Cardioversion is a medical procedure by which an abnormally fast heart rate (tachycardia) or cardiac arrhythmia is converted to a normal rhythm using electricity or drugs. Synchronized electrical cardioversion uses a therapeutic dose of electric current to the heart at a specific moment in the cardiac cycle. (Defibrillation uses a therapeutic dose of electric current to the heart at a random moment in the cardiac cycle, and is the most effective resuscitation measure for cardiac arrest associated with ventricular fibrillation and pulseless ventricular tachycardia.) Pharmacologic cardioversion, also called chemical cardioversion, uses antiarrhythmia medication instead of an electrical shock.