An electrocardiogram (ECG) records 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).
BASIC CARDIAC ELECTROPHYSIOLOGY
Before discussing the basic ECG patterns, we will review some elementary aspects of cardiac electrophysiology. Fortunately, only certain simple principles are required for clinical interpretation of ECGs. In addition, it is worth mentioning now that no special knowledge of electronics or electrophysiology is necessary despite the connotations of the term “electrocardiography.”
In simplest terms the function of the heart is to contract 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 specialized nervous conducting tissue within the heart and by the heart muscle itself. The ECG records the currents produced by the heart muscle.
Normally the signal for cardiac electrical stimulation starts in the sinus node (also called the sinoatrial or SA node). The sinus 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 spontaneously generating electrical stimuli (signals). From the sinus node, this electrical stimulus spreads first through the right atrium and then into the left atrium. In this way the sinus node functions as the normal pacemaker of the heart.
The first phase of cardiac activation consists of the electrical stimulation of the right and left atria, electrical stimulation, in turn, signals the atria to contract and to pomp blood simultaneously through the tricuspid and mitral valves into the right and left ventricles respectively. 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 carry the stimulus to the ventricular muscle cells.
The AV junction, which functions as an electrical “bridge” connecting the atria and ventricles, is located at the base of the interatrial septum and extends into the ventricular septum. It has two subdivisions; the upper (proximal) part is the AV node. (in older texts the terms “AV node” and “AV junction” are used synonymously.) the lower (distal) segment 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 brings the electrical stimulus to the right ventricle, and the left bundle branch, which brings the electrical stimulus to the left ventricle.
The electrical stimulus spreads simultaneously down the left and right bundle branches into the ventricular muscle itself (ventricular myocardium). The stimulus spreads itself into the ventricular myocardium by way of specialized conducting cells, called Purkinje fibers, located in the ventricular muscle.
Under normal circumstances, when the sinus node is pacing the heart (normal sinus rhythm), the AV junction appears to function primarily as a shuttle, directing the electrical stimulus into the ventricles. However, under some circumstances (described later) the AV junction can also function as an independent pacemaker of the heart. For example, if the sinus node fails to function properly, the AV junction may act as an escape pacemaker. In such cases an AV junctional rhythm (and not sinus rhythm) is present. This produces a distinct ECG pattern Just as the spread of electrical stimuli through the atria leads to atrial contraction, so the spread of the electrical stimuli through the ventricles leads to ventricular contraction with pumping of blood to the lungs and into the general circulation.
In summary, the electrical stimulation of the heart normally follows a repetitive sequence of five steps:
Production of a stimulus from pacemaker cells in the sinus node (in the right atrium)
stimulation of the tight and left atria
spread of the stimulus to the AV junction AV node and bundle of His)
stimulation of the left and right ventricular myocardium
CARDIAC CONDUCTIVITY AND AUTOMATICITY
The speed with which the electrical impulses are conducted through different parts of the heart varies. For example, conduction speed or 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 the other major electrical feature of the heart is automaticity. Automaticity refers to the capacity of certain myocardial cells to function as pacemakers, to spontaneously generate electrical impulses that spread throughout the heart. Normally, as mentioned earlier, the sinus node is the pacemaker of the heart because of its inherent automaticity. Under special conditions other cells outside the sinus node (in the atria, the AV junction. Or the ventricles) can also act as independent pacemakers. For example, as mentioned before, if the automaticity of the sinus node id depressed, the AV junction may function as an escape pacemaker. In other conditions the automaticity of pacemakers outside the sinus node may become abnormally increased, and these ectopic (non-sinus) pacemakers may compete with the sinus node for control of the heartbeat. Ectopy is discussed in detail in Part II of this book (in the section on cardiac arrhythmias).
If you understand the normal physiologic stimulation of the heart, you have the basis for understanding the abnormalities of heart rhythm (arrhythmias) and conduction that produce distinctive ECG patterns. For example, failure of the sinus node to stimulate the heart properly may result in various rhythm disturbances, such as sinoatrial block (SA block)... Similarly, blockage of the spread of the stimulus through the AV junction produces various degrees of AV heart block. Disease of the bundle branches may produce left or right bundle branch block .. Finally, any disease process that involves the ventricular muscle itself (for example, destruction) also produces marked changes in the normal ECG patterns.