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 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.

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 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.

The picture on this page compares the size of a typical ICD generator to a quarter.

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 and the heart. This new, less invasive 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?

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. (Note: the pacemaker aspect of subcutaneous ICDs is very limited - this is one of the disadvantages of these less invasive devices.)

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 any time its settings need to be adjusted.

But while ICDs may be able to 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 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.

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.

Read more about ICDs:

Sources:

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.

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