In this review of eight randomized controlled trials that reported on mortality and 76 observational studies that examined safety or effectiveness, ICDs reduced death from all causes in patients with left ventricular systolic dysfunction by 20 percent. This reduction came mostly from a 54 percent relative reduction in sudden cardiac deaths, which are usually caused by dangerous heart rhythms.

ICDs are small machines placed under the skin below the collarbone. They monitor the heart’s rhythm and give the heart an electrical shock if a dangerous rhythm occurs. Left ventricular systolic dysfunction -- heart muscle weakened from coronary artery disease or heart attack -- carries a high risk for sudden cardiac death. -American College of Physicians