The Full form of ICD is Implantable Cardioverter-Defibrillator. An Implantable Cardioverter-Defibrillator (ICD) is a device that detects any abnormal, life-threatening or rapid heartbeat. This abnormal heartbeat is called an arrhythmia. If it occurs in a person, the ICD quickly sends an electrical shock to the heart. The shock changes the rhythm back to normal. This is called defibrillation. An ICD is made of some parts which are pulse generator, electrodes, pacemaker or subcuntaneous ICD. The pulse generator is usually about the size of a large pocket watch. It contains a battery and electrical circuits that read the electrical activity of heart. The electrodes are wires that go through veins to heart. They connect heart to the rest of the device. Your ICD may have 1, 2, or 3 electrodes, most ICDs have a built-in pacemaker. The heart may need pacing if it is beating too slowly or too fast, or if a person has had a shock from the ICD. There is a special kind of ICD called a subcutaneous ICD. This device has a lead that is placed in the tissue to the left of the breastbone rather than in the heart.