The implantable cardioverter-defibrillator (ICD) is a surgically implanted electronic device that directs an electric charge directly into the heart to treat life-threatening heartbeat irregularities.
Purpose
The implantable cardioverter-defibrillator is used to detect and stop serious arrhythmias and restore a normal heartbeat. The exact indications for the implantation of the device are controversial, but patients suffering from ventricular fibrillation (unproductive heartbeat), ventricular tachycardia (abnormally fast heartbeat), long QT syndrome (an inherited heart disease), or others at risk for sudden cardiac death are potential candidates for this device. A study by the National Institute for Heart, Lung, and Blood of the National Institutes of Health indicated a significant increase in survival for patients suffering from ventricular arrhythmias when ICD implant is compared to medication. Several follow-up studies indicate that this may be due to the marked increase in survival for the sickest patients, generally defined as those having a heart weakened to less than 50% of normal, as measured by the ability of the left side of the heart to pump blood. Overall, studies have documented a very low mortality rate of 1–2% annually for persons implanted with the device, compared to approximately 15–25% for patients on drug therapy.
Description
Similar in structure to a pacemaker, an ICD has three main components: a generator, leads, and an electrode. The generator is encased in a small rectangular container, usually about 2 in (5 cm) wide and around 3 oz (85 g) in weight. Even smaller generators have been developed, measuring 1 in (2.54 cm) in diameter and weighing about half an ounce (14.17 g). The generator is powered by lithium batteries and is responsible for generating the electric shock. The generator is controlled by a computer chip that can be programmed to follow specific steps according to the input gathered from the heart. The programming is initially set and can be changed using a device (called a wand programmer) that communicates by radio waves through the chest of the patient after implantation.
One or two leads, or wires, are attached to the generator. These wires are generally made of platinum with an insulating coating of either silicone or polyurethane. The leads carry the electric shock from the generator. At the tip of each lead is a tiny device called an electrode that delivers the necessary electrical shock to the heart. Thus, the electric shock is created by the generator, carried by the leads and delivered by the electrodes to the heart. The decision of where to put the leads depends on the needs of the patient, but they can be located in the left ventricle, the left atrium, or both.
According to the American College of Cardiology, more than 100,000 persons worldwide currently have an ICD. The battery-powered device rescues the patient from a life-threatening arrhythmia by performing a number of functions in order to reestablish normal heart rhythm, which varies with the particular problem of the patient. Specifically, if encountered with ventricular tachycardia, many devices will begin treatment with a pacing regimen. If the tachycardia isn't too fast, the ICD can deliver several pacing signals in a row. When those signals stop, the heart may go back to a normal rhythm. If the pacing treatment is not successful, many devices will move onto cardioversion. With cardioversion, a mild shock is sent to the heart to stop the fast heartbeat. If the problem detected is ventricular fibrillation, a stronger shock called a defibrillation is sent. This stronger shock can stop the fast rhythm and help the heartbeat go back to normal. Finally, many ICDs can also detect heartbeats that are too slow. It can act like a pacemaker and bring the heart rate up to normal.
ICDs that defibrillate both the ventricles and the atria have also been developed. Such devices not only provide dual-chamber pacing but also can distinguish ventricular from atrial fibrillation. Patients that experience both atrial and ventricle fibrillation or atrial fibrillation alone that would not be controlled with a single chamber device are candidates for this kind of ICD.
Operation
ICD insertion is considered minor surgery and can be performed in either an operating room or an electrophysiology laboratory. The insertion site, in the chest, will be cleaned, shaved and numbed with the injection of a medication (local anesthetic). Generally, left handed persons have ICDs implanted on the right side and visa versa, to speed return to normal activities. Two small cuts (incisions) are made, one in the chest wall and one in a vein just under the collarbone. The wires of the ICD are passed through the vein and attached to the inner surface of the heart. The other ends of the wires are connected to the main box of the ICD, which is inserted into the tissue under the collarbone and above the breast. Once the ICD is implanted, the physician will test it several times before the anesthesia wears off by causing the heart to fibrillate and making sure the ICD responds properly. The doctor then closes the incision with sutures (stitches), staples or surgical glue. The entire procedure takes about an hour.
Immediately following the procedure, a chest x ray will be taken to confirm the proper placement of the wires in the heart. The ICD's programming may be adjusted by passing the programming wand over the chest. After the initial operation, the physician may induce ventricular fibrillation or ventricular tachycardia one more time prior to the patient's discharge, although recent studies suggest that this final test is not generally necessary.
