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ECG tutorial: Physiology of the conduction system

ECG tutorial: Physiology of the conduction system
Literature review current through: Jan 2024.
This topic last updated: Dec 03, 2021.

INTRODUCTION — The cardiac conduction system is designed for electrical impulse creation and propagation. It allows for initiation of impulses in the atrium, slowed conduction in the atrioventricular (AV) node, and rapid propagation through the His-Purkinje system to allow synchronous contraction in the ventricles. Layers of redundancy occur, so that if one portion is damaged, there may be other areas that can compensate for the loss of function.

Cardiac cells have the inherent property of spontaneous depolarization, which creates the cardiac impulse. Cells within the sinus node have the fastest rate of spontaneous depolarization, and, therefore, the sinus node is the main pacemaker region of the heart. The AV node has the second fastest rate of spontaneous depolarization, which allows it to create an escape rhythm if the sinus node is diseased.

ATRIAL ACTIVATION — The sinus node (the most proximal part of the conduction system) exhibits the most automaticity and functions as the dominant pacemaker to start the heartbeat in normal circumstances. This structure generates a slow action potential, mediated by calcium currents, that exits the node and activates the atrial myocardium (see "Cardiac excitability, mechanisms of arrhythmia, and action of antiarrhythmic drugs"). The atrial myocardium action potential has a rapid upstroke, mediated by sodium ions (figure 1) that help to quickly transmit the signal.

Several preferential tracts exist in the atria to more quickly spread electrical signals [1]. In the right atrium, these include the crista terminalis and pectinate muscles. The Bachman bundle begins anterior to the superior vena cava and crosses the superior interatrial septum to facilitate right to left atrial conduction. A superior pulmonary bundle and septo-atrial bundle speed conduction in the left atrium.

As the atrium is depolarized, a P wave is transcribed on the surface electrocardiogram (ECG) (figure 2). Since the sinus node is in the superior right atrium, the signal goes from superior to inferior, anterior to posterior, and right to left. The P wave is upright and slightly notched in all of the limb leads, with the exception of aVR which has a negative P wave. The precordial leads also show an upright P wave, although in leads V1 and V2 the P wave is usually biphasic; an initial positive followed by a negative deflection reflects depolarization of the right atrium (which is anterior) and then the left atrium (which is posterior).

ATRIOVENTRICULAR NODE AND BUNDLE OF HIS ACTIVATION — After atrial activation, the impulse reaches the AV node. This structure generates a slow calcium mediated action potential (figure 1). Thus, there is a delay in impulse transmission through this structure.

Once the action potential traverses the AV node, it activates the proximal portion of the bundle of His, a specialized conducting tissue that generates a fast action potential. Thus, impulse conduction through this structure is rapid. This period of time from the end of the P wave to the beginning of the QRS, during which there is activation of both the AV node and bundle of His, is termed the PR segment. The PR interval, in contrast, includes the P wave (atrial activation) as well as the PR segment (figure 2). Because the AV node and bundle of His are small, there is no electrical activity manifest on the surface ECG from their conduction. The electrical activity that is occurring during the PR interval can be measured, however, using intracardiac electrodes during electrophysiologic testing (see "Invasive diagnostic cardiac electrophysiology studies"). Abnormalities of conduction in the AV node and bundle of His are manifest on the surface ECG as first-, second-, or third-degree AV block. (See "ECG tutorial: Atrioventricular block".)

VENTRICULAR ACTIVATION — After impulse transmission through the bundle of His, the impulse is conducted to the right and left bundle branches; these are extensions of the His bundle that generate a fast action potential (figure 1). The left bundle splits into left anterior and posterior fascicles. Sometimes there is a left septal fascicle (see "Left septal fascicular block"). The impulse travels through the bundles, fascicles, and the Purkinje fiber network, generating a fast action potential and resulting in rapid activation and depolarization of the myocardium of the right and left ventricles. The impulse enters the ventricular myocardium first from a septal portion of the left bundle. Thus, the left intraventricular septum is the first part of the ventricle to become depolarized, resulting in the following ECG appearance of the QRS (figure 2):

An initial small septal Q wave on the surface ECG leads that measure electrical activity toward the left side of the heart (leads I, aVL, and V4 to V6) since the impulse is moving away from these leads.

A small initial R wave in the leads pointed toward the right side of the heart (aVR, V1, and V2) since the impulse is moving in the direction of these leads (figure 1).

Since the mass of the left ventricle is much greater than that of the right, the normal ECG primarily reflects left ventricular depolarization (figure 2). The impulse spreads from the septum to the lateral wall in a right to left and superior to inferior direction, thereby generating the following ECG appearance:

A tall upright R wave in the limb leads I, II, aVF and the precordial leads V3 to V6 since the impulse is moving towards these leads.

A deep negative deflection or S wave in the leads that are directed to the septum and right side of the heart (aVR, V1, and V2) since the impulse is moving away from these leads.

The last part of the left ventricle to become depolarized is the high lateral region. Thus, the last part of the QRS may be reflected on the surface ECG as a small terminal S wave in leads I, II, aVF and V4 to V6, and a small r' in lead aVR and occasionally V1 and V2.

REPOLARIZATION — After the entire myocardium of the left and right ventricles completes depolarization, there is a period of time immediately prior to the final phase of repolarization or recovery during which there is no apparent electrical activity on the surface ECG, the isoelectric ST segment phase. During this period, corresponding to phase 2 of the action potential, the ventricular myocytes are at about the same potential, so no net current flow occurs. Thereafter, the ventricular myocardium undergoes the final phase of repolarization, generating a T wave on the surface ECG (figure 2). Since the wave of repolarization occurs from the epicardium to the endocardium and is of opposite electrical charge as depolarization, it is usually positive on the ECG. The last wave form sometimes seen and usually most apparent in the mid-precordial leads is the U wave, which is of uncertain origin but may represent repolarization of the His-Purkinje system or of the mid-myocardial M cells.

SUMMARY

Cardiac cells possess inherent spontaneous automaticity. The tissue that possesses the greatest degree of automaticity (eg, has the fastest rate of spontaneous depolarization) functions as the dominant pacemaker; it generates a spontaneous action potential that is conducted along the rest of the conduction system, activating the myocardium in a uniform fashion. (See 'Introduction' above.)

The sinus node exhibits the fastest automaticity and initiates each beat in most cases. Electrical activity spreads from the sinus node to the atrium, causing a P wave on the electrocardiogram (ECG). (See 'Atrial activation' above.)

After atrial activation, the impulse reaches the atrioventricular (AV) node. There is relatively slow transmission of the electrical current here. Once the action potential traverses the AV node, it activates the proximal portion of the bundle of His. Although there is no manifestation of AV nodal or bundle of His activation on the surface ECG, conduction through these structures occurs during the PR segment. The PR interval, which also includes atrial conduction, is comprised of the P wave as well as the PR segment. (See 'Atrioventricular node and bundle of His activation' above.)

Sequential activation of different parts of the left and right ventricles follows and causes the characteristic QRS complex. As the left ventricular muscle mass is far greater than that of the right ventricle, the QRS complex primarily represents left ventricular depolarization.

After the QRS, the ventricular myocytes normally remain at about the same potential, so the surface ECG returns to baseline-ST segment phase. Thereafter, the ventricular myocardium undergoes the final phases repolarization, generating a T wave and sometimes a small U wave. (See 'Repolarization' above.)

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