ELECTROPHYSIOLOGY

Depolarization of the heart is the initiating event for cardiac contraction. The electric currents that spread through the heart are produced by three components: cardiac pacemaker cells, specialized conduction tissue, and the heart muscle itself.The ECG,however, records only the depolarization (stimulation) and repolarization (recovery) potentials generated by the atrial and ventricular myocardium.

The depolarization stimulus for the normal heartbeat originates in the sinoatrial (SA) node, or sinus node, a collection of pacemaker cells.These cells fire spontaneously; that is, they exhibit automaticity. The first phase of cardiac electrical activation is the spread of the depolarization wave through the right and left atria, followed by atrial contraction. Next, the impulse stimulates pacemaker and specialized conduction tissues in the atrioventricular (AV) nodal and His-bundle areas; together, these two regions constitute the AV junction. The bundle of His bifurcates into two main branches, the right and left bundles, which rapidly transmit depolarization wavefronts to the right and left ventricular myocardium by way of Purkinje fibers. The main left bundle bifurcates into two primary subdivisions, a left anterior fascicle and a left posterior fascicle. The depolarization wavefronts then spread through the ventricular wall, from endocardium to epicardium, triggering ventricular contraction.

Since the cardiac depolarization and repolarization waves have direction and magnitude, they can be represented by vectors. Vectorcardiograms that measure and display these instantaneous potentials are no longer used much in clinical practice.However, the general principles of vector analysis remain fundamental to understanding the genesis of normal and pathologic ECG waveforms.Vector analysis illustrates a central concept of electrocardiography—that the ECG records the complex spatial and temporal summation of electrical potentials from multiple myocardial fibers conducted to the surface of the body.This principle accounts for inherent limitations in both ECG sensitivity (activity from certain cardiac regions may be canceled out or may be too weak to be recorded) and specificity (the same vectorial sum can result from either a selective gain or a loss of forces in opposite directions).