A 70-year-old woman is brought to the emergency department 20 minutes after an episode of light-headedness while jogging. She has a 2-year history of hypertension, treated with losartan. Her pulse is 38/min and blood pressure is 80/50 mmHg. Prominent 'A' waves are seen in the jugular venous pulse. The lungs are clear to auscultation. No murmurs are heard. An ECG shows dissociation of the P waves and QRS complexes. The QRS complexes are wide. The atrial rate is 88/min and the ventricular rate is 40/min. Which of the following is the most appropriate next step in management?