ECG and the Heart’s Internal Conduction

System

18-19 March 2015

What is surprising about this video of a diseased heart that has been removed from a patient prior to transplant surgery?

The heart continues to beat after being removed from the body!

Unlike skeletal muscle, the heart does NOT need to be stimulated by the nervous system to contract. Instead, the heart has its own pacemakers and intrinsic conduction system to regulate contraction.

What events lead to skeletal muscle contraction?◦ Nerve releases neurotransmitter

into synaptic cleft

◦ Neurotransmitter causes action potential along sarcolemma

◦ Action potential causes sarcoplasmic reticulum to release calcium

◦ Calcium exposes the myosin binding sites on actin, leading to muscle contraction

Quick Review of Skeletal Muscle Contraction

What part of this process involves an electric current?

The action potential!

During the action potential, the membrane first depolarizes then repolarizes.

Contraction occurs shortly after the depolarization of the membrane.

SA node is the heart’s natural pacemaker. ◦ It depolarizes ~75 times / min, to start each heartbeat.

Instrinsic conduction system

Impulse travels throughout the atria and to the AV nodeo Travels cell-to-cell through

gap junctions in intercalated discs – another unique feature of cardiac muscle

AV node delays impulse transmission to allow atria to finish contracting

The AV bundle (aka bundle of His) transmits the impulse to the bundle branches and Purkinje fibers.

Instrinsic conduction system

o These bundles and fibers speed the transmission of the impulse throughout the ventricles, to coordinate ventricular contraction

Interestingly, all the cells of the conduction system are autorhythmic. That is, all will depolarize at a certain rate. The SA node has the fastest rate of depolarization, though, so it sets the pace for the entire heart.

What is the function of:◦ AV node – delays impulse so atria can finish contracting, and conducts to AV bundle◦ Gap junctions –

Allow impulse to spread cell-to-cell

◦ SA node heart’s pacemaker – depolarizes to start contraction◦ Purkinje fibers carry impulse to cells of ventricles

Quick Review: Instrinsic conduction system

Name 3 differences between cardiac and skeletal muscle contraction & explain why each is important to cardiac function.

1. Cardiac muscles do not need to be stimulated by nerves. This allows heart to beat even if nervous system doesn’t function well.

2. Cardiac muscles have autorhythmic / natural pace-makers to keep the heart beating an an appropriate rate.

3. Cardiac muscle has special features to speed impulse conduction from one cell to another, including special cells (the AV bundle, branch bundles and Purkinje fibers) and gap junctions between cells.

Quick Review: Instrinsic conduction system

An electrocardiogram (ECG or EKG) records the electrical current through the heart.

A normal ECG has three distinct waves:◦ P wave – atrial depolarization◦ QRS complex – ventricular

depolarization AND atrial repolarization

◦ T wave – ventricular repolarization

Electrocardiogram

Watch me!

A normal ECG has three distinct waves:

◦ P wave – atrial depolarization◦ QRS complex – ventricular

depolarization AND atrial repolarization

◦ T wave – ventricular repolarization

Electrocardiogram - Questions

What is responsible for these delays?• Delay between P wave and QRS complex is caused

by AV node• Delay between T and next P wave is determined by

heart rate / speed at which SA node depolarizes

Examine this normal ECG. When would the ventricles be in systole? When would they be in diastole? When would the heart sounds occur, and

what do the heart sounds correspond to?

Electrocardiogram Questions

systole systolediastole diastole

“LUB” - closing of AV valves as ventricle begins to contract

“DUP” - closing of SL valves as ventricle begins to relax

Abnormal ECGs Identify how the abnormal ECGs differ from

the normal one, and match them to their disease.

a)

All waves present and normal shape.Heart beat is about twice as fast.

TACHYCARDIA

Abnormal ECGs Identify how the abnormal ECGs differ from

the normal one, and match them to their disease.

b)

No relationship between p waves and QRS complex

AV block

Abnormal ECGs Identify how the abnormal ECGs differ from

the normal one, and match them to their disease.

c)

Waves are more-or-less correct shape. Heart rate is about 1.5 times slower than normal.

Bradycardia

Abnormal ECGs Identify how the abnormal ECGs differ from

the normal one, and match them to their disease.

d)

No obvious p waves, wandering baseline (not so obvious)

Atrial fibrillation

Regulation of Heart RateEarlier, we said that the SA node depolarizes at a rate of ~75 beats per minute, and that this acts as a pace maker for heart contraction.

Does this mean our hearts always beat at ~75 bpm?

Of course not!Our hearts do not require stimulation from the

nervous system to beat, however, they do respond to stimulation from the nervous system by either

speeding up or slowing down.

Regulation of Heart RateEarlier, we said that the SA node depolarizes at a rate of ~75 beats per minute, and that this acts as a pace maker for heart contraction.

Does this mean our hearts always beat at ~75 bpm?

Of course not!Our hearts do not require stimulation from the

nervous system to beat, however, they do respond to stimulation from the nervous system by either

speeding up or slowing down.

Regulation of Heart RateWhat are the two branches of the autonomic nervous system, and how do they affect heart rate?

Sympathetic (fight-or-flight / stress) – increases HR Parasympathetic (rest & digest) – decreases HR

Regulation of Heart RateOther factors that influence heart rate Age (fastest in fetus, young children) Gender (faster in females) Temperature (faster in heat) Exercise (faster with exercise) Ions imbalances & medicines (faster or slower) Weak / damaged heart (can be either faster / slower)

Closure

Think back to the list of questions we made last class. Were any answered today?

Have you developed any new questions?

Remember HW assignment: research one question and be ready to answer.

Exit Ticket

1) What serves as the heart’s natural pacemaker?2) What event does a T wave correspond with?3) Name one the stimulation of cardiac muscle differs

from skeletal muscle.4) What does tachycardia refer to?5) Which system slows down the heart?

E.C. What does ECG stand for?