Cardioversion is delivering an electrical shock to a person s heart to rapidly restore an abnormal heart rhythm back to normal. There are two types of cardioversion -- external and internal. EXTERNAL CARDIOVERSION An external cardioversion is often performed in an emergency situation. When a person s heart is beating abnormally and inadequately pumping blood to the body, an electrical shock is given with a defibrillator to revert the heart rate to a normal rhythm. An internal cardioversion is delivered by a device similar to a pacemaker, called an implantable cardioverter defibrillator (ICD). This device is used for treating ventricular (lower heart chamber) arrhythmias, such as ventricular tachyarrhythmias or fibrillation. The severity of these arrhythmias can cause sudden death because of the dangerously fast heart rate. and a Defibrillation is a process in which an electronic device gives an electric shock to the heart. This helps reestablish normal contraction rhythms in a heart having dangerous arrhythmia or in cardiac arrest. In recent years small portable defibrillators have become available. These are called automated external defibrillators or AEDs.