A defibrillator is used to restore a normal heart rhythm by delivering an electrical shock to the heart when the heartbeat is dangerously fast due to ventricular tachycardia or ventricular fibrillation. Although few deaths are recorded as caused by ventricular fibrillation, it is the actual cause of death in most cases of coronary artery disease. Patients who have coronary artery disease or have suffered from a previous heart attack are more likely to go into cardiac arrest, a condition in which the heart stops pumping blood suddenly and circulation ceases. Cardiac arrest is also known as sudden cardiac death.

It is estimated that sudden cardiac death claims more than 330,000 people every year outside of hospitals. Other patients at high risk of arrhythmias requiring defibrillation include those with cardiomyopathy, long QT syndrome, arrhythmogenic right ventricular dysplasia and survivors of sudden cardiac death.

Defibrillation works by delivering a strong jolt of electricity to the heart. This is often enough to "restart" the heart's automatic pumping action, restoring blood flow to the body. Some defibrillators are external (e.g., defibrillator paddles in an emergency room), while some may be implanted in the patient’s chest (e.g., an implantable defibrillator). Although the devices range in size and appearance, their goal is the same: to convert an unhealthy cardiac rhythm to a more normal rhythm.

There are three basic types of defibrillators: