Defibrillator. Nurse holding a defibrillator used to treat a heart attack, or myocardial infarction. During a heart attack, the heart's ventricles (lower chambers) start to beat chaotically fast (ventricular fibrillation), leading to impaired pumping of blood around the body. This is swiftly fatal. An electric current is passed through the heart between the two plates seen here. The jolt of electricity settles the heart down into its normal rhythm. It is also used to treat arrhythmia (irregular heartbeat) and tachycardia (fast heartbeat).