A 68-year-old woman presents to the emergency department with sudden-onset palpitations and mild shortness of breath starting 2 hours ago. ECG shows atrial fibrillation with a ventricular rate of 150 bpm. She is haemodynamically stable with blood pressure 130/80 mmHg, no chest pain and normal consciousness. This is her first documented episode and she has no history of heart disease. What is the most appropriate initial management strategy?