An implantable cardioverter-defibrillator (ICD) is a device designed to quickly detect an abnormal heart rhythm (arrhythmia) and suppress it immediately. The ICD can convert the arrhythmia back to normal rhythm by delivering an electrical shock to the heart. This way the ICD can effectively prevent sudden death.

An ICD is a minicomputer, about the size of a small cell phone, that consists of:

• a pulse generator with batteries and a capacitor that can send a powerful shock to the heart
• an electronic logic circuit to tell the ICD when to discharge.
• lead electrodes placed in the heart (or, rarely, small electrodes placed under the skin), to sense the cardiac rhythm and deliver the shock to the heart muscle.

Modern ICDs also include pacing capabilities so that the device can act as a pacemaker. Pacing may be necessary to support the heart in some instances (for example, after a shock) or as a method to terminate certain arrhythmias.

The ICD permanently tests the heartbeats for regularity, but remains inactive until it senses a rhythm deemed life-threatening. Then, the ICD delivers a powerful electric shock to the heart. Each shock uses energy from the generator, transformed before the shock by a capacitor. The ICD batteries are designed to last 5 to 8 years.

During the implantation of an ICD, electrodes are inserted through the veins into the heart, and the pulse generator is surgically buried under the skin of the chest, generally below the left collarbone.

Who needs an ICD?

In some people, the heart either suddenly starts beating in a disorganized manner (ventricular fibrillation or VF), or it starts beating very rapidly (ventricular tachycardia or VT). Both VF and VT can make the heart stop pumping blood, leading the person to a sudden loss of consciousness or death. Arrhythmias like VT and VF require emergency treatment and are best treated in hospital coronary care units. In these units, VT and VF can be converted back to the normal rhythm by an electric shock given across the chest (external defibrillation).

Many episodes of VT or VF, however, occur outside hospitals and people without ICDs often die because defibrillation is not immediately available.

Candidates for ICDs are people at risk of sudden death, including:

• those who have survived a cardiac arrest due to VF not triggered by a recent heart attack
• those with life-threatening episodes of VT
• survivors of a heart attack
• those who have structural defects of the heart muscle, such as dilated cardiomyopathy and hypertrophic cardiomyopathy, especially when unexplained fainting episodes have occurred
• people with a reduced pumping function of the heart, often assessed as a left ventricular ejection fraction (LVEF) of 30% or less

Before a decision is made about an ICD, many people will require an electrophysiological study (EPS). The EPS is a special test conducted by a cardiologist with a specialization in arrhythmias. The EPS measures how much the heart is at risk for an event that could prove fatal. (For more information, see Electrophysiologic Study.)

Benefits of the ICD

Among patients considered good candidates for an ICD, the ICD has shown to prolong life by preventing sudden death. For many people with ICDs the quality of life can also improve. The device provides peace of mind, relieving concerns about fainting spells. Most patients adapt to the uncertainty of the shocks. Most patients also can resume driving.

Potential risks of the ICD

The ICD is implanted through a surgical procedure which in itself might result in death, but this is very rare.

The most common problem with the ICD is that it sometimes delivers shocks when you do not need them. Each shock -- even though it lasts a very short time -- will be usually quite evident to you. Every shock you feel should be notified to your doctor.

When the ICD batteries are about to run down, the ICD generator must be replaced. This requires only minor surgery.

Potential ICD malfunctioning can now be prevented by the incorporation of audible alerts that can be programmed by your electrophysiologist. If your ICD alert goes off, you should promptly contact your doctor.

What to expect after ICD implantation

The recovery time after the ICD implantation is quite short. Hospital stays are rarely longer than 1 day; and patients quickly return to prior activity levels. Full recovery of activity can be expected in about 4 to 6 weeks.

You will be discharged from the hospital with a wallet card listing the specifications of your ICD and providing contact information for emergencies. This wallet card should always be with you when you go out.

You will need to make regular visits to the doctor so the ICD can be monitored. The monitoring shows whether the device is sensing the heartbeat properly, how many shocks have been delivered, and how much power is left in the batteries. When batteries are about to run out, your ICD generator can be replaced.

The ICD generator is metal and will be detected by metal detectors such as those in airports or public places with safety checks. For air travel, public events, and admission to certain areas you will need to show your wallet card to the security personnel to prevent setting off any alarms.

People with ICDs still need to follow their doctor's advice about medication, diet, and exercise.