Electrocardiogram (ECG)

A few selected examples of ECGs are displayed herein. In the normal ECG, as taken from standard lead II, there are three upward or positive deflections, P, R, and T and two downward negative deflections, Q and S. The P wave indicates atrial depolarization. The QRS complex shows ventricular activity. The S-T segment as well as the T wave indicate ventricular repolarization. There are atrial repolarization waves but they are too low in voltage to be visible (Figure 3).

The time line on the X axis is real time. The recording paper is read on this line as 0.04 seconds for each small vertical subdivision if the paper is running at 0.98 in (25 mm) per second. At the end of each group of five of these, which corresponds to 0.2 seconds, the vertical line is darker on the ruled paper. If the pulse rate is found to be 75 per minute, the duration of a cardiac cycle is 60/75 or 0.8 seconds. Variations in expected normal times for any part of a cycle indicate specific cardiac abnormalities. This is used to diagnose arrhythmias which have a basis in time deviation.

On the Y axis, every 0.4 in (10 mm) corresponds to 1 mV of activity in the heart. Although time on the X axis is real, the mV on the Y axis cannot always be taken literally. Voltages may partly lose significance in that a fatty person can to some extent insulate cardiac currents from reaching the skin.