A 67-year-old man with a previous anterior myocardial infarction develops palpitations while being monitored in hospital. He is alert, has a blood pressure of 118/76 mmHg and a regular pulse of 174 beats per minute. The ECG shows a broad-complex rhythm with atrioventricular dissociation, occasional capture beats and a uniform QRS morphology. Which one of the following is the most appropriate treatment?