A short stay in the hospital is usually required following ICD insertion but this varies with the patient's age and condition. If there are no complications, complete recovery from the procedure will take about four weeks. During that time, the wires will firmly take hold where they were placed. In the meantime, the patient should avoid heavy lifting or vigorous movements of the arm on the side of the ICD, or else the wires may become dislodged.
After implantation, the implantable cardioverterdefibrillator is programmed to respond to rhythms above the patient's exercise heart rate. Once the device is in place, many tests will be conducted to ensure that the device is sensing and defibrillating properly. About 50% of patients with ICDs require a combination of drug therapy and the ICD.
Safety
Environmental conditions that can affect the functioning of the ICD after installation include:
strong electromagnetic fields, such as those used in arcwelding
contact sports
shooting a rifle from the shoulder nearest the installation site
cell phones used on that side of the body
magnetic mattress pads, such as those believed to treat arthritis
Environmental conditions often erroneously thought to affect ICDs include:
microwave ovens (the waves only affect old, unshielded pacemakers and do not affect ICDs)
airport security (although metal detector alarms could be set off, so patients should carry a card stating they have an ICD implanted)
anti-theft devices in stores (although patients should avoid standing near the devices for prolonged periods)
Patients should also be instructed to memorize the manufacturer and make of their ICD. Although manufacturing defects and recalls are rare, they do occur and a patient should be prepared for that possibility.
Maintenance
In general, if the condition of the patient's heart, drug intake, and metabolic condition remain the same, the ICD requires only periodic checking every two months or so for battery strength and function. This is done by placing a special device over the ICD that allows signals to be sent over the telephone to the doctor, a process called trans-telephonic monitoring.
If changes in medications or physical condition occur, the doctor can adjust the ICD settings using a programmer, which involves placing the wand above the pacemaker and remotely changing the internal settings. One relatively common problem is the so-called "ICD storm," where the machine inappropriately interprets an arrhythmia and gives a series of shocks. Reprogramming can sometimes help alleviate that problem.
When the periodic testing indicates that the battery is getting low, an elective ICD replacement operation is
KEY TERMS
Arrhythmia—A variation of the normal rhythm of the heartbeat.
Cardioverter—A device to apply electric shock to the chest to convert an abnormal heartbeat into a normal heartbeat.
Defibrillation—An electronic process which helps re-establish a normal heart rhythm.
Ventricles—The two large lower chambers of the heart which pump blood to the lungs and the rest of the human body.
Ventricular fibrillation—An arrhythmia in which the heart beats very fast but blood is not pumped out to the body that can become fatal if not corrected.
Ventricular tachycardia—An arrhythmia in which the heart rate is more than 100 beats per minute.
scheduled. The entire signal generator is replaced because the batteries are sealed within the case. The leads can often be left in place and reattached to the new generator. Batteries usually last about four to eight years.
Health care team roles
Electrophysiologists are specially trained cardiologists or thoracic surgeons who study and treat problems with the heart conduction system. They often implant the ICD system and oversee the programming or reprogramming of the device. They are assisted in the operating room by specially trained nurses, who can help with the testing of the ICD, and the anesthesiologist, who is responsible for numbing the area of the incision and keeping the patient comfortable. ICD manufacturers often send representatives to be present for the implantation and initial programming.
The maintenance of the ICD can be overseen by the electrophysiologist or cardiologist and associated staff, which can include specially trained cardiac medical assistants as well as nurses.
Training
The training for implantation of ICDs and their use occurs during medical training (medical or nursing school) and on the job. Physicians, nurses, and other allied health professionals can also receive training about ICDs as part of their continuing education courses. Continuing education concerning ICD tends to be in specific subject areas, such as the psychological effects of ICD firing, the interpretation of clinical trials in the area, or comparisons between appropriate and inappropriate firing rates.
BOOKS
Gersh, Bernard J., ed. Mayo Clinic Heart Book. New York: William Morrow and Company, Inc., 2000.
PERIODICALS
Moss, A. "Implantable Cardioverter-defibrillator Therapy: The Sickest Patients Benefit Most." Circulation. 101 (April 2000): 1638–1640.
Sears, Samuel F. Jr. et al. "Fear of Exertion Following ICD Storm: Considering ICD Shock and Learning History." Journal of Cardiopulmonary Rehabilitation. 21 (January/February 2001): 47.
ORGANIZATIONS
American Heart Association. National Center. 7272 Greenville Avenue, Dallas, TX, 75231-4596. (214) 373-6300. <http://www.americanheart.org>.
North American Society of Pacing and Electrophysiology. 6 Strathmore Road, Natick, MA, 01760-2499. (508) 647-0100. <http://www.naspe.org/index.html>.
