Premature beats refer to the early discharge of the heart's impulses. Premature beats can be divided into atrial premature beats, ventricular premature beats, and junctional premature beats. Atrial premature beats occur when the impulse is prematurely emitted from the atrium; ventricular premature beats occur when the impulse is prematurely emitted from the ventricles; junctional premature beats occur when the impulse is prematurely emitted from the atrioventricular junction. Atrial fibrillation is characterized by the atria losing their normal pattern of contraction and relaxation, replaced by irregular quivering. Both premature beats and atrial fibrillation are common arrhythmias that can be identified on an electrocardiogram.