The cardiac nurse urgently asks you to assess a 59-year-old woman on the ward who has developed sudden palpitations and is feeling faint. On arrival, she is awake but looks pale and sweaty. Her vital signs are: pulse 185 beats per minute, blood pressure 80/48 mmHg. Her ECG shows a wide-complex tachycardia consistent with ventricular tachycardia. What is the most appropriate immediate intervention?