Electrocardiogram is a machine which records electrical activity of the heart. It record the changes in voltages at the body surface produced by alteration in the intracellular potential within myocardium.
At rest, cardiac cell are electrically polarized, i.e., intracellular potential is negative as compared to extracellular fluid. This polarity is maintained by membrane pumps that ensure the appproriate distribution of ions to keep the insides of these cells relatively electronegative. When activated cardiac cells lose their internal negativity by the process of depolarization. Depolarization is propagated from cell to cell, producing a wave of depolarization that is transmitted across the entire heart. After depolarization is complete, the polarity of cardiac cell is restored by the membrane pump,through a process of repolarization. This changes of intracellular potential produced electrical currents that radiate through the surrounding tissues to the skin, and can be detected by electrodes placed on the surfaced of the body.
Notes: Electrocardiogram does not record potential changes in individual cell, instead ECG record the summated effect of the electrical changes in all the myocardial cells.