An Overview of the Implantable Defibrillator

An implantable defibrillator (also called an implantable cardioverter defibrillator, or ICD) is a surgically placed medical device that monitors your heart rhythm and automatically delivers lifesaving treatment should you suddenly develop the dangerous heart arrhythmias known as ventricular fibrillation and ventricular tachycardia. ICDs are recommended for people who are known to have a high risk of sudden death from cardiac arrest.

Internal defibrillator on white surface

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What an ICD Looks Like

Most ICDs consist of a small, thin, battery-driven titanium "generator," which is inserted beneath the skin just below the collarbone, and from one to three "leads" (wires) that are attached to the generator. The leads are passed through nearby blood vessels and positioned at specific locations within the heart.

Recently, a subcutaneous ICD has been developed, in which both the generator and the leads are placed under the skin, rather than in the blood vessels or the heart. This newer type of ICD has several advantages, and some disadvantages, compared to a standard ICD.

The ICD generator contains a battery, capacitors, a computer, and other sophisticated electronics. The leads transmit the heart's tiny electrical signals (the signals that control the heart rhythm) back to the generator, where they are continuously analyzed. If a dangerous arrhythmia is detected, the ICD immediately treats it by either pacing or shocking the heart through the leads.

What an ICD Does

ICDs accomplish several important functions, including the following.

Cardioversion and defibrillation: The main job of an ICD is to prevent sudden cardiac death from cardiac arrest caused by ventricular fibrillation.

An ICD will automatically detect the sudden onset of ventricular fibrillation, and within 10 to 20 seconds automatically deliver a large electrical discharge (that is, a shock) to the heart, which stops the arrhythmia and allows the normal heart rhythm to return.

ICDs are highly effective. A properly implanted, well-functioning ICD will stop these life-threatening arrhythmias more than 99% of the time.

Anti-tachycardia pacing: Ventricular tachycardia is another potentially life-threatening cardiac arrhythmia that is treated automatically by an ICD. In many cases, the heartbeat is rapid and unstable enough that it must be treated with a shock, just as ventricular fibrillation is treated. However, in some cases, ventricular tachycardia can be terminated by applying brief, rapid bursts of pacing. ICDs are often programmed to deliver a few sequences of this kind of anti-tachycardia pacing when ventricular tachycardia occurs, in the attempt to stop the arrhythmia without delivering a shock. If the anti-tachycardia pacing fails to stop the arrhythmia, a shock will then be delivered automatically.

Bradycardia pacing: In addition to its ability to terminate lethal arrhythmias, ICDs can also function as standard pacemakers to prevent heart rates that are too slow (bradycardia).

Cardiac resynchronization therapy: Certain specialized ICDs, in addition to treating rapid and slow cardiac arrhythmias, can also provide cardiac resynchronization therapy, which can improve symptoms in people who have heart failure.

Monitoring and storing ECGs: ICDs have the capacity to monitor the heart rhythm at all times as well as store ECGs from any unusual arrhythmias, including any episodes that required treatment. Most modern ICDs can also transmit these ECGs wirelessly via the internet to your healthcare provider so they can review any arrhythmias that may have occurred and make any necessary adjustments in the ICD’s function.

All ICDs are "programmable," which means that, with a programmer device that wirelessly communicates with the ICD, the healthcare provider can easily change the way the device functions any time its settings need to be adjusted.

Who Should Receive an ICD

ICDs are indicated in many people who have a high risk of death or injury due to episodes of ventricular tachycardia or ventricular fibrillation. Typically, these are people with serious underlying heart disease. However, some people with inherited cardiac disorders may have an increased risk of sudden death from cardiac arrhythmias, even though they are otherwise quite healthy. For instance, people with long QT syndrome or Brugada syndrome are sometimes treated with ICDs.


The surgery to implant an ICD is considered minimally invasive and is usually done by a cardiologist using local anesthesia in a cardiac catheterization laboratory. A small incision is made beneath the collarbone, where the leads are inserted and positioned into the heart using fluoroscopy (an X-ray "video") as a guide. The leads are then attached to the ICD generator, the generator is placed beneath the skin, and the incision is closed.

Once the ICD has been implanted, the healthcare provider may test the device to ensure it will work as designed, if and when a cardiac arrest should occur. This is done by putting the patient into a light sleep with a short-acting sedative, then inducing an arrhythmia and allowing ICD to detect and stop the arrhythmia automatically.

The insertion procedure generally takes about an hour or so. In most cases, the patient can go home the same day.


Most people who receive ICDs have no serious complications. However, as with any type of invasive therapy, complications do occur.

Surgical complications include infections, bleeding, and damage to blood vessels during lead insertion. Complications related to the ICD itself include receiving inappropriate shocks, lead dislodgement, and erosion of the generator.


After an ICD is implanted, the healthcare provider will often see the patient in two weeks to make sure the surgical site is fully healed. Long-term follow-up usually requires office visits two to four times per year. During all these visits, the ICD is wirelessly "interrogated" using its programmer. This interrogation gives the practitioner vital information about how the ICD is functioning, the status of its battery, the status of the leads, and whether and how often the ICD has needed to deliver both pacing therapy and shock therapy.

As noted above, many modern ICDs also have the capacity to wirelessly send this kind of information to the practitioner from home, through the Internet. This "remote interrogation" feature allows the healthcare provider to evaluate a person’s ICD whenever needed, without requiring the patient to come to the office.

A Word From Verywell

ICDs are designed to monitor your heart rhythm continuously, and, if a potentially lethal cardiac arrhythmia occurs, to automatically deliver life-saving treatment. For people who are at particularly high risk for sudden death, ICDs should be strongly considered.

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  • Al-Khatib SM, Stevenson WG, Ackerman MJ, et al. 2017 AHA/ACC/HRS Guideline for Management of Patients With Ventricular Arrhythmias and the Prevention of Sudden Cardiac Death: A Report of the American College of Cardiology/American Heart Association Task Force on Clinical Practice Guidelines and the Heart Rhythm Society. J Am Coll Cardiol 2018; 72:e91.

  • Wilkoff BL, Fauchier L, Stiles MK, et al. 2015 HRS/EHRA/APHRS/SOLAECE Expert Consensus Statement on Optimal Implantable Cardioverter-Defibrillator Programming and Testing. Heart Rhythm 2016; 13:e50.

  • Russo AM, Stainback RF, Bailey SR, et al. ACCF/HRS/AHA/ASE/HFSA/SCAI/SCCT/SCMR 2013 appropriate use criteria for implantable cardioverter-defibrillators and cardiac resynchronization therapy: a report of the American College of Cardiology Foundation appropriate use criteria task force, Heart Rhythm Society, American Heart Association, American Society of Echocardiography, Heart Failure Society of America, Society for Cardiovascular Angiography and Interventions, Society of Cardiovascular Computed Tomography, and Society for Cardiovascular Magnetic Resonance. J Am Coll Cardiol 2013; 61:1318.

By Richard N. Fogoros, MD
Richard N. Fogoros, MD, is a retired professor of medicine and board-certified in internal medicine, clinical cardiology, and clinical electrophysiology.