Clinical Electrocardiography: A Simplified Approach, 7th Edition (2006)

Part I. BASIC PRINCIPLES AND PATTERNS

Chapter 1. Introductory Principles

The electrocardiogram (ECG or EKG) is a graph that records the electrical activity of the heart.

The ECG records these cardiac electrical currents (voltages, potentials) by means of metal electrodes placed on the surface of the body.[*] These metal electrodes are placed on the arms, legs, and chest wall (precordium).

*  As discussed in Chapter 3 , the ECG actually records the differences in potential between these electrodes.
BASIC CARDIAC ELECTROPHYSIOLOGY

Before the basic ECG patterns are discussed, some elementary aspects of cardiac electrophysiology must be reviewed. Fortunately, only certain simple principles are required for clinical interpretation of ECGs.

The function of the heart is to contract rhythmically and pump blood to the lungs for oxygenation and then to pump this oxygenated blood into the general (systemic) circulation. The signal for cardiac contraction is the spread of electrical currents through the heart muscle. These currents are produced both by pacemaker cells and specialized conducting tissue within the heart and by the working heart muscle itself.

The ECG records only the currents produced by the working heart muscle.

ELECTRICAL STIMULATION OF THE HEART

In simplest terms, the heart can be thought of as an electrically timed pump. The electrical “wiring” of the heart is outlined in Figure 1-1 . Normally, the signal for cardiac electrical stimulation starts in the sinus node, also called the sinoatrial (SA) node. This node is located in the right atrium near the opening of the superior vena cava. It is a small collection of specialized cells capable of automatically generating an electrical stimulus (signal). From the sinus node, this stimulus spreads first through the right atrium and then into the left atrium. Thus the sinus node functions as the normal pacemaker of the heart.

FIGURE 1-1  Normally, the cardiac stimulus is generated in the sinoatrial (SA) node, which is located in the right atrium (RA). The stimulus then spreads through the RA and left atrium (LA). Next, it spreads through the atrioventricular (AV) node and the bundle of His,which comprise the AV junction. The stimulus then passes into the left and right ventricles (LV and RV) by way of the left and right bundle branches, which are continuations of the bundle of His. Finally, the cardiac stimulus spreads to the ventricular muscle cells through the Purkinje fibers.

The first phase of cardiac muscle activation is electrical stimulation of the right and left atria. This in turn signals the atria to contract and pump blood simultaneously through the tricuspid and mitral valves into the right and left ventricles. The electrical stimulus then spreads to specialized conduction tissues in the atrioventricular (AV) junction, which includes the AV node and bundle of His, and then into the left and right bundle branches, which transmit the stimulus to the ventricular muscle cells.

The AV junction, which acts as a sort of electrical “bridge” connecting the atria and ventricles, is located at the base of the interatrial septum and extends into the interventricular septum (see Fig. 1-1 ). The upper (proximal) part of the AV junction is the AV node. (In some texts, the terms AV node and AV junction are used synonymously.) The lower (distal) part of the AV junction is called the bundle of His after the physiologist who described it. The bundle of His then divides into two main branches: the right bundle branch, which distributes the stimulus to the right ventricle, and the left bundle branch,[*] which distributes the stimulus to the left ventricle (see Fig. 1-1 ).

The electrical stimulus spreads simultaneously down the left and right bundle branches into the ventricular myocardium (ventricular muscle) by way of specialized conducting cells called Purkinje fibers.These fibers are located in the ventricular myocardium.

Under normal circumstances, when the sinus node is pacing the heart (normal sinus rhythm), the AV junction appears to serve primarily as a shuttle, directing the electrical stimulus into the ventricles. Under some circumstances, however, the AV junction can also act as an independent pacemaker of the heart. For example, if the sinus node fails to function properly, the AV junction can act as an escapepacemaker. In such cases, an AV junctional rhythm (and not sinus rhythm) is present. This produces a distinct ECG pattern (see Chapter 14 ).

Just as the spread of electrical stimuli through the atria leads to atrial contraction, so the spread of stimuli through the ventricles leads to ventricular contraction, with pumping of blood to the lungs and into the general circulation.[*]

*  The left bundle branch has two major subdivisions called fascicles. (These small bundles are discussed in Chapter 7 along with the hemiblocks.)
*  The initiation of cardiac contraction by electrical stimulation is referred to as electromechanical coupling. A key part of this contractile mechanism is the release of calcium ions inside the atrial and ventricular heart muscle cells, which is triggered by the spread of electrical activation.
CARDIAC CONDUCTIVITY AND AUTOMATICITY

The speed with which electrical impulses are conducted through different parts of the heart varies. For example, conduction is slowest through the AV node and fastest through the Purkinje fibers. The relatively slow conduction speed through the AV node is of functional importance because it allows the ventricles time to fill with blood before the signal for cardiac contraction arrives.

