What Is an Implantable Defibrillator (ICD)?

Most people picture a defibrillator on a wall in a gym, or in a paramedic's hands — pads on the chest, a shouted warning, one big shock. An implantable cardioverter defibrillator, usually shortened to ICD, does the same essential job from inside the body. It is a small device fitted under the skin near the collarbone that watches every heartbeat, day and night, and treats a dangerously fast heart rhythm automatically, within seconds, if one ever starts.

The short video at the top of this page, "Pacemakers & Implantable Defibrillators" (about two and a half minutes), introduces the two implanted device families side by side — and, as its description puts it, ICDs earn their place by preventing sudden death in people who have had sustained ventricular tachycardia or fibrillation, and by protecting high-risk patients who have not yet had a cardiac arrest. Because the two devices are so often mentioned in one breath, this article separates them out. Telling an ICD apart from a pacemaker — and from the external defibrillator on the wall — is the quickest way to understand what each one is actually for.

The emergency it stands guard against

Your heart's rhythm is an electrical event. Each beat starts as a signal that spreads through the heart muscle in an orderly wave. The problems an ICD exists for begin in the ventricles, the heart's two main pumping chambers. In ventricular tachycardia, the ventricles take over and beat far too fast to fill and pump properly. In ventricular fibrillation, the orderly wave collapses into quivering chaos and the heart effectively stops pumping altogether — that is cardiac arrest, and it is fatal within minutes unless the rhythm is reset.

Two things about these rhythms shape everything that follows. First, they are frequently unpredictable: they can strike someone who felt perfectly well an hour earlier. Second, they are different from a heart attack. A heart attack is a plumbing problem — a blocked artery starving heart muscle — while these are electrical failures. The two are connected, because a heart attack, or heart muscle weakened for other reasons, can leave the kind of scarred, stretched tissue in which dangerous rhythms arise. That is one reason our guide to what heart failure is makes a useful companion read: a weakened heart is one of the settings in which an ICD is most often discussed.

Three devices people mix up

A pacemaker treats a slow heart. If your heart's natural timing falters and the rate drops too low, a pacemaker sends out tiny electrical pulses to keep it ticking over at a sensible rate. Those pulses are far too small to feel. Our guide to what a pacemaker is and how it is fitted covers that device in full.

An ICD treats a fast, dangerous heart. It does everything described below, and its defining extra is the shock: a much larger pulse, delivered inside the chest, that resets a runaway rhythm. Most ICDs can also do a pacemaker's job for slow rhythms, which is exactly why the two get confused — the ICD is the pacemaker's bigger, watchful sibling, not a different species altogether.

An AED treats a stranger's heart, once. The automated external defibrillator in public places is a one-time rescue tool used by whoever is standing nearby during a collapse. An ICD is better thought of as carrying that rescue inside you permanently, with a clinician's judgement about your personal risk built into the decision to fit it.

What is actually inside, and how it responds

An ICD has two parts. The pulse generator — a smooth metal case, roughly the size of a large matchbox, holding a battery and a small computer — sits in a pocket formed under the skin just below the collarbone. One or more thin insulated wires, called leads, run from the generator through a vein into the heart itself. Through those leads the device listens to your heart's electrical activity continuously, and it can also deliver treatment along the same path.

When a dangerous rhythm starts, the response is stepped. For many fast rhythms, the device first tries a burst of painless pacing pulses, timed to interrupt and take over the rhythm — a technique called anti-tachycardia pacing, which most people never notice happening. If the rhythm is too chaotic for that, or the pacing doesn't settle it, the device charges for a few seconds and delivers its shock internally. People who have felt one often describe it as a sudden strong thump or kick in the chest: unmistakable, and over in a moment. Either way, the whole episode — detection, decision, treatment — is handled in seconds, which matters, because with ventricular fibrillation there is no time to fetch help.

The version that never touches the heart

There is also a newer variation worth knowing about. A subcutaneous ICD, or S-ICD, keeps everything outside the heart and blood vessels: its generator sits at the side of the chest, and its lead is tunnelled just under the skin alongside the breastbone, sensing and shocking from there. You can see how that arrangement is implanted in "S-ICD Implant Procedure" (just over two minutes), whose description makes the design's point plainly — it leaves the heart and vasculature untouched, and it can be implanted using anatomical landmarks, without X-ray guidance during the procedure. The trade-off is that an S-ICD cannot provide ongoing pacing support, so it suits people who need shock protection but not pacing or anti-tachycardia pacing. Which design fits is a specialist judgement, made case by case.

