An arrhythmia is an abnormal heart rhythm. In an arrhythmia the heartbeats may be too slow, too rapid, too irregular, or too early. Rapid arrhythmias (greater than 100 beats per minute) are called tachycardias. Slow arrhythmias (slower than 60 beats per minute) are called bradycardias. Irregular heart rhythms are called fibrillations (as in atrial fibrillation and ventricular fibrillation). When a single heartbeat occurs earlier than normal, it is called a premature contraction.
Atrial arrhythmias are abnormal heart rhythms due to electrical disturbances in the atria (the upper chambers of the heart) or the AV node (an electrical relay station at the juncture of the atria and ventricles), leading to fast heart beats. Examples of atrial arrhythmias includes atrial fibrillation, atrial flutter, and paroxysmal atrial tachycardia (termed PAT).
Ventricular arrhythmias are abnormal rapid heart rhythms (arrhythmias) that originate in the lower chambers of the heart (the ventricles). Ventricular arrhythmias include ventricular tachycardia and ventricular fibrillation. Both are life threatening arrhythmias most commonly associated with heart attack or scarring of the heart muscle from previous heart attack.