A 62-year-old man has a regular broad-complex tachycardia at 190 beats per minute three hours after treatment for an acute myocardial infarction. He is conscious and has a palpable pulse, but reports chest pressure, looks pale and has a blood pressure of 74/42 mmHg. The QRS complexes have the same appearance beat to beat, and a reliable synchronisation marker is visible on the monitor. Which one of the following is the most appropriate immediate treatment?