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 CRT functions if needed.