Atrial fibrillation (AF) is a chaotic rhythm of the upper two chambers of the heart (the atria). It results in an irregular heart rhythm that is usually rapid (100-150 beats per minute). AF is often persistent and symptomatic, and may warrant attempts to convert the chaotic atrial rhythm back to normal “sinus” rhythm, thus slowing and regularizing the pulse. Electrical cardioversion involves a synchronized shock, applied with a standard defibrillator, that converts the rhythm of the heart back to normal sinus rhythm. This procedure takes 15 minutes and requires a general anesthesia that lasts five minutes. Recovery time is a couple of hours.

During AF, the atria are not contracting in a coordinated fashion. This allows blood to pool in the atria and potentially form clots. When the AF has persisted 48 hours or more, the risk of a blood clot must be reduced by treating with anti-clot (anticoagulant) drugs for three weeks prior to, and at least four weeks after, the shock. (The patient can avoid taking anticoagulants before the shock if the presence of a blood clot is ruled out with echocardiographic (ECG) imaging. Anticoagulant medication taken for at least four weeks afterward remains necessary to protect against development of a stroke.)

Direct-current cardioversion may not be needed if the patient is without symptoms; in such a case, simple rate control (keeping the heart rate about 80 beats per minute during persistent AF) and anticoagulation drugs may be appropriate. Cardioversion is not advised if the patient cannot receive anticoagulation medications, except in cases where the episode of AF is under 48 hours or in emergency situations.

If neither medication nor cardioversion effectively treats their AF, patients should consider a catheter-based procedure called AF ablation or a surgery called maze.

Conversion of AF to normal rhythm, at least transiently, is successful in 90 percent of patients. However, maintaining a normal rhythm depends on a number of factors, including underlying heart disease and heart-rhythm-stabilization therapy. AF usually recurs with time, and other treatments may be necessary. For unknown reasons, cardioversion does not correct the heart rhythm in a third of patients.

Risks of cardioversion are low as long as anticoagulation issues are handled appropriately. There is a small (less than 1 percent) risk of stroke. Risks of the electrical shock include mild muscle soreness and minor skin "burn" at the site of electrode placement.

If cardioversion is not performed, the AF can persist indefinitely. Permanent AF may be acceptable when symptoms are mild or nonexistent. Moreover, it is sometimes impossible to maintain a normal rhythm even with cardioversion. (AF does not lead to—and should not be confused with—the potentially lethal heart rhythm called ventricular fibrillation.)

Cardioversion is rarely an emergency, and its urgency depends partly on the patient’s tolerance of the arrhythmia. Usually there is ample time to optimize anticoagulation and schedule the procedure. On the other hand, the sooner that normal rhythm is restored, the sooner symptoms can be reduced. Furthermore, animal studies have shown that "AF begets AF,” meaning that long-standing AF may be more difficult to control.