A 29-year-old man presents with palpitations and light-headedness. His pulse is irregular at 190 beats per minute and the blood pressure is 126/74 mmHg. The ECG shows an irregular broad-complex tachycardia with beat-to-beat variation in QRS width; a previous ECG in sinus rhythm showed a short PR interval and slurred initial QRS upstroke. Which one of the following is the most appropriate immediate treatment?