Out-of-hospital cardiac arrest (OHCA) causes approximately 310,000 deaths before the patient reaches the hospital Emergency Department each year in the United States. Sudden cardiac death occurs more often in patients who are recovering from a heart attack or whose hearts have a reduced ability to pump blood. In most cases, OHCA is caused by ventricular tachycardia or ventricular fibrillation, a sudden change in heart rhythm that affects the large chambers of the heart. During ventricular fibrillation, the heart muscles do not contract simultaneously, the heart stops pumping blood effectively, and death occurs rapidly unless the patient receives treatment to restore a normal heartbeat.