An implanted cardioverter defibrillator (ICD) is an electrical cardiac device that is used to treat very fast heart rates and can stop life-threatening abnormal heart rhythms. The ICD monitors the heart rhythm at all times and will notice any abnormal rhythm. Within seconds, it can try to get the heart back into a normal rhythm by delivering painless pacing impulses or a more noticeable shock to the heart. An ICD is implanted into the upper chest with lead wires to the heart, similar to a regular pacemaker.