The heart is divided into four chambers, two upper chambers called atria and two lower chambers called ventricles. A healthy heart (Figure 1), at rest, typically beats between 60 and 100 times per minute and pumps over 1,400 gallons of blood per day. Each normal heartbeat is the result of electrical signals originating at a precise area in the right atrium. These electrical signals cause a physical contraction of the atria, which then pumps blood into the ventricles. The electrical impulses then continue to the ventricles, causing them to contract and distribute blood throughout the body.

Figure 1. Normal Heart Rate

One life-threatening arrhythmia is ventricular fibrillation (Figure 2), wherein the heart's electrical signals become disorganized and erratic. During ventricular fibrillation, the heart ceases to pump sufficient blood to the body, resulting in unconsciousness within seconds. Consequently, damage is done to the heart, brain and other organs from lack of oxygen and nutrients.

Figure 2. Ventricular Fibrillation

Ventricular fibrillation is often preceded by ventricular tachycardia (Figure 3) , which is an abnormal, rapid heartbeat originating within the ventricles. While medication and surgery are used to control ventricular tachycardia, these alternatives are only partially effective and in some cases actually increase the risk of SCA. The ICD is a highly effective method of treating ventricular tachycardia and ventricular fibrillation, preventing most SCA.

Figure 3. Ventricular Tachycardia

Asystole, the lack of any ventricular contraction, is another cause of SCA. Like ventricular fibrillation, death can occur quickly since no blood is pumped to the brain and other organs. Asystole can be treated with a pacemaker.