There are three main components to an ECG (electrocardiogram) : 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.
Source: https://www.instagram.com/p/CXYFKmJMVgQ/?igshid=YmMyMTA2M2Y=