The purpose of this study is to evaluate the safety and efficacy of vernakalant injection in subjects with recent onset (AF > 3 hours to <= 7 days), symptomatic atrial fibrillation and no evidence or history of congestive heart failure.

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.

Inclusion Criteria:

• Females must be not pregnant or nursing and if pre-menopausal, must be using an effective form of birth control from time of screening until 30 days post study treatment
• Subject must have recent onset (> 3 hours to <= 7 days) symptomatic AF to be best managed by acute conversion to SR
• Subject must have adequate anticoagulant therapy
• Subject must have systolic blood pressure (SBP) above 90 mmHg and less than 160 mmHg and diastolic blood pressure (DBP) less than 95 mmHg at screening and baseline
• Subject must have a body weight between 45 and 136 kg, inclusive (99 and 300 lbs)

Exclusion Criteria:

• Subject has a history of heart failure or documentation of left ventricular dysfunction
• Subject has known or suspected prolonged QT or uncorrected QT interval of > 0.440 sec
• Subject has symptomatic bradycardia or ventricular rate less than 50 bpm at Screening, unless controlled by a pacemaker
• Subject has bradycardia (heart rate less than 50 bpm) or hypotension (SBP less that 90 mmHg) after receiving a loading, bolus dose, or sustained infusion of any rate control medication during Screening
• Subject has a QRS interval > 0.14 sec., unless subject has a pacemaker
• Subject had a myocardial infarction (MI), acute coronary syndrome, cardiac surgery (including percutaneous transluminal coronary angioplasty (PTCA) or stent placement), within 30 days prior to enrollment or subject has evidence of new ischemic changes on Screening 12-lead ECG
• Subject has troponin I or T levels beyond the upper limit of normal for the local lab
• Subject has significant valvular stenosis, hypertrophic obstructive cardiomyopathy, restrictive cardiomyopathy or constrictive pericarditis
• Subject has failed electrical cardioversion for AF at anytime
• Subject has failed pharmacologic conversion with an intravenous Class I or Class III antiarrhythmic drug for this episode of AF
• Subject has any known reversible causes of AF such as alcohol intoxication, pulmonary embolism, hyperthyroidism, acute pericarditis or hypoxemia
• Subject has uncorrected electrolyte imbalance
• Subject has clinical evidence of digoxin toxicity
• Subject has a history of clinically significant illness (e.g. neurological, gastrointestinal, renal, hepatic, pulmonary, metabolic, endocrine, hematological, or psychiatric), medical condition or laboratory abnormality within 4 weeks prior to Screening