After a heart attack, some patients develop a dangerous heart rhythm called sustained ventricular arrhythmia. If the patient is in the hospital, doctors or nurses apply an electric shock to the chest to stop the rhythm (external cardioversion). It is now possible to place a device called an implantable cardioverter defibrillator (ICD), which monitors heart rhythm, inside the chest. When it detects a dangerous rhythm, it applies an electrical shock to restore a normal heart rhythm. The ICD improves the survival of people who have had sustained ventricular arrhythmias. These dangerous rhythms could also be prevented by treatment with a drug, such as amiodarone. However, the balance between the costs of these treatments and their potential benefits is not known.