Participants received a 10-minute intravenous (IV) infusion of vernakalant (3 mg/kg) or an equivalent amount of normal saline (placebo), followed by a 15-minute observation period. If the participant was still in atrial fibrillation or atrial flutter, a second 10-minute IV infusion of vernakalant (2 mg/kg) or an equivalent amount of placebo was administered unless the participant experienced any dose-stopping criteria after the start of the first infusion. If a participant converted to sinus rhythm during the first or second infusion, that infusion was completed.