An Implantable Cardioverter Defibrillator (ICD) can send two types of electrical impulses to keep the heart in rhythm. Low-energy pacing delivers small, painless electrical impulses to the heart like a pacemaker or series of impulses (over-drive pacing) to correct certain abnormal fast rhythms (pathological tachycardias). High-energy defibrillation (shock) delivers a strong, brief shock when the heart goes into a life-threatening rhythm such as ventricular fibrillation (VF) or ventricular tachyc