A 58-year-old man with a history of anterior wall myocardial infarction 2 years ago presents to the emergency department with palpitations and lightheadedness. His blood pressure is 110/70 mmHg and he is alert. ECG shows a regular tachycardia at 180 bpm. The structure marked **A** in the diagram—a wide QRS complex (≥120 ms) with regular rhythm—is observed. On closer inspection, independent P waves are visible at a slower rate than the QRS complexes, and occasional narrow QRS complexes are seen interspersed among the wide beats. Given these findings, which of the following is the most appropriate immediate management?