The electrical activity associated with any muscle action travels through various tissues and ultimately reaches the surface of the body where it can be detected by electrodes applied to the skin. The record obtained from the depolarisation and repolarisation voltages of the heart muscle is called an electrocardiogram or ECG.
The diagram below shows an ECG trace. Each 5 mm square in the grid represents 0.2 seconds duration and 0.5 mV amplitude. The trace is 8 seconds in length: each second is 25 mm (5 squares) long, as indicated by the black lines (arrowed).
In order to understand and interpret the ECG, specific terminology is used to define:
The ECG can be used to detect normal activity, cardiac arrhythmia, and heart disease. To detect different cardiac problems a range of ECG tests are used.
The precise derivation of the ECG from the actions of the heart muscle is described here.