ICDs have been very useful in preventing sudden death in patients with known, sustained ventricular tachycardia or fibrillation. Studies have shown that they may have a role in preventing cardiac arrest in high-risk patients who haven't had, but are at risk for, life-threatening ventricular arrhythmias.

How is an ICD implanted?

A battery-powered pulse generator is implanted in a pouch under the skin of the chest or abdomen, often just below the collarbone. The generator may be a little larger than a man’s wallet. Wires or leads run from the pulse generator to positions on the surface of or inside the heart. Newer devices are smaller and have simpler lead systems. They can be installed through blood vessels, eliminating the need for open-chest surgery.

When the ICD detects ventricular tachycardia or fibrillation, it shocks the heart to restore the normal rhythm. New devices also provide “overdrive” pacing to electrically convert a sustained ventricular tachycardia (fast heart rhythm) and "backup" pacing if bradycardia (slow heart rhythm) occurs. They also offer a host of other sophisticated functions such as storage of detected arrhythmic events and the ability to perform electrophysiologic testing.

The American Heart Association recommends that before a patient is considered to be a candidate for an implanted defibrillator, the arrhythmia in question must be life-threatening and correctable causes of the arrhythmia must be ruled out. These include:

• Acute myocardial infarction (heart attack)
• Myocardial ischemia (inadequate blood flow to the heart muscle)
• Electrolyte imbalance and drug toxicity