A 74-year-old woman with hypertension and diabetes presents with palpitations and breathlessness. ECG shows atrial fibrillation with a ventricular rate of 140 beats per minute. She has no chest pain, blood pressure is 130/80 mmHg and there are no signs of heart failure. The arrhythmia has been present for at least 48 hours. What is the most appropriate initial rate-control therapy?