Even though there continues to be new technologies developed for the diagnostic evaluation of patients with cardiovascular disease, the electrocardiogram (ECG) retains its central role. The ECG is the most important test for interpretation of the cardiac rhythm, conduction system abnormalities, and for the detection of myocardial ischemia. The ECG is also of great value in the evaluation of other types of cardiac abnormalities including valvular heart disease, cardiomyopathy, pericarditis, and hypertensive disease. Finally, the ECG can be used to monitor drug treatment (specifically antiarrhythmic therapy) and to detect metabolic disturbances.
A systematic approach to interpretation of the ECG is important in order to avoid overlooking important abnormalities. Pattern recognition can be useful, but only after certain salient features have been determined. This topic review provides the framework for a systematic analysis of the ECG.
The ECG is a plot of voltage on the vertical axis against time on the horizontal axis. The electrodes are connected to a galvanometer that records a potential difference. The needle (or pen) of the ECG is deflected a given distance depending upon the voltage measured.
The ECG waves are recorded on special graph paper that is divided into 1 mm2 grid-like boxes (figure 1). The ECG paper speed is ordinarily 25 mm/sec. As a result, each 1 mm (small) horizontal box corresponds to 0.04 second (40 ms), with heavier lines forming larger boxes that include five small boxes and hence represent 0.20 sec (200 ms) intervals. On occasion, the paper speed is increased to 50 mm/sec to better define waveforms. In this situation there are only six leads per sheet of paper. Each large box is therefore only 0.10 sec and each small box is only 0.02 sec. In addition, the heart rate appears to be one-half of what is recorded at 25 mm/sec paper speed, and all of the ECG intervals are twice as long as normal.
Vertically, the ECG graph measures the height (amplitude) of a given wave or deflection, as 10 mm (10 small boxes) equals 1 mV with standard calibration. On occasion, particularly when the waveforms are small, double standard is used (20 mm equals 1 mv). When the wave forms are very large, half standard may be used (5 mm equals 1 mv). Paper speed and voltage are usually printed on the bottom of the ECG.