A 59-year-old woman is admitted after vomiting for two days and taking sotalol for recurrent atrial fibrillation. She has several brief episodes of faintness associated with a rapidly changing broad-complex rhythm on the monitor; between episodes she is conscious and her pulse is regular at 84/min. The ECG shows a markedly prolonged QT interval, and the potassium concentration is 3.0 mmol/L (3.5–5.2). Which one of the following is the most appropriate immediate treatment for the recurrent rhythm?