An implantable cardioverter-defibrillator (ICD) is a device designed to quickly detect a life-threatening, rapid heartbeat coming from the bottom chamber of the heart. It converts the abnormal rhythm back to normal by delivering an electrical shock to the heart. An ICD can effectively prevent sudden cardiac death.
An ICD is a minicomputer that consists of:
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a pulse generator with batteries and a capacitor that can send a powerful shock to the heart
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an electronic logic circuit to tell the ICD when to discharge.
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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.
All ICDs have a built-in pacemaker. Pacing may be necessary to support the heart in some instances, for example, after a shock, or during a very slow heart beat. The pacemaker also allows the device to correct heartbeats that are too fast.
The ICD continously monitors the heartbeats for regularity. It only delivers a shock to the heart when it senses a life-threatening rhythm.
During the implantation of a pacemaker-type ICD, electrodes are inserted through the veins into the heart. 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 lower chamber of 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. Abnormal rhythms, 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:
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those who have survived a cardiac arrest due to VF not triggered by a recent heart attack
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those with life-threatening episodes of VT
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survivors of a heart attack with weakened pumping function
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those who have structural defects of the heart muscle, such as dilated cardiomyopathy and hypertrophic cardiomyopathy, especially when unexplained fainting episodes have occurred
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people with a reduced pumping function of the heart, often assessed as a left ventricular ejection fraction (LVEF) of 35% or less
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Occasionally, an electrophysiological study (EPS) is required to make the decision about ICD implantation. 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.
