The Implantable Defibrillator

Implantable Device Monitors Heart Rhythm, Reduces Risk of Sudden Death

Typical Modern ICD Generator.

The implantable defibrillator — also called the implantable cardioverter defibrillator (ICD) — is a surgically-implanted medical device that monitors your heart rhythm and automatically delivers lifesaving treatment should you suddenly develop the dangerous heart arrhythmiasknown 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.

What Does An ICD Look Like?

Most ICDs consist of a small, thin, battery-driven titanium "generator," that 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 to specific locations within the heart.

The picture on this page shows a typical ICD generator, along with a quarter so you can estimate its size.

Recently, a subcutaneous ICD has been developed, in which both the generator and the leads are placed under the skin, and not in the blood vessels or the heart. This newer type of ICD has several advantages, and some disadvantages, compared to the standard ICD. This article specifically addresses only the standard ICDs, but you can read about subcutaneous ICD here.

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 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 Does an ICD Do?

ICDs can accomplish several important functions. These include:

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 will 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 the ICD. In many cases, 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 (CRT), which can improve symptoms in people who have heart failure.

Monitoring and storing ECGs. ICDs have an extensive capacity to monitor the heart rhythm at all times, and store ECGs from any unusual arrhythmias, including any episodes that required treatment. Most modern ICDs have the capacity to transmit these ECGs wirelessly via the internet, so that doctors can review any arrhythmias that may have occurred, and then make any necessary adjustments in the ICD’s function.

All ICDs are "programmable," which means that with a special programmer device that wirelessly communicates with the ICD, the doctor 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 sometimes are treated with ICDs.

How Is an ICD Inserted?

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, and the leads are inserted and positioned into the heart using flouroscopy (an x-ray "video") as a guide. Then the leads are attached to the ICD generator; the generator is placed beneath the skin; and the incision is closed.

Once the ICD has been implanted, the doctor may test the device to assure that 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, and in most cases the patient can go home the same day.

Complications With an ICD

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.

What Is Follow-Up Like With An ICD?

After an ICD is implanted, the doctor will see the patient in four to six 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 the programmer. This interrogation gives the doctor vital information on 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 therapy - both pacing therapy and shocking therapy.

Many modern ICDs have the capacity to wirelessly send this kind of information to the doctor from home, through the Internet. This "remote interrogation" feature allows the doctor 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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Article Sources

  • 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.