An implantable cardioverter-defibrillator (ICD), also called defibrillator, is a cardiac device implanted in order to detect and stop dangerously fast heart rhythms (ventricular tachycardia and ventricular fibrillation). It is a battery-powered device which continuously monitors the heart rhythm. When an abnormally fast ventricular heart rhythm is detected, the ICD delivers either painless short and fast electrical stimuli (often only 8 beats) or a painful electric shock in order to restore a normal heart rhythm.
A transvenous ICD has a pacemaker ability as well. This means that once an ICD is implanted, both slow and fast rhythms can be treated. Some ICDs may also have Cardiac Resynchronisation Therapy (CRT) functions if needed.
The size and shape of an ICD may vary depending on the manufacturer and model. ICDs are in general larger than pacemaker/ biventricular pacemaker, as the generator box has additionally defibrillating function and is larger.