Cardiac arrest — the sudden failure of the heart to pump blood — claims about 335,000 lives each year in the United States, according to the American Heart Association.

The most common heart-rhythm disorders (arrhythmias) the lead to cardiac arrest are:

• Ventricular tachycardia — a dangerously fast heartbeat
• Ventricular fibrillation — a rapid and chaotic heartbeat that causes the ventricle to quiver ineffectively, unable to supply blood to your body

An implantable cardioverter-defibrillator (ICD) — a pager-sized device implanted in your chest like a pacemaker — may reduce your risk of dying of cardiac arrest by detecting and stopping these dangerous arrhythmias. An ICD continuously monitors your heartbeat and delivers precisely calibrated electrical shocks to restore a normal heart rhythm.

You're a prime candidate for an ICD if you've had ventricular tachycardia, survived a cardiac arrest or have fainted from a ventricular arrhythmia. You may also benefit from an ICD if you have:

• A history of coronary artery disease and prior heart attack that has led to a weak heart.
• A heart condition that involves abnormal heart muscle, such as enlarged (dilated cardiomyopathy) or thickened (hypertrophic cardiomyopathy) heart muscle.
• An inherited heart defect that adversely affects your heart's electrical system. These include long QT syndrome, which can cause ventricular fibrillation and death even in young, active people with no signs or symptoms of heart problems, and other rare conditions such as Brugada syndrome and arrhythmogenic right ventricular dysplasia.

To determine whether you need an ICD, your doctor may perform any of these diagnostic tests:

• Electrocardiography (ECG), a noninvasive test that measures your heart's electrical activity.
• Echocardiography, a noninvasive ultrasound test that shows how well your heart pumps blood.
• Electrophysiology study (EPS), a procedure during which electrodes are guided through blood vessels to your heart and used to test the function of your heart's electrical system, locate short circuits and identify your potential for heart-rhythm problems.
• Holter monitoring, a noninvasive test that requires you to wear a device that records your heart's electrical activity for 24 to 48 hours.
• Event recorder, a pager-sized device that records your heart activity. Unlike a Holter monitor, it doesn't operate continuously and you turn it on only when you feel your heart is beating abnormally.

You may have seen TV shows in which a hospital worker or paramedic "shocks" an unconscious person out of cardiac arrest with a pair of electrified paddles. An ICD does the same thing, only internally and automatically.

This small, battery-powered device is surgically placed under your skin, usually below your left collarbone. One or two flexible, insulated wires (leads) run from the ICD through your veins to the lower chambers of your heart.

Because the ICD is constantly on guard for abnormal heart rhythms and instantly attempts to correct them, it helps treat cardiac arrest even if you're hours away from the nearest hospital.

The surgery to implant an ICD is usually relatively minor. It can be performed with local anesthesia and a sedative that puts you in a relaxed state but allows you to remain aware of your surroundings.

The procedure typically takes one to three hours. Testing the ICD requires shocking your heart and for that, general anesthesia is used. You stay in the hospital one or two days, and the ICD may be evaluated one more time before you're discharged. Any additional studies are usually performed through the device via radio waves and are nonsurgical.

After surgery you may have some pain in the incision area, which can remain swollen and tender for a few days or weeks. Pain medication often is prescribed, and you can take nonaspirin pain relievers as the severity lessens. Unless your doctor instructs you to, don't take pain medication containing aspirin because it may increase the risk of bleeding.

After implantation, your doctor programs the ICD to treat your specific heart-rhythm problem.

Two main ICD functions

• Cardioversion. When the ICD detects ventricular tachycardia, it delivers an electrical shock that converts the fast heartbeat into a slower, normal heartbeat.
• Defibrillation. Sometimes cardioversion fails, and ventricular tachycardia either worsens or leads to ventricular fibrillation. Other times, ventricular fibrillation develops spontaneously. When the ICD detects either of these life-threatening rhythms, it delivers a stronger electrical shock that resets (defibrillates) your heart to start beating normally.

An ICD can also be programmed to perform additional functions, which include:

Additional ICD functions

• Anti-tachycardia (tak-ih-KAHR-de-uh) pacing. If you experience an unusually fast heart rate, the ICD delivers painless, low-energy impulses that pace or stimulate the heart to beat at a rate that prompts it to return to a normal rhythm. This can prevent the need for cardioversion or defibrillation.
• Anti-bradycardia (brad-e-KAHR-de-uh) pacing. When the heartbeat is abnormally slow (bradycardia) because of a heart condition or medication, a standard pacemaker is the typical treatment. People with ICDs, however, sometimes develop bradycardia as a result of the shock the ICD delivers in response to ventricular tachycardia or ventricular fibrillation. Under these circumstances, the ICD can sense the slow heart rate and function as a pacemaker, delivering low-energy impulses that stimulate the heart to beat normally.
• Recording heart activity. The ICD records information about variations in your heart's electrical activity and rhythm. This information helps your doctor evaluate your heart-rhythm problem and, if necessary, reprogram your ICD.
• Biventricular pacing. Unlike a standard pacemaker, which stimulates only one side of your heart's main pumping chamber (the right ventricle), a biventricular pacemaker stimulates both the right and left ventricles to make the heart beat more efficiently. A special type of ICD — a combined biventricular pacemaker with ICD — can do the same thing. Biventricular pacing is particularly valuable for some people with heart failure whose hearts' electrical systems don't work normally.

Because of these capabilities, ICDs have become standard treatment for anyone who has survived cardiac arrest and are used increasingly in individuals who are at high risk of sudden cardiac arrest. If you have an ICD, your risk of sudden death from cardiac arrest is significantly lower than it would be if you were treated solely with anti-arrhythmic medications.