A 58-year-old man with a history of anterior wall myocardial infarction 3 months ago presents to the emergency department with palpitations and lightheadedness. His blood pressure is 85/50 mmHg, heart rate 180 bpm, and respiratory rate 22/min. Physical examination reveals an unresponsive patient. A 12-lead ECG shows a wide complex tachycardia with a rate of 180 bpm; the QRS duration is 160 ms. Cardiac biomarkers are negative. Chest X-ray shows mild cardiomegaly. What is the most appropriate immediate management?