A 35-year-old woman presents to the emergency department with sudden onset palpitations and mild dyspnoea of 3 hours' duration. ECG confirms atrial fibrillation with a ventricular rate of 150 beats/min. She is haemodynamically stable, has no structural heart disease, and no history of arrhythmia. CHADS-65 criteria are negative. According to Canadian emergency medicine best practice for recent-onset atrial fibrillation, which is the most appropriate initial management strategy?