Vernakalant (Oral) Prevention of Atrial Fibrillation Recurrence Post-Conversion Study

Inclusion Criteria: * Comprehend and sign a written informed consent form, (per local and national regulations, as applicable) * Be 18 to 85 years of age * Women must not be pregnant, be non-nursing and if pre-menopausal, must be using an effective form of birth control from time of screening until 3 months after the last dose of medication. Methods of birth control considered to be effective may include hormonal contraception (the pill), an intrauterine device (IUD), condoms in combination with a spermicidal cream, total abstinence or sterilisation. Men should be advised not to conceive a child and are advised to use an effective form of birth control from admission until 3 months after the last dose of study medication * Have symptomatic AF that has been sustained for greater than 72 hours and less than 6 months duration and is clinically indicated for cardioversion; * Have adequate anticoagulant therapy for cardioversion in accordance with standard of practice as recommended by ACC/AHA/ESC guidelines (Fuster V. et al, 2006); * Be haemodynamically stable (100 mmHg \< systolic blood pressure \< 190 mmHg) at screening and on Day 1 before dosing (while taking rate control drugs, if required). After resting supine for 3 minutes, blood pressures should be measured 3 times in 5 minutes with at least 1 minute between assessments; * Have a body weight between 45 and 113 kg (99 and 250 lbs). Exclusion Criteria: * Have known prolonged QT syndrome or QTcB interval of \>0.500 sec as measured at screening on a 12 lead ECG; familial long QT syndrome; previous Torsades de Pointes; ventricular fibrillation; or sustained ventricular tachycardia (VT). * Have a QRS \>0.140 sec; * Documented previous episodes of second or third-degree atrioventricular block; * Have clinically significant persistent bradycardia with ventricular rate below 50 beats/min, sick-sinus syndrome or pacemaker; * Have clinically significant moderate or severe aortic valvular stenosis (gradient \>25 mmHg), hypertrophic obstructive cardiomyopathy, restrictive cardiomyopathy or constrictive pericarditis; * Have Class III or Class IV congestive heart failure at screening or admission, or have been hospitalized for heart failure in the previous 6 months; * Have a myocardial infarction (MI), cardiac surgery, angioplasty, unstable angina or acute coronary syndrome within 30 days prior to entry into the study; h) Have serious pulmonary, hepatic, metabolic, renal (serum creatinine \> 2.0 mg/dl), gastrointestinal, central nervous system (CNS) or psychiatric disease, end-stage disease states, or any other disease that could interfere with the conduct or validity of the study or compromise subject safety; * Have known concurrent temporary secondary causes of AF such as alcohol intoxication, pulmonary embolism, hyperthyroidism, pneumonia, hypoxemia (oxygen saturation \< 90% on room air), acute pericarditis, or myocarditis; * Potassium (K+) \<3.5 mmol/L or \>5.5 mmol/L or magnesium (Mg2+) below the lower limit of normal (Mg2+\< 0.65 mmol/L in subjects 65 years or younger and \<0.80 mmol/L in subjects 66 years or older). (Both K+ and Mg2+ should be corrected prior to dosing); * Have clinical evidence of digoxin toxicity; * Have received an oral Class I or Class III antiarrhythmic agent (including sotalol) within 3 days of randomisation or oral amiodarone within 4 weeks, or have received intravenous Class I or Class III antiarrhythmic agent or i.v. amiodarone within 24 hours prior to start of dosing; * Have any other surgical or medical condition that, in the judgment of the clinical Investigator might warrant exclusion or be contraindicated for safety reasons; * Be concurrently participating in another drug study or have received an investigational drug within 30 days prior to screening; * Be unable to communicate well with the Investigator and to comply with the requirements of the entire study;