An ICD or implantable cardioverter-defibrillator is a small device that looks similar to a pacemaker and may be required if a person’s heart is found to be at risk of going into a dangerous heart rhythm, such as ventricular tachycardia or ventricular fibrillation. ICDs work by sensing and stopping these dangerous arrhythmias by producing extra beats or electrical shocks to restore the heart to a normal rhythm.