A 62-year-old man with a history of anterior wall myocardial infarction 6 months ago presents to the emergency department with palpitations and lightheadedness. His ECG shows a wide-complex tachycardia at 180 bpm. The structure marked **C** in the diagram—AV dissociation with P waves marching through the QRS complex at an independent rate—is noted. Which of the following is the most appropriate immediate management for this patient?