Conduction System of Heart

The normal electrical conduction in the heart allows the impulse that is generated by the sinoatrial node (SA node) be propagated to and stimulate the myocardium. It is the ordered stimulation of the myocardium that allows efficient contraction of the heart.

In order to maximize the efficiency of contraction and cardiac output, the conduction system of the heart has:

a) Substantial atrial to ventricular delay. It allows the atria to completely empty their contents into the ventricles. The atria connected only via the AV node which briefly delays the signal.

b) Coordinated contraction of ventricular cells. The ventricles must maximize systolic pressure to force blood through the circulation.

c) Absence of tetany. After contracting the heart must relax to fill up again.

Signals arising in the SA node stimulate the atria to contract and travel to the AV node. After a delay, the stimulus is conducted through the bundle of His to the Purkinje fibers and the endocardium at the apex of the heart, and then finally to the ventricular epicardium[50].

The heart is a functional syncytium. In a functional syncytium, electrical impulses propagate freely between cells in every direction, so that the myocardium functions as a single contractile unit. This property allows rapid, synchronous depolarization of the myocardium. While normally advantageous, this property can be detrimental as it

potentially allows the propagation of incorrect electrical signals. These gap junctions can close to isolate damaged or dying tissue, as in a myocardial infarction.

Fig. 2.7 conduction system of heart[22].

A typical ECG tracing of the cardiac cycle consists of a P wave, a QRS complex, a T wave, and a U wave.[50]

Table 2.1 wave and intervals. [52]

description

RR interval The interval between an R wave and the next R wave: Normal resting heart rate is between 60 and 100 bpm.

P wave During normal atrial depolarization, the main electrical vector is directed from the SA node towards the AV node, and spreads from the right atrium to the left atrium. This turns into the P wave on the ECG.

PR interval The PR interval is measured from the beginning of the P wave to the beginning of the QRS complex. The PR interval reflects the time the electrical impulse takes to travel from the sinus node through the AV node and entering the ventricles. The PR interval is, therefore, a good estimate of AV node function.

J-point The point at which the QRS complex finishes and the ST segment begins, it is used to measure the degree of ST elevation or depression present.

ST segment The ST segment connects the QRS complex and the T wave. The ST segment represents the period when the ventricles are depolarized. It is isoelectric.

T wave The T wave represents the repolarization (or recovery) of the ventricles. The interval from the beginning of the QRS complex to the apex of the T wave is referred to as the absolute refractory risk factor for ventricular tachyarrhythmias and sudden death. It varies with heart rate and for clinical relevance requires a correction for this, giving the QTc.

U wave The U wave is hypothesized to be caused by the repolarization of the interventricular septum. They normally have low amplitude, and even more often completely absent. They always follow the T wave and also follow the same direction in amplitude. If they are too prominent, suspect hypokalemia, hypercalcemia or hyperthyroidism usually[24].

In normal conditions, electrical activity is generated by the SA node. This electrical impulse is propagated throughout the right atrium, and through Bachmann's bundle to the left atrium, stimulating the myocardium of the atria to contract. The conduction of the electrical impulse throughout the atria is seen on the ECG as the P wave. As the electrical activity is spreading throughout the atria, it travels via specialized pathways, known as internodal tracts, from the SA node to the AV node.

The AV node functions as a critical delay in the conduction system. Without this delay, the atria and ventricles would contract at the same time, and blood wouldn't flow effectively from the atria to the ventricles. The delay in the AV node forms much of the PR segment on the ECG. And part of atrial repolarization can be represented by PR segment.

The distal portion of the AV node is known as the Bundle of His. The Bundle of His splits into two branches in the interventricular septum, the left bundle branch and the right bundle branch. The left bundle branch activates the left ventricle, while the right bundle branch activates the right ventricle. The left bundle branch is short, splitting into the left anterior fascicle and the left posterior fascicle. The left posterior fascicle is relatively short and broad, with dual blood supply, making it particularly resistant

to ischemic damage. The left posterior fascicle transmits impulses to the papillary muscles, leading to mitral valve closure. As the left posterior fascicle is shorter and broader than the right, impulses reach the papillary muscles just prior to depolarization, and therefore contraction, of the left ventricle myocardium. This allows pre-tensioning of the chordae tendinae, increasing the resistance to flow through the mitral valve during left ventricular contraction. This mechanism works in the same manner as pre-tensioning of car seatbelts.

The two bundle branches taper out to produce numerous Purkinje fibers, which stimulate individual groups of myocardial cells to contract. The spread of electrical activity through the ventricular myocardium produces the QRS complex on the ECG.

The last event of the cycle is the repolarization of the ventricles. It is the restoring of the resting state. In the ECG, repolarization includes the J wave, ST-segment, and T- and U-waves[51].

Cardiac muscle is a syncytium, and the heart is composed of two syncytiums: the atrial syncytium that constitutes the walls of the two atria, and the ventricular syncytium that constitutes the walls of the two ventricles[50]. The atria are separated

from the ventricles by fibrous tissue that surrounds the atrioventricular (A-V) valvular openings. Potentials are conducted only by way of a specialized conductive system:

A-V bundle. This division of the muscle of the heart into two functional syncytiums allows the atria to contract a short time ahead of ventricular contraction, which is important for effectiveness of heart pumping. The action potential recorded in a ventricular muscle fiber, averages about 105 millivolts. The intracellular potential rises from a very negative value, about -85 millivolts, between beats to a slightly positive value, about +20 millivolts, during each beat. After the initial spike, the membrane remains depolarized for about 0.2 second, exhibiting a plateau, followed at the end of the plateau by abrupt repolarization.

In cardiac muscle, the action potential is caused by two types of channels[50]: (a) the fast sodium channels as the same in skeletal muscle and (b) another entirely different population of slow calcium channels, which are also called calcium-sodium channels.

During this time, a large quantity of both calcium and sodium ions flows through these channels to the interior of the cardiac muscle fiber, and this maintains a prolonged period of depolarization, causing the plateau in the action potential. Further, the calcium ions that enter during this plateau phase activate the muscle contractile

process, while the calcium ions that cause skeletal muscle contraction are derived from the intracellular sarcoplasmic reticulum.