Pacemakers are devices that are implanted into the body, just below the collarbone, to take over the job of the heart’s own electrical system and prevent slow heart rates. Although they weigh only an ounce and are the size of a large wristwatch face, a pacemaker contains a computer with memory and electrical circuits, a powerful battery (generator), and special wires called “leads.” The generator creates electrical impulses that are carried by the leads to the heart muscle, signaling it to pump.

Defibrillation, or shock, can be the only way to stop certain heart arrhythmias, or irregular heartbeats, before they kill. If the heart beats too quickly, the chambers, or ventricles, will not have enough time to fill with blood and pump blood to the rest of the body, which can cause death. For people at high risk for the deadliest forms of arrhythmias – called ventricular tachycardia and ventricu­lar fibrillation – an internal “shocking” device may be the best protection against sudden cardiac arrest (SCA).