An electrocardiogram (ECG or EKG) is a graphic representation of electrical activity generated by the heart. The signals, detected by means of metal electrodes attached to the extremities and chest wall, are amplified and recorded by the electrocardiograph. ECG leads (derivations) are configured to display the instantaneous differences in potential between specific pairs of electrodes. The utility of the ECG derives from its immediate availability as a noninvasive, inexpensive, and highly versatile test. In addition to its use in detecting arrhythmias and myocardial ischemia, it may reveal findings related to life-threatening metabolic disturbances or to increased susceptibility to sudden cardiac arrest (see also Chaps. 299 and 401).
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 records only the depolarization (stimulation) and repolarization (recovery) potentials generated by the “working” atrial and ventricular myocardium (see also Chaps. 239 and 241).
The stimulus initiating the normal heartbeat originates in the sinoatrial (SA) node (Fig. 235-1), which possesses spontaneous automaticity. Spread of the depolarization wave through the right and left atria induces contraction of these chambers. Next, the impulse stimulates specialized conduction tissues in the atrioventricular (AV) nodal and His-bundle areas; together, these two regions constitute the AV junction. The bundle of His fans bifurcates into two main branches, the right and left bundles, which rapidly transmit depolarization wavefronts in a synchronous way to the right and left ventricular myocardium by way of Purkinje fibers. The main left bundle bifurcates into left anterior and left posterior fascicle subdivisions. The depolarization wavefronts then spread through the ventricular wall, from endocardium to epicardium, triggering coordinated ventricular contraction. Since the cardiac depolarization and repolarization waves have directions and magnitudes, they can be represented by vectors.
Schematic of the cardiac conduction system.
ECG WAVEFORMS AND INTERVALS
The ECG waveforms are labeled alphabetically, beginning with the P wave, which represents atrial depolarization (Fig. 235-2). The QRS complex represents ventricular depolarization, and the ST-T-U complex (ST segment, T wave, and U wave) represents ventricular repolarization. The J point is the junction between the end of the QRS complex and the beginning of the ST segment. Atrial repolarization waveforms (ST-Ta) are usually of too low in amplitude to be detected, it may become apparent in acute pericarditis, atrial infarction, and AV heart block.
Basic ECG waveforms and intervals. Not shown is the RR interval, the time between consecutive QRS complexes.