The Normal Electrocardiogram
The electrocardiograph is a galvanometer calibrated to produce a deflection with an amplitude of 1 cm for every millivolt (mV) of voltage. The electrocardiogram (ECG) is recorded on graph paper divided into small 1-mm squares. At the standard paper speed of 25 mm/s, each small square corresponds to 40 ms (Fig. 3.1). Before the ECG is recorded, the machine must be calibrated to ensure that it is functioning correctly, and the square (or rectangular) calibration mark must be recorded on the tracing.