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Implantable Cardiac Defibrillator (cont.)

How are ICDs designed?

An ICD consists of one or more leads (conducting wires insulated with silicone or polyurethane) and a defibrillator unit. The defibrillator unit is a small titanium case containing a microchip computer, a capacitor, and a battery.

The leads carry electrical signals between the heart and the defibrillator unit. One end of a lead is placed on the inner wall of the heart while the other end is attached to the defibrillator unit. The leads help the defibrillator unit monitor the natural heart rhythm. The leads also deliver electrical shock(s) from the defibrillator unit to the heart when tachycardias occur.

The microchip computer runs the defibrillator, monitors the natural heart rhythm, instructs the capacitor to send electrical shock(s) when tachycardias occur, determines the strength of the shock(s) sent, and also keeps a record of the heart rhythms as well as the shock(s) sent by the defibrillator.

ICDs have programmable features that allow the doctor to change the cutoff heart rate for activating the defibrillator. Tachycardias with rates higher than the cutoff heart rate activate the firing of shocks by the defibrillator. The doctor can also adjust the strength (amount of energy delivered) of each shock, and the number of shocks delivered with each tachycardia episode.

Most defibrillators now have built-in pacemakers as well. The newer defibrillators can have very sophisticated pacing devices equipped with the ability to pace both the atrium and the ventricle (dual chamber pacers). Cardioverter defibrillators have rapid pacing capabilities. Rapid pacing can sometimes convert a tachycardia to normal rhythm without administering electric shock(s).

The electric pulses and shocks delivered by the ICDs are of such low energy that they do not harm the patient or family members in physical contact with the patient.

How are ICDs implanted?

Implantation of an ICD is similar to that of a permanent pacemaker. The procedure, which lasts 1-2 hours, is considered minor in that it does not involve major heart surgery. Patients are typically sedated during the procedure. A local anesthetic is injected under the skin over the area where the ICD will be placed, usually in the right or left upper chest near the collarbone. The lead is then inserted into a vein located in the upper chest near the collarbone. The tip of the lead is placed on the inner wall of the heart with the visual guidance of x-rays. If there is more than one lead, the process is repeated. The other end of the lead (or leads) is connected to the defibrillator unit, which is then inserted under the skin at the incision site. Because there are no nerve endings inside the blood vessels and the heart, the patient usually does not feel the placement of the lead(s).

Heavy sedation is used during the procedure when the defibrillator is tested for proper functioning. Testing an ICD involves inducing a rapid heart rhythm and allowing the defibrillator to detect the abnormal rhythm and then terminate it with a shock (just as the device would be expected to operate in a real-life tachycardia episode).

Medically Reviewed by a Doctor on 1/31/2014

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