Electrocardiography is the process of producing an electrocardiogram (ECG), recording and, a graph of voltage for the electrical activity of the heart using electrodes placed on the skin. These electrodes detect a small electrical change in cardiac cycle that are caused by the depolarization of cardiac muscle followed by repolarization. Changes in the normal ECG pattern occur in numerous cardiac abnormalities, including cardiac rhythm disturbances (atrial fibrillation and ventricular tachycardia), inadequate coronary artery blood flow (myocardial ischemia and myocardial infarction), and electrolyte disturbances (hypokalemia and hyperkalemia).

In a conventional 12-lead ECG, ten electrodes are placed on the patient’s limbs and on the surface of the chest. The overall magnitude of the heart’s electrical potential is then measured from twelve different angles (“leads”) and is recorded over a period of time (usually ten seconds). In this way, the overall magnitude and direction of the heart’s electrical depolarization is captured at each moment throughout the cardiac cycle.

There are three main components of ECG: the P wave, which represents the depolarization of the atria; the QRS complex, which represents the depolarization of the ventricles; and the T wave, which represents the repolarization of the ventricles.