Before we can discuss arrhythmias, we need to briefly review the parts of the heart that are involved in the normal depolarization of the heart. Depolarization of the heart begins in the sinoatrial node, or SA node for short, which is located in the right atrium of the heart. The SA node spontaneously depolarizes, generating a depolarization impulse that travels down the heart and depolarizes the heart. The SA node normally spontaneously depolarizes 60–100 times a minute, leading to the normal heart rate of 60–100 beats per minute. The rhythm that results from normal depolarization of the heart is called normal sinus rhythm.
After the SA node depolarizes, the depolarization impulse travels down and across the atria, depolarizing the right and left atria, and arrives at the AV node. The impulse is briefly delayed in the AV node, contributing to the delay between the P wave and the QRS complex (this distance on the ECG you may remember is denoted as the PR interval).
Once the depolarization impulse passes through the AV node, it then makes its way into and throughout the ventricles. The depolarization impulse first passes through specialized cells called the His bundle (or “Bundle of His”), which is located near the junction of the atria and ventricles. The depolarization impulse then travels down the left bundle branch (LBB) and the right bundle branch (RBB) into the ventricles.2
Figure 1.1: Normal depolarization of the heart. Spontaneous depolarization of the SA node leads to a depolarization impulse traveling down the atria to the AV node. The depolarization impulse then travels through the His bundle and down the left bundle branch (LBB) and right bundle branch (RBB) and is distributed into the cells of the ventricles via the Purkinje fibers. The term His-Purkinje system is used to denote the system of specialized cells that transmit the depolarization impulse into and throughout the ventricles, and includes the His bundle, left and right bundle branches, and the Purkinje fibers
Impulse conduction down the left bundle branch leads to depolarization and contraction of the left ventricle. Impulse conduction down the right bundle branch leads to depolarization and contraction of the right ventricle. The depolarization impulse is then distributed into the cells of the left and right ventricle by specialized cells call Purkinje fibers. The term His-Purkinje system refers to all the specialized cells that conduct the depolarization impulse into and throughout the ventricles, and includes the His bundle, left and right bundle branches, and the Purkinje fibers.3
As we will discuss in the next chapter and other chapters, although the SA node is the normal “pacemaker” of the heart, if spontaneous depolarization of the SA node dramatically slows or stops completely, other parts of the heart and conduction system can assume the role of the hearts’ pacemaker. Also, if other parts of the heart begin to abnormally spontaneously depolarize at a rate faster than that of the SA node, then that part of the heart takes over as the heart's pacemaker.
Throughout this book, Heartman will offer clinical pearls on diagnosing and treating arrhythmias. This information will be displayed in highlighted box. While the additional information in these clinical pearls will add to your understanding of arrhythmias, the information contained in these clinical pearls is not necessary for you to obtain a basic understanding of diagnosing and treating arrhythmias.
Heartman will offer clinical pearls throughout the book on diagnosing and treating arrhythmias