Fibrillation is the rapid, irregular, and unsynchronized contraction of the muscle fibers of the heart. There are two major classes of fibrillation, atrial fibrillation and ventricular fibrillation.

• Atrial fibrillation can be a chronic condition, usually treated with anticoagulation and sometimes with conversion to normal sinus rhythm.

• Ventricular fibrillation is rapidly fatal if not reversed by defibrillation.