Implantable cardioverter-defibrillators are commonly used in patients who have life-threatening heart rhythms (ventriculararrhythmias). With these implanted electronic devices, slow heart rhythms (bradyarrhythmias) and fast heart rhythms (tachyarrhythmias) can be recognized promptly and treated with electrical pacing or electrical shock therapy (defibrillation). Implantable cardioverter-defibrillators have been shown to reduce the incidence of sudden cardiac death in patients with known life-threatening ventricular arrhythmias. Technologic advances have allowed transvenous implantation of cardiac leads, removing the need for open heart surgery and thereby lowering the risk of perioperative complications.