Devices that are used to correct dangerously abnormal heart rhythms; usually, ventricular or sudden muscle spasms or involuntary contractions of a heart muscle or group of muscles or to restart hearts by depolarizing their electrical conduction systems and delivering brief measured electrical shocks to the chest wall or more directly to the heart muscles: The patient's doctor applied a defibrillator to counteract the irregular movements of his heart muscles and and to restore his normal heartbeats by applying a brief electric shock.