Sudden cardiac death kills an estimated 325,000 people each year when their hearts electrical system goes haywire. Their heart races (tachycardia) and quivers (fibrillates), preventing the coordinated contractions that enable it to pump. Without a high-energy shock, sudden cardiac death causes death in minutes. ICDs work like internal shock paddles to jolt a racing heart back into normal rhythm. ICDs are highly effective in preventing sudden cardiac death in high-risk patients, including survivors of sudden cardiac death, ventricular tachycardia (VT) and ventricular fibrillation (VF); those who are genetically predisposed to sudden cardiac death and those with a left ventricular ejection fraction less than or equal to 35 percent, usually due to dilated cardiomyopathy.