A 32-year-old woman with no significant past medical history presents with episodes of palpitations, sweating and tremor over several months. Examination reveals a fine tremor and a small diffuse goitre. TSH is suppressed and free T4 is elevated. ECG shows atrial fibrillation with a ventricular rate of 130 beats/min. What is the most appropriate initial pharmacological management of her atrial fibrillation?