A 64-year-old man with a history of coronary artery disease presents to the emergency department with sudden onset palpitations and lightheadedness. He is awake and speaking in full sentences. Vital signs: T 37.0°C (98.6°F), BP 128/76 mm Hg, HR 176/min, RR 18/min, SpO2 98% on room air. ECG shows a regular wide-complex tachycardia at 176/min with no clear P waves. He has no chest pain, no dyspnea, and no signs of shock. Which of the following is the most appropriate next step in management?