Health A-Z

Medical Content Created by the Faculty of the Harvard Medical School


What Is It?

An implantable cardioverter defibrillator (ICD) is a potentially lifesaving medical device that is placed inside the body. An ICD treats life-threatening abnormal heart rhythms (called arrhythmias), including ventricular fibrillation, which makes the heart's large muscular chambers (the ventricles) quiver without actually squeezing and pumping. When this happens, there is no real heartbeat and not enough blood flows to the brain or other organs, including the heart. As a result, a person with ventricular fibrillation passes out and can die within minutes.

An ICD is made of two parts. The pulse generator looks like a small box. It is implanted under the skin below the collarbone. The box contains a lithium oxide battery (which lasts about five to nine years) and electrical components that analyze the heart's electrical activity. Connected to the pulse generator are one or more electrodes, which travel to the heart. When the ICD senses an abnormal heart rhythm, it administers a brief, intense electrical shock to the heart, correcting the abnormal rhythm. Many people say that the shock feels like being punched in the chest, although the amount of discomfort varies.

In addition to "zapping" the heart back to a normal rhythm, ICDs also can generate milder electrical impulses. These impulses can artificially regulate or "pace" the heartbeat if the heart develops other types of arrhythmias. For example, ICD impulses can help to slow down the heart when a person has ventricular tachycardia, an abnormally fast heartbeat. ICD impulses also can speed up the heartbeat in cases of bradycardia, an abnormally slow heartbeat.

An ICD also keeps a record of its actions. This record helps your doctor to monitor how often you have arrhythmias and how dangerous they are. It also allows your doctor to see how well the ICD is working.

ICDs must be checked periodically. Surgery isn't required. A special radio transmitter can receive information from the ICD. Also, ICDs can be reprogrammed to improve performance. Adjustments are made with a small, wand like instrument held near the body.

To prevent an unexpected loss of power, ICDs have a built-in warning signal that tells the doctor when the battery is low. This signal appears several months before the battery expires. It can be detected in the doctor's office during a routine ICD checkup.

The first ICDs in the 1980s were fairly simple. They only treated ventricular fibrillation, and they had a relatively large pulse generator (about the size of a pack of cigarettes). Implanting them required major, open-heart surgery, followed by a long hospital stay. Newer ICDs have pulse generators that are approximately 1 inch (2.5 centimeters) in diameter and weigh less than 1.4 ounces (about 40 grams). Their pulse generators are inserted under the skin through a small incision, and the electrodes can be threaded through veins without surgically opening the chest. Because of these improvements, ICD implantation is now classified as minor surgery.

ICDs are implanted in a hospital operating room or in a cardiac electrophysiology laboratory.

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From Health A-Z, Harvard Health Publications. Copyright 2007 by the President and Fellows of Harvard College. All rights reserved. Written permission is required to reproduce, in any manner, in whole or in part, the material contained herein. To make a reprint request, contact Harvard Health Publications. Used with permission of StayWell.

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