An implantable cardioverter-defibrillator (ICD) - a pager-sized device that's implanted into your chest - may reduce your risk of dying if the lower chambers of your heart (ventricles) go into a dangerous rhythm and stop beating effectively (cardiac arrest). You may need an implantable cardioverter-defibrillator if you have a dangerously fast heartbeat (ventricular tachycardia) or a chaotic heartbeat that makes it so your heart can't supply enough blood to the rest of your body (ventricular fibrillation).Implantable cardioverter-defibrillators work by detecting and stopping abnormal heartbeats (arrhythmias). An implantable cardioverter-defibrillator continuously monitors your heartbeat and delivers extra beats or electrical shocks to restore a normal heart rhythm when necessary. An ICD differs from a pacemaker - another implantable device sometimes used to treat less dangerous heart rhythms, such as those that occur in the upper chambers of your heart (atria).