An implanted cardiac defibrillator (ICD) is a small device that is used to help control a heart rhythm that is too fast (ventricular tachycardia or ventricular fibrillation).
An ICD senses the heart rhythm. If the ICD senses ventricular tachycardia or ventricular fibrillation, it will deliver a small electrical shock to change the heartbeat back to a normal rhythm.