Apparatus that delivers a controlled electric shock to restore normal heart rhythm in patients whose hearts have developed VENTRICULAR FIBRILLATION or have stopped beating. The shock is delivered by electrodes placed on the chest wall or directly to the heart after the chest has been surgically opened. Defibrillators are a standard item of equipment for paramedical staff in ambulances, and some airlines now routinely carry the apparatus. They are also often available in large public areas such as shopping malls or railway termini. (See also HEART, DISEASES OF.)