What factors determine the chances of successful elective cardioversion from atrial fibrillation?

Elective cardioversion should only be undertaken when the precipitant (e.g. hypoxia, ischaemia, thyrotoxicosis, hypokalaemia and hypoglycaemia) has been treated and the patient is metabolically stable. With this proviso, the success of cardioversion depends not so much on the ability to restore sinus rhythm (success rates of 70–90% are usual), but rather on the capacity to sustain sinus rhythm.

Cardioversion of unselected patients will result in consistently high rates of relapse: at one year 40 to 80% of patients will have reverted to atrial fibrillation. Early cardioversion, particularly in those patients in whom a clear trigger of atrial fibrillation has been
effectively treated and in whom there is little or no evidence of concomitant cardiac disease, is associated with the best long term outcome. This may reflect the finding (well described in animal models) that sustained atrial fibrillation modifies atrial electrophysiology
so that, with time, there is a predisposition to continued and recurrent AF. If early cardioversion is not feasible, then the extent of underlying cardiac disease may be a more important determinant of long term outcome than the duration of AF.

The presence of severe structural cardiac disease is associated with a high relapse rate and sometimes an inability to achieve cardioversion, e.g. severe ventricular dysfunction, markedly enlarged atria and valvular disease.

Certain categories of patients justify specific mention:

• Obese patients may be especially resistant to cardioversion from the external route but not necessarily using electrodes positioned within the heart.
• A proportion of patients with paroxysmal atrial fibrillation will eventually develop chronic atrial fibrillation: for many this provides a paradoxical reprieve from their symptoms. If cardioverted their propensity to atrial fibrillation remains and they are likely to relapse.
• The prognosis of patients with structurally normal hearts who develop atrial fibrillation as a result of thyrotoxicosis is excellent: once the thyrotoxicosis has been treated a high proportion revert to sinus rhythm and the remainder are sensitive to cardioversion with a relatively low relapse.