

3. delayed at non-conductingatrioventricular septum

4. wave of excitation is sent from AVN5. passes along bundles of His/Purkine

fibres to the base of the ventricleswhich then contract

6. before spreads over the walls of theventricles causing them to contract

1

2

2

2

3

4 5

55

6

6

Extract from Chief Examiner’s MarkschemeA surprising number of candidates were confused about the roles of the SAN, AVN and bundle of His. Many thought that the only roleof the SAN was to relay impulses from the CNS. Few candidates realized that the AVN produces the electrical activity that causes theventricles to contract and a quarter of the candidates thought that the bundle of His was responsible for the contraction of theventricles from their bases.

Typical Exam QuestionRead through all of the following passage and then fill in the spaces with the most appropriate word or words.The cardiac cycle is initiated and controlled by the heart itself. Cardiac muscle is said to be ......................... since it will contract andrelax of its own accord. The beat is initiated by the .........................which is situated in the wall of the ............... Waves of depolarisationtravel through the atria causing atrial .............. The waves of depolarisation can only travel to the ventricles via the ................... situatedat the top of the ventricular septum. From here the waves travel to the apex of the heart through the ..................... which is made ofspecialised conducting cells called ........................... These then carry the waves of depolarisation through the ventricle walls causingboth ventricles to contract simultaneously. At this stage the ..................... are open and the ........................... are shut so that blood canbe forced into the arches.

Markschememyogenic; sinoatrial node/pacemaker; right atrium; systole/contraction; atrioventricular node; bundle of His/left + rightbundle branches;Purkinje fibres; semilunar valves; atrioventricular valves;

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www.curriculum-press.co.uk234. Electrocardiograms (ECGs)

Electrocardiograms (ECGs)ECGs are a record of the electrical currents that cause the heartmuscle to contract. Electrodes placed on the skin detect the electricalactivity. Electrode jelly ensures good electrical contact and thepatient is required to keep still because movement causes interferenceto the trace due to the electrical impulses from the muscles (Fig 2).

Fig. 2 Typical ECG for a normal cardiac cycle

• The PR interval = the time taken for an electrical impulse totravel from the atria to the ventricles

• The QT interval = the contraction time because theventricles are contracting

• The interval between T of one cardiac cycle and Q of thefollowing cycle = the filling time because blood is firstlyfilling the atria, then the ventricles

electricalpotential /mV

0 0.2 0.60.4 0.8time (s)

P

Q

S

R

T

P: atrial systole (contraction)the wave of depolarisation in the walls of the atria i.e. atrialsystole. The atria contract and blood flows into the ventricles.The heart rate can be calculated from the interval betweensuccessive P waves

QRS: ventricular systole (contraction)the wave of depolarisation in the walls of the ventricles i.e.ventricular systole. The ventricles contract and the valvebetween atrium and ventricle closes

T : ventricular diastoleThe ventricles relax

Fig. 3 shows an abnormal ECG trace from a patient who has suffereda heart block. The ECG trace shows a much longer than normal timeinterval between the P and R waves. This could be a result of damageto the Purkine tissue or improper functioning of the AV node.

Fig.3 ECG of patient with heart block

Extract from Chief Examiner’s MarkschemeFew candidates realised that ECGs shows electrical activityrather than blood flow. Only about half the candidates knewthat the P wave corresponded to electrical activity prior toatrial systole and that the QRS corresponded to electricalactivity prior to ventricular systole.

Fig.4 shows an abnormal ECG trace from a patient suffering who hassuffered ventricular fibrillation caused either by an acute heart attackor electrical shock.

Fig. 4 ECG showing ventricular fibrillation

Atrial fibrillation causes blood flow to slow down and it may evenstop. This can cause ‘pooling’ of the blood in certain places in thecirculatory system.

Fig.5 shows an ECG trace from a patient suffering tachycardia inwhich the heart beat much faster than normal - exceeding 100bpmeven when the patient is at rest. Greater voltages are also generated.

Fig. 5 ECG showing tachycardia

Typical Exam QuestionSuggest what effects tachycardia could have on cardiac output.

MarkschemeCardiac output could increase if ventricles have time to fillsufficiently;But it could decrease if there was insufficient time to fill theventricles between contractions;Net effect depends on whether the decrease in stroke volumeis compensated by the increase in heart rate;

1 mv

1 sec

Key

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www.curriculum-press.co.uk234. Electrocardiograms (ECGs)

Fig.6 shows an ECG trace from a patient suffering from hypothermia– the heart beat is much slower than normal but retains the shape ofthat of a normal individual

Fig.6 ECG trace showing hypothermia

0.2 0.4 0.60.0 0.8 1.0 1.2 1.4 1.6 1.8

Exam Hint:- Do not confuse electrical activity with blood flow.ECGs show electrical activity, NOT blood flow

Fig.7 shows an ECG trace from a patient suffering from a damagedAVN. Because it is the AVN that produces electrical activity, there isa much lower spike and the low AVN spike results in less electricalactivity in the rest of the heart.

