The devices are connected to leads positioned inside the heart or on the surface. The leads deliver electrical shocks, monitor the cardiac rhythm and can pace the heart. When the ICD detects the need, it delivers a shock to the heart to restore normal rhythm.
ICDs have been very useful in preventing sudden death in patients who suffer from recurrent, sustained ventricular tachycardia or fibrillation. Studies have shown they may also prevent cardiac arrest in patients at high risk for ventricular arrhythmias.