A cardioversion is a procedure which is used to convert an abnormal heart rhythm (usually atrial fibrillation or atrial flutter) back to a normal heart rhythm (normal sinus rhythm).  It is usually performed when the arrhythmia is not expected to stop on its own.  There are two methods of cardioversion available:  chemical and electrical. If a chemical cardioversion is to be attempted first, the intravenous drug (usually procainamide or ibutilide) is infused via an IV and the rhythm is continuously monitored.  The success rate of a chemical cardioversion depends on many factors - one of the most important being the duration that the patient is in the arrhythmia.  For patients who have been in the arrhythmia for less than 48 hours, the conversion rates can be as high as 50-60%.  The average conversion rate that is typically quoted is ~ 30%.  If the chemical cardioversion is successful, the patient is typically monitored for at least a few hours, if not overnight, to watch the rhythm. If the chemical cardioversion fails to restore a normal rhythm or, if the physician feels that a chemical cardioversion is unwarranted, an electrical cardioversion will be performed.  Special pads are placed on the chest and back and attached to a defibrillator.  The patient is sedated with medication via the intravenous line.  The defibrillator then delivers an electrical shock via the pads through the heart muscle, to restore a normal rhythm.  The initial success rates of electrical cardioversion are very high (~95%).