Fig. 7 ECG comparing healthy heart to one with damagedAVN

healthy

damagedAVN

healthy heart

diseasedheart

MarkschemesSAN is in the (right) atrium;ECG from the healthy person is identical to the trace for thediseased heart in the region of the atria;

electricalactivity inmuscles of atria

electrical activity inmuscles of ventricles

Typical Exam QuestionThe diagram shows ECGs from a healthy heart and from a diseasedheart.

Looking at the ECGs, a consulting cardiologist was reasonablycertain that the damage caused to the diseased heart had notaffected the sinoatrial node. Explain why (2)

Typical Exam Question(a) The electrocardiogram (ECG) trace below was recorded over a

period of 5 seconds from a healthy person.

Count the number of complete cardiac cycles, and thencalculate the heart rate in beats per minute (2).

(b) The following ECG trace was taken from a different person.

Identify one irregularity shown in the trace (1).

(c) A woman was found to have collapsed in the street. Aparamedic obtained the following trace using a portable ECGmonitor.

Explain why the person collapsed (4).

Markscheme(a)5 complete cardiac cycles;

5 × 12 = 60 (beats per minute;

(b)frequency/distance between peaks varies/ arrhythmia;

(c)Heart attack / myocardial infarction/ cardiac arrest;Branch of coronary artery blocked with blood clot/ atheroma;Reduced oxygen supply (to heart muscle;Ventricular fibrillation / ventricles quiver;Random / haphazard depolarisation of cells in ventricle;Uncoordinated waves /No coordinated heart beat;Atria and ventricles not contracting simultaneously;Rapid drop in blood pressure;

However, because even patients with heart disease can display anormal ECG trace modifications of the ECG procedure are routinelyused:

1. 24 hour ‘ambulatory’ monitoring of the ECG.The patient wears the heart monitor and takes readings about every20 minutes. The data are stored and the patient keeps a diary of howthey are feeling, enabling feelings of breathlessness etc to be linkedto changes in their ECG trace.

2. Exercise tolerance test or stress testThe ECG electrodes are attached to the patient who then walks ona slowly – moving treadmill set at a slight incline. Both the inclineand the speed of the treadmill incline are gradually increased sothat the patient’s heart rate increases.

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www.curriculum-press.co.uk234. Electrocardiograms (ECGs)

Exam Hint:- Don’t go into the exam hall if you can’t explainFig.8!

Fig.8 An electrocardiogram (ECG) and associated pressure changes

1. Atria contracting so blood flowing into ventricle

2. Ventricles start to contract so ventricular pressure > atrial pressure and the atrioventricular valve closes

3. Ventricular pressure > aortic pressure forcing open aortic valve so blood flows from the ventricle into the aorta

4. Ventricular pressure falls below aortic pressure, so aortic valve closes

5. Ventricular pressure falls below atrial pressure so that blood flows from the atria to the ventricles and ventricular volume risesrapidly

6 Atrium filling with blood from pulmonary vein until atrial pressure> ventricular pressure and blood flows from atrium to ventricle

aorticpressure

ventricularpressure

atrialpressure

pressure(mm Hg)

120

100

0

80

60

40

20

time (s)0 0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8

ECG traceP

Q

R

S

T

semilunarvalve opens

semilunarvalve closes

atrioventricularvalve open

atrioventricularvalve closed

1

2

3

6

4

5

Left atrium

Left ventricle

Aorta

bicuspid(atrioventricular)valve

semilunar(aortic) valve

atrialsystole

ventricular systole Atrial and ventricular diasole

P: atrial systole (contraction)The wave of depolarisation in the walls of the atria i.e. atrialsystole. The atria contract and blood flows into the ventricles.

QRS: ventricle systole (contraction)the wave of depolarisation in the walls of the ventricles i.e.ventricular systole. The ventricles contract and the valvebetween atrium and ventricle closes

T : ventricular diastole. The ventricles relax

Acknowledgements:This Factsheet was researched and written by Kevin Byrne and Martin Griffin.Curriculum Press, Bank House, 105 King Street, Wellington, Shropshire, TF1 1NU.Bio Factsheets may be copied free of charge by teaching staff or students, provided that their school is a registered subscriber. No part of these Factsheets may be reproduced, stored in a retrieval system, or transmitted, in any other form orby any other means, without the prior permission of the publisher. ISSN 1351-5136

Linking electrical activity to blood flowFig. 8 shows the pressure, volume and electrical changes that occurduring the cardiac cycle.