Sudden cardiac death is believed to account for a substantial proportion of deaths in patients with evidence of cardiovascular disease. The exact proportion of cardiac deaths that are classified as being sudden varies depending on the population, the underlying disease, and the definition of sudden death. Various estimates suggest that about 500,000 sudden cardiac deaths occur annually in the United States alone. The majority of sudden cardiac deaths are thought to be due to ventricular fibrillation (VF) or tachycardia (VT).