Atrial fibrillation is the most common cardiac arrhythmia and is expected to affect about 30 million North Americans and Europeans by 2050. Atrial fibrillation is associated with increased cardiovascular morbidity and mortality, with stroke being an especially important and potentially devastating complication. Many studies have investigated the efficacy of different drugs in converting atrial fibrillation to sinus rhythm. There are numerous randomized controlled trials that have tested the efficacy of agents against placebo and some trials that directly compared the efficacy of two or more different drugs. The class III antiarrhythmic drug Ibutilde is approved for the acute termination of atrial fibrillation and atrial flutter of recent onset and has been shown to be superior to sotalol and equivalent to flecainide in this indication. Recently, the relatively atrial selective antiarrhythmic agent vernakalant has been approved by the European Commission for the rapid conversion of recent onset AF to sinus rhythm in adults. The investigators hypothesize that the period of time needed for cardioversion to sinus rhythm and the efficacy of cardioversion within 90 minutes is different between vernakalant and ibutilide in patients with recent-onset atrial fibrillation.

Inclusion Criteria:

• Symptoms of atrial fibrillation since no longer than 48 hours
• Age 18 - 90 years

Exclusion Criteria:

• Need for immediate electrical cardioversion due to hemodynamic instability (hypotension: systolic blood pressure < 100 mmHg, dyspnea, loss of consciousness, unstable angina)
• Moderate to severe heart failure (NYHA III/IV) and patients with previously documented left ventricular ejection fraction (LVEF) ≤ 35%
• History or signs of acute coronary syndromes (acute myocardial infarction, unstable angina) within the last 30 days
• Resting ventricular rate < 80 beats per minute without pace maker back-up
• QT interval of > 440 milliseconds
• Wolff-Parkinson-White (WPW) syndrome
• History of Torsade de pointes arrhythmias or other polymorphic ventricular tachycardias
• Signs of thyreotoxicosis
• Sick Sinus Syndrome or atrioventricular block greater than first degree
• Severe valvular heart disease, clinically meaningful hypertrophic obstructive cardiomyopathy, restrictive cardiomyopathy, or constrictive pericarditis
• Serious disorders of the hepatic, renal (Creatinine > 2.5mg/dl), pulmonary, gastrointestinal, hematologic or central nervous system and serious psychiatric disorders
• Abnormal serum electrolytes despite adequate therapy (especially potassium <3.5 mmol/l or > 5.5 mmol/l)
• Intravenous use of other Class I or III antiarrythmic drugs within 4 hours of study drug application
• Pregnancy (a β-HCG test will be performed in all female subjects apart from women > 50 years and with amenorrhea for at least 12 month (absence of other causes of amenorrhoea)
• Known hypersensitivity to study medication