In addition to conductivity, a major electrical feature of the heart is automaticity. Automaticity refers to the capacity of certain cardiac cells to function as pacemakers by spontaneously generating electrical impulses that spread throughout the heart. As mentioned earlier, the sinus node normally is the primary pacemaker of the heart because of its inherent automaticity. Under special conditions, however, other cells outside the sinus node (in the atria, AV junction, or ventricles) can also act as independent pacemakers. For example, if the automaticity of the sinus node is depressed, the AV junction can act as a backup (escape) pacemaker.

The term sick sinus syndrome is used clinically to describe patients who have severe depression of sinus node function (see Chapter 20 ). These patients may experience light-headedness or even syncope (fainting) because of excessive bradycardia (slow heartbeat). (Sick sinus syndrome and other causes of bradycardia are discussed in Part 2 in the section on cardiac arrhythmias.)

In other conditions, the automaticity of pacemakers outside the sinus node may be abnormally increased, and these ectopic (non-sinus) pacemakers may compete with the sinus node for control of the heartbeat. A rapid run of ectopic beats results in an abnormal tachycardia. (Ectopy is also discussed in detail in Part 2.)

If you understand the normal physiologic stimulation of the heart, you have the basis for understanding the abnormalities of heart rhythm and conduction that produce distinctive ECG patterns. For example, failure of the sinus node to stimulate the heart properly can result in various rhythm disturbances associated with sick sinus syndrome. Blockage of the spread of stimuli through the AV junction can produce various degrees of AV heart block (see Chapter 17 ). Disease of the bundle branches can produce left or right bundle branch block (see Chapter 7 ). Finally, any disease process that involves the ventricular muscle itself (e.g., injury of the heart muscle by myocardial infarction) can produce marked changes in the normal ECG patterns.

The first part of this book is devoted to explaining the basis of the normal ECG and then examining the major conditions that cause abnormal depolarization (P and QRS) and repolarization (ST-T and U) patterns. (This alphabet of ECG terms is defined in Chapter 2 .) The second part is devoted to describing various abnormal rhythms and AV conduction disturbances (arrhythmias). The third part is a review and extension of material covered in earlier chapters. The fourth part is a collection of “unknowns” for self-assessment. In addition, review questions are included at the end of each chapter. Selected publications are cited in the Bibliography , including freely available online resources.

 

REVIEW

An electrocardiogram (ECG or EKG) records the electrical voltages (potentials) produced in the heart. It does this by means of metal electrodes (connected to an electrocardiograph) placed on the patient's chest wall and extremities. The potentials recorded on the ECG are produced by the working atrial and ventricular muscle fibers themselves.

Normally, the cardiac stimulus starts in pacemaker cells of the sinus node, also called the sinoatrial (SA) node, located high in the right atrium near the opening of the superior vena cava. From there the stimulus spreads downward and to the left, through the right and left atria, and reaches the atrioventricular (AV) node, located near the top of the interventricular septum (see Fig. 1-1 ). After a delay, the stimulus spreads through the AV junction (AV node and bundle of His). The bundle of His then subdivides into right and left bundle branches. The right bundle branch runs down the interventricular septum and into the right ventricle. From there the small Purkinje fibers rapidly distribute the stimulus outward into the main muscle mass of the right ventricle. Simultaneously, the left main bundle branch carries the stimulus down the interventricular septum to the muscle mass of the left ventricle, also by way of the Purkinje fibers.

This repetitive sequence of stimulation of the heart is the normal basic process. Disturbances in this process may produce abnormalities of heart rhythm, termed cardiac arrhythmias.

 

QUESTIONS

 

1.   

Label the major parts of the cardiac conduction system shown in this diagram; then trace the spread of the normal cardiac stimulus from the atria to the ventricles.

 

2.   

What is an electrocardiogram?

 

3.   

True or false. The ECG directly records only the electrical activity of working heart muscle cells, not that of the pacemaker cells or of the specialized conduction system.