The difference between cardioversion and defibrillation is based on whether the synchronization mode on the cardioverter-defibrillator is turned on or off. In cardioversion, the electrical shock is delivered in synchronization (“turned on”) with the QRS complex and can result in the termination of a variety of arrhythmias. Atrial fibrillation is one of the most frequently cardioverted rhythms and usually requires at least 300 J of energy on traditional defibrillators and between 75 J and 200 J on the newer biphasic defibrillators. Urgent cardioversion is undertaken in patients with hemodynamic compromise and in stable patients after anticoagulation unless the arrhythmia has lasted less than 48 hours. Direct-current cardioversion is also used for atrial flutter and most practitioners initially try 50 to 100 J with a good rate of success. Supraventricular tachycardias and ventricular tachycardias may also be cardioverted but this is done far less frequently secondary to success of intravenous antiarrhythmics and unrecognizable R waves.