This study will compare vernakalant therapy to amiodarone therapy in the acute management of recent onset atrial fibrillation.

Inclusion Criteria:

• Symptomatic AF (duration of current episode is ≤7 days), which is hemodynamically stable, and participant has no other condition at the time of screening that may result in acute hospitalization
• If female and of reproductive potential, the patient agrees to remain abstinent or use 2 acceptable methods of birth control from time of screening until 30-day follow-up.
• Weigh at least 45 kg
• Receiving adequate anticoagulant therapy

Exclusion Criteria:

• Hypersensitivity to the vernakalant or amiodarone or to citric acid, sodium chloride, sodium hydroxide, or iodine
• Severe aortic stenosis
• Systolic blood pressure <100 mmHg
• New York Heart Association (NYHA) Class III or IV heart failure
• Severe bradycardia, sinus node dysfunction, or second and third degree heart block in the absence of a pacemaker
• Use of IV rhythm control anti-arrhythmics (Class I and III) within 4 hours of study drug administration
• Acute coronary syndrome (including myocardial infarction) within previous 30 days
• History of thyroid dysfunction
• Severe acute respiratory failure or cardiovascular collapse
• Participating in another drug study or has received an investigational drug within 30 days prior to enrollment
• Pregnant or breast-feeding, or expecting to conceive from time of screening until 30-day follow-up