A 52-year-old man with a history of hypertension and left ventricular hypertrophy presents to the emergency department with palpitations and dyspnea. His ECG shows a regular narrow-complex tachycardia at 180 bpm. Carotid sinus massage fails to terminate the arrhythmia. Intravenous adenosine 6 mg is administered but produces only transient slowing of the heart rate without termination. What is the most appropriate next step in management?