The heart's sequence of openings and closings provides optimal timing for it to operate. In much the same way that a spark plug that fails to fire on time decreases the mechanical ability of a car, so too does the proper timing of the heart's electrical system determine critical aspects of its overall function: the muscular pumping function and the normal operation of the valves.

An irregular heartbeat can arise from anywhere within the heart—the atrial or ventricular chambers or the connections that lie in between. We refer to arrhythmic beats that occur in the atrial chambers as either atrial premature depolarizations (APDs), or supraventricular depolarizations. Arrhythmic beats that stem from the ventricle are known as ventricular premature depolarizations (VPDs). More sustained irregularities, such as atrial or ventricular fibrillation, are more important medically. These conditions may be lethal or predispose the patient to stroke if not recognized and treated.