The electrocardiogram (ECG or EKG) is a recording of the electrical activity of the heart. This electrical activity (depolarization and repolarization) gives rise to contraction and relaxation of heart muscle. The ECG therefore represents the sum of the myocardial muscle fibre action potentials.

Recording
The ECG is recorded by placing electrodes on the body surface, since the body fluids are excellent conductors of electricity. To enchance electrical contact a jelly, containing an electrolyte, is put on the skin where electrodes are attached.