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Why it's done

By Mayo Clinic staff

Heart-Healthy Living

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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 implantable cardioverter-defibrillator (ICD) does the same thing, only internally and automatically when it detects your heartbeat is abnormal.

An ICD 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 constantly monitors 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.

How an ICD works
When you experience a rapid heartbeat, the wires from your heart to the device transmit signals to the ICD to send electrical pulses to regulate your heartbeat. Depending on the problem with your heartbeat, your ICD could be programmed for these therapies:

• Low-energy pacing therapy. You may feel either nothing or a painless fluttering in your chest when your ICD responds to mild disruptions in your heartbeat.
• Cardioversion therapy. A higher energy shock is delivered to deal with a more serious heart rhythm problem. You might feel as if you're being thumped in the chest.
• Defibrillation therapy. This is the strongest form of electrical therapy used to restore a normal heartbeat. During this therapy you may feel as if you're being kicked in the chest. It may knock you off your feet. The pain from this therapy typically lasts only a second. There should be no discomfort after the shock is over.

Usually, only one shock is needed to restore a normal heartbeat. Sometimes, however, you may have two or more such shocks during a 24-hour period. Frequent shocks in a short time period are known as ICD storms, and they may understandably cause you to worry. If you experience ICD storms, you should seek emergency care to see if your ICD is working properly or if you have a problem that's making your heart beat more abnormally. If necessary, the ICD can be adjusted to deliver the appropriate number of shocks. Additional medications may be needed to make your heart beat regularly and decrease the chance of an ICD storm.

Who needs an ICD?
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 makes your heart beat abnormally. 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. Having other rare conditions such as Brugada syndrome and arrhythmogenic right ventricular dysplasia also may mean you need an ICD.

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