Who is offered one — and what an ICD does not do

Broadly, ICDs are discussed for two groups. The first is people who have already survived sustained ventricular tachycardia, ventricular fibrillation, or a cardiac arrest not caused by a temporary, fixable problem. The second is people who have not had such an event but are judged to be at high risk of one — most often because the heart muscle pumps much less strongly than it should, for example after heart damage or with established heart failure. Rhythm recordings, scans of the heart's pumping strength, and your wider medical picture all feed into that judgement, which is made with a cardiologist who specialises in heart rhythm. An ECG and longer rhythm monitoring are usually part of the evidence they work from.

It is equally worth being clear about the limits. An ICD does not prevent dangerous rhythms from starting; it treats them when they occur. It does not repair weakened heart muscle, unblock arteries, or replace medicines — and for some rhythm problems a different approach, such as cardiac ablation, is part of the plan as well. Think of it as insurance that travels with you, not as a cure for the underlying condition.

Living with one

Day to day, most people are barely aware of their device. Follow-up happens at a device clinic — and for many patients partly from home, with remote monitoring equipment that sends readings to the clinic between visits. The battery is designed to last for years; when it eventually runs low, the generator is replaced in a procedure that is usually smaller than the first implant, and the leads are normally left in place. Your clinic will give you specific guidance on driving, exercise, travel security scanners and medical procedures — carry your device identification card, and tell any clinician treating you, including your dentist, that you have an ICD fitted. If the device ever delivers a shock, the standard advice is to sit or lie down, note how you feel, and contact your device clinic promptly for a check; repeated shocks, or feeling unwell, mean urgent medical help.

References & further reading

MedlinePlus — Pacemakers and Implantable Defibrillators: https://medlineplus.gov/pacemakersandimplantabledefibrillators.html

British Heart Foundation — Implantable cardioverter defibrillator (ICD): https://www.bhf.org.uk/informationsupport/treatments/implantable-cardioverter-defibrillator

NHS — Pacemaker implantation: https://www.nhs.uk/conditions/pacemaker-implantation/

National Heart, Lung, and Blood Institute — Pacemakers: https://www.nhlbi.nih.gov/health/pacemakers

Frequently Asked Questions

Is an ICD the same thing as a pacemaker?
No, though they look similar and most ICDs can also pace. A pacemaker's job is to stop the heart beating too slowly, using tiny pulses you cannot feel. An ICD does that too, but adds the defining extra: it can deliver a strong internal shock to reset a dangerously fast rhythm such as ventricular fibrillation.

Does an ICD shock hurt?
People who have experienced one usually describe a sudden, strong thump or kick in the chest that is over in a moment. It is startling rather than prolonged. Many of the fast rhythms an ICD treats are actually ended first by painless pacing pulses, which are not felt at all.

Can an ICD prevent a heart attack?
No. A heart attack is caused by a blocked artery, and an ICD does not unblock arteries or prevent blockages. It treats dangerous heart rhythms — which a heart attack can sometimes trigger. Chest pain or other heart-attack symptoms are always an emergency, ICD or not.

Will I feel the device working day to day?
Almost certainly not. The sensing is silent, pacing pulses are too small to feel, and the device itself sits quietly under the skin, where you may feel its outline under the collarbone. You would only notice it if it delivered a shock.

What happens when the battery runs out?
The battery is checked at every follow-up, so replacement is planned, never a surprise. When it runs low after some years, the pulse generator is swapped for a new one in a procedure usually smaller than the original implant; the leads are normally left undisturbed.

MedicalVideos.com has published trusted medical videos since 2007. The site was founded by Dr. Mohamed Ibrahim, MD, PhD — physician-scientist.

Educational only — this article is general information about implantable cardioverter defibrillators (ICDs), not medical advice and not a diagnosis. Decisions about whether an ICD is right for you belong with your own cardiology team, who can assess your heart's rhythm and pumping strength.