Heart rhythm disorders: Abnormal rhythm of the heartbeat due to irregularities in transmission of the electrical signals that normally control heart rate and rhythm. Also known as arrhythmias. With an arrhythmia, the heartbeats may be irregular or too slow (bradycardia), too rapid (tachycardia), or occur too early. When a single heartbeat occurs earlier than normal, it is called a premature contraction. An abnormal heart rhythm can be due to electrical disturbances in the upper chambers of the heart (atria), the atrioventricular (AV) node, or the lower chambers of the heart (ventricles). Examples of atrial arrhythmias include atrial fibrillation, atrial flutter, and paroxysmal atrial tachycardia. Ventricular arrhythmias include ventricular tachycardia and ventricular fibrillation.