Electrical cardioversion is used to manage patients with cardiac arrhythmias that involve re-entrant circuits, such as ventricular tachycardia or atrial fibrillation. In an emergent setting any patient with a ventricular rate greater than 150 who is unstable (i.e., chest pain, hypotension, unresponsive) should be treated with synchronized cardioversion. Cardioversion may also be performed on stable patients who have an arrhythmia that is refractory to pharmacologic intervention.

Cardioversion theoretically stops the arrhythmia by depolarizing the re-entrant circuit and making it refractory to propagation. Cardioversion delivers energy that is synchronous with the early part of the QRS complex. If the energy is delivered in an asynchronous manner, it can induce ventricular fibrillation. This occurs when the energy is delivered during the early phase of repolarization. When the cardioverter is placed in synchronous mode, it automatically discharges a current that coincides with a large R or S wave, thereby avoiding the period of repolarization when ventricular fibrillation can occur. In synchronous mode there is always a delay in energy delivery while the cardioverter searches for the R or S wave. It is important to switch the mode to asynchronous delivery if the patient develops ventricular fibrillation.