Key Inclusion Criteria:

   - Must weigh at least 40 kg

   - Possess the β1389 Arg/Arg genotype

   - Left Ventricular Ejection Fraction (LVEF) < 0.50 assessed within 12 months prior to
   Screening

   - At least one episode of symptomatic paroxysmal or persistent AF within 180 days of
   Screening

   - Clinically appropriate for electrical cardioversion (ECV) if AF/AFL is present after
   study drug initiation

   - Receiving appropriate anticoagulation therapy prior to Randomization

Key Exclusion Criteria:

   - NYHA Class IV symptoms at the time of Randomization

   - Significant fluid overload at Randomization

   - Permanent AF at Screening

   - More than two previous ECV within 6 months of Randomization or if the most recent ECV
   failed to produce SR

   - Presence of an LVAD, or likely to requirement LVAD placement within 6 months of
   Randomization

   - History of a successful atrioventricular (AV) node ablation

   - History of an AF/AFL ablation within 30 days of Randomization

   - Evidence of an appropriate firing of an implanted cardioverter-defibrillator (ICD)
   device for ventricular tachycardia (VT) or ventricular fibrillation (VF) within 90
   days of Randomization