Cardiac arrest is a sudden failure of the heart to pump blood. The most common reason for cardiac arrest is an electrical problem called ventricular fibrillation. Ventricular fibrillation causes the lower chambers (ventricles) of the heart to rapidly contract in an uncoordinated manner. This prevents the heart from pumping blood. Death usually occurs within 5 minutes unless pumping of the heart is maintained by external cardiac massage or the heartbeat is returned to its normal rhythm by an electrical shock (electrical defibrillation). Risk for cardiac arrest is increased in various people, including patients who have been resuscitated from previous arrest, patients with symptoms from abnormally fast ventricular beats (ventricular tachycardia), patients with heart failure, and patients with a recent heart attack. Doctors can treat patients at risk for cardiac arrest with drugs that help prevent abnormal heart rhythms (antiarrhythmic agents) or with electrical devices that are inserted under the skin (implantable cardioverter defibrillators [ICDs]). The ICDs detect abnormally fast heartbeats and give an electronic shock to the heart to return it to normal rhythm. Although ICDs can prevent deaths from cardiac arrest, it is unclear which types of patients are most likely to benefit from them.