Similar in structure to a pacemaker, an ICD has three main components: a generator, leads, and an electrode.
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.
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.