cardiac conducting system in health and disease

7/29/2019 Cardiac Conducting System in Health and Disease

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Lecture 9- Cardiac conducting system in health and disease

1. Introduction- Cardiac muscle is a functional syncytium[confluent of adjacent cell]- Action potentials are propagated from one cell to the next adjacent cell- Gap junctions facilitate the propagation of action potential- After depolarisation the cell membrane cant be depolarised for a defined period of

time to ensure one-way propagation of action potentials across myocardial tissue

- The heart contains a specialised conduction system that directs and facilitates spreadingof original action potential from the SA node to the entire heart


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2. Sequence of cardiac excitation

- Action potential initiated at the SA node spread throughout the myocardium, passingfrom cell to cell by way of gap junctions.

- Depolarisation first spread through the muscle of cells of the atria, with conductionrapid enough that the right and left atria contract at essentially the same time.


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- The action potential spreading through muscle cells of the right atrium causesdepolarisation of the AV node.

- Autonomic nervous system regulates the AV conduction(sympathetic andparasympathetic)

- Autorhytmic cells in the AV node can function as pacemakers at a rate of 25-40beats/min

- The propagation of action potential through the AV node is relatively slow in order toallow atrial contraction to be completed before ventricular excitation occur. However,

most of ventricular filling is completed during early diastole that is long before cardiac

excitation starts in the SA node)

- After leaving the AV node, action potential propagated down the wall [interventricularseptum] between the two ventricles. This pathway has conducting-system fibers called

the Bundle of His

- Within the interventricular septum, the bundle of His divides into right and left bundlebranches- which eventually leave the septum to enter the walls of both ventricles.

- These fibres in turn make contact with Purkinje fibres, large conducting cells that rapidlydistribute the impulse throughout much of the ventricles

- Purkinje fibers have the highest conduction velocity of all cardiac cells. This allows for arapid spread of the excitation throughout the ventricles

- Purkinje fibers make contact with ventricular myocardial cells, which spread the impulsethrough the rest of ventricles

- Early contraction of the papillary muscles prevents eversion of the AV valves into theatria during systole

- Purkinje fibers have a long refractory period which can block conduction of prematureexcitation of atria.

- Purkinje fibers have a potential for automacity and rhythm generation


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- Autorhythmic cells become the back-up system. AV node has the capacity ofdepolarising but dont depolarise as fast as the SAN. Just enough to maintain the

pumping action.

3. AV conduction defects can be diagnosed using the ECG

- Partial atrioventricular block : disease of the electrical conduction system of the heart. It refers to a conduction

block between the atria and ventricles.

The presence of second-degree AV block is diagnosed when one or more (butnot all) of the atrial impulses fail to conduct to the ventricles due to impaired

conduction.

is characterized on a surface ECG by intermittently nonconducted P waves notpreceded by PR prolongation and not followed by PR shortening.

- Total atrioventricular block: medical condition in which the impulse generated in the SA node in the atrium

does not propagate to the ventricles.

two independent rhythms can be noted on the electrocardiogram (ECG). The P waves with a regular P to P interval represents the first rhythm. The QRS complexes with a regular R to R interval represent the second

rhythm. The PR interval will be variable

- Cardiac arrhythmia Tissue damage can cause changes in local ion concentrations and in ion channel

function leading to arrhythmia

Changes in extracellular K+ levels can induce arrhythmia through effects onresting membrane potential

An arrhythmia is a disturbed heart rhythm. Some arrhythmias don't affect youroverall health, while others are more serious and life-threatening. Palpitations


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are a sensation or awareness of your heart beating. Palpitations may feel like

your heart is 'racing', thumping or skipping beats. They can be triggered by

exercise, emotional stress, caffeine and nicotine. Palpitations are usually

associated with an abnormal heart rhythm (arrhythmia).

http://watchlearnlive.heart.org/CVML_Player.php?moduleSelect=arrhyt A cardiac impulse may re excite some myocardial region through which it had

passed previously and which is no longer refractory.

This phenomenon is called re entry and can cause cardiac arrhythmia The Wolff-Parkinson-White syndrome is a disease where a small amount extra

electrical conductive tissue is present between atria and ventricles which may

lead to dangerous tachycardias and arrhythmias.

4. Pacemaker therapy of AV conduction defects- is a medical device that uses electrical impulses, delivered by electrodes contacting the

heart muscles, to regulate the beating of the heart

- The primary purpose of a pacemaker is to maintain an adequate heart rate, eitherbecause the heart's native pacemaker is not fast enough, or there is a block in the

heart's electrical conduction system

- This pacemaker is made to be able to detect the depolarisation of the atrium induced bythe SA node, and after an adequate time-delay to deliver an electrical impulse to the

apex of heart.

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cardiac conducting system in health and disease

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