THE HEART’S CONDUCTION SYSTEM AND “NATURAL PACEMAKER”
The heart has its own built-in electrical system, called the conduction system (figure 1). The conduction system sends electrical signals throughout the heart that determine the timing of the heartbeat and cause the heart to beat in a coordinated, rhythmic pattern. The conduction system stimulates precise contractions of the heart’s chambers to ensure that blood is pumped effectively.
The electrical signals, or impulses, of the heart are generated by specialized tissue called the sinoatrial (SA) or sinus node (figure 1). The sinus node is sometimes called the heart’s “natural pacemaker.” Each time the sinus node generates a new electrical impulse; that impulse spreads out through the heart’s upper chambers, called the right atrium and the left atrium. This electrical impulse stimulates the atria to contract, pumping blood into the lower chambers of the heart (the right and left ventricles).
The electrical impulse then spreads to another area of specialized tissue located between the atria and the ventricles, the atrioventricular (AV) node. The AV node momentarily slows down the spread of the electrical impulse, to allow the left and right atria to finish contracting.
From the AV node, the impulse spreads into a system of specialized fibers called the bundle of His and the right and left bundle branches (figure 1). These fibers distribute the electrical impulse rapidly to all areas of the right and left ventricles, stimulating them to contract in a coordinated way. With this contraction, blood is pumped from the right ventricle to the lungs, and from the left ventricle throughout the body.
The heart’s conduction system must function normally for the heart to beat properly and to pump blood effectively to meet the body’s needs. Problems with the flow of electrical impulses in the heart are called arrhythmias, which is a general term meaning that there is an abnormality in the pattern of electrical conduction or electrical rhythm.
Bradyarrhythmias — Bradyarrhythmias are heart rhythm abnormalities that cause an abnormally slow heartbeat. Most bradyarrhythmias are due to one of two kinds of problems: sinus bradycardia or heart block.
Sinus bradycardia occurs when the heartbeat is too slow because the heart’s “natural pacemaker” is operating too slowly. Although some people (for example, competitive athletes) may have a slow heartbeat as a result of good health, in others sinus bradycardia is an abnormal condition that requires treatment.
Heart block is a term for a delay or interruption in the heart’s conduction system, causing the electrical impulses to travel too slowly or to be stopped. There are several kinds of heart block, classified according to location (where in the conduction system the block occurs) and degree (whether the block is mild, causing delayed conduction, or severe, causing conduction to stop). These conditions require Pacemaker implantation if they cause symptoms or are severe.
In first-degree atrioventricular (AV) block, all electrical impulses reach the ventricles from the atria, but are abnormally slowed as they pass through the AV node.
In second-degree AV block, some atrial impulses fail to reach the ventricles (“dropped beats”), resulting in a slow or an irregular heart rate.
In third-degree AV block, the most serious form, no atrial impulses are conducted to the ventricles. This condition is sometimes called complete heart block. For the heart to continue to beat, a separate electrical impulse (called an escape rhythm) may be generated in the ventricles. Without an escape rhythm, the ventricles (the chambers that pump blood throughout the body) stop beating.●In right bundle branch block (RBBB), impulses are not conducted by the right bundle branch. Electrical impulses reach the right ventricle only by traveling through the heart muscle from the left ventricle. As a result, activation of the right ventricle is delayed.
In left bundle branch block (LBBB), impulses are not conducted by the left bundle branch. Electrical impulses reach the left ventricle only by traveling through the heart muscle from the right ventricle. As a result, activation of the left ventricle is delayed.
Tachyarrhythmias — Sometimes, the electrical impulses “short circuit” the normal pathway and travel across the heart in an abnormal way. Abnormal pathways, or routes, for the impulses can develop, causing abnormalities in the heartbeat. In other cases, arrhythmias arise when areas other than the sinus node become active and begin to send out impulses that either compete with or take over the pacemaker function of the sinus node. Typically, the result of these abnormalities is a heartbeat that is too fast. This may develop in any location within the atria, AV node, or ventricles. When the fast rhythm requires tissue from the upper part of the heart, it is known as supraventricular tachycardia. When it requires tissue from the lower chambers only, it is known as ventricular tachycardia.
Most of the tachyarrhythmias require treatment by Electrophysiological Stimulation and Radiofrequency Ablation (EPS and RFA or simply Catheter Ablation).
Catheter ablation may be recommended for:
Paroxysmal supraventricular tachycardia, which includes:
AV nodal reentrant tachycardia
Inappropriate sinus tachycardia
Ventricular tachycardia and premature ventricular contractions
Figure 1 – Conduction system of the heart.
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