Last updated: 02/17/2009

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 (2.5 cm) in diameter and weighing about 0.5 oz (14 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 wand programmer, a device 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 is not 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 return to normal. Finally, many ICDs can also detect heartbeats that are too slow; they 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 fibrillations, 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 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 cardioverter-defibrillator 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.