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The Implantable Defibrillator

Implantable Device Monitors Heart Rhythm, Reduces Risk of Sudden Death


Updated June 12, 2014

Written or reviewed by a board-certified physician. See About.com's Medical Review Board.

The Implantable Defibrillator

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 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.

The ICD itself consists of a small, thin, battery-driven titanium "generator," about 2/3 the size of an iPod Nano, that is inserted beneath the skin just below the collarbone, and two or three "leads" (wires) that are attached to the generator. The wires are passed through nearby blood vessels and positioned to specific locations within the heart. (See the figure, which compares the size of a typical ICD generator to a quarter.)

The generator itself 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?

The main job of an ICD is to prevent sudden cardiac death from cardiac arrest caused by ventricular tachycardia or ventricular fibrillation.

An ICD will automatically detect the sudden onset of these dangerous arrhythmias, 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.

In addition to delivering shocks that stop cardiac arrest, ICDs can also function as pacemakers. Pacemakers use tiny electrical discharges to stimulate the heart to beat when the heart rate is too slow. In some patients, the pacemaker function of ICDs can also be used to stop episodes of ventricular tachycardia (but not ventricular fibrillation), thus avoiding the need to give a shock. Finally, some ICDs can also provide cardiac resynchronization therapy (CRT), which can improve symptoms in people who have heart failure.

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 anytime its settings need to be adjusted.

But while ICDs can do all these different things, their core function is to prevent sudden cardiac death in people who are at increased risk for cardiac arrest.

How Is an ICD Inserted?

The surgery to implant your 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 your collarbone, and the leads are inserted and positioned into your 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, most doctors will test the device to assure that it will work as designed, if and when a cardiac arrest should occur. So, after putting you into a light sleep with a short-acting sedative, the doctor induces an arrhythmia and the ICD is allowed to detect and stop the abnormal heart rhythm automatically.

The insertion procedure generally takes an hour or two, and in most cases you can go home the same day.

What Is Follow-Up Like with ICDs?

After your ICD is implanted, the doctor will want to see you four to six weeks after surgery to make sure the surgical site is fully healed and to answer any additional questions that may have occurred to you in the interim. Afterward, the doctor will usually want to see you in the office two to four times per year. During all these visits, your ICD will be 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.

Some modern ICDs have the capacity to wirelessly send this kind of information to the doctor from your home, through the Internet. This "remote interrogation" feature allows the doctor to evaluate your ICD whenever needed, without requiring you to come to the office. Even if your ICD has this remote feature, however, the doctor will want to see you in the office at least once a year.

Read more about ICDs:


Epstein, AE, DiMarco, JP, Ellenbogen, KA, et al. ACC/AHA/HRS 2008 Guidelines for Device-Based Therapy of Cardiac Rhythm Abnormalities: a report of the American College of Cardiology/American Heart Association Task Force on Practice Guidelines (Writing Committee to Revise the ACC/AHA/NASPE 2002 Guideline Update for Implantation of Cardiac Pacemakers and Antiarrhythmia Devices): developed in collaboration with the American Association for Thoracic Surgery and Society of Thoracic Surgeons. Circulation 2008; 117:e350.

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