Atrial fibrillation (abnormal rhythm in the upper chamber of the heart) is a common supraventricular arrhythmia (abnormal heart rhythm) for which antiarrhythmic therapy is often prescribed. The primary goals of therapy are to maintain sinus rhythm (normal heart rhythm) and to reduce the occurrence of episodes of atrial fibrillation. Azimilide may have an effect on increasing the time to first recurrence of symptomatic atrial fibrillation or atrial flutter and symptomatic paroxysmal supraventricular tachycardia (other types of abnormal heart rhythms).

This double-blind, placebo-controlled study is designed to evaluate the efficacy and safety of azimilide compared with placebo in maintaining sinus rhythm in patients who require cardioversion (electric shock to correct heart rhythm) to reduce atrial fibrillation. Once this phase of the study is completed, a second phase with a different study design will be conducted. The second phase is an open-label follow-up phase to the study. This follow-up phase will continue to evaluate the long-term efficacy and safety of a daily oral dose of azimilide in patients who complete the double-blind, placebo-controlled phase of this study.

Inclusion criteria:

• Documented (12-lead ECG) history of symptomatic atrial fibrillation occurring between 48 hours and 6 months before screening
• Require the procedure of cardioversion (electric shock to correct heart rhythm)
• In the investigator's opinion, be likely to maintain sinus rhythm after cardioversion.
• Be anticoagulated according to the recommendations of the Study Group on Atrial Fibrillation of the European Society of Cardiology guidelines.

Exclusion criteria:

• Previously unsuccessful electrical cardioversions
• Failed to respond to any Class III antiarrhythmic drugs
• Qualifying arrhythmia due to acute reversible illness, acute myocardial infarction, and/or cardiac or thoracic surgery within one month prior to randomization