A 65-year-old man with chronic atrial fibrillation and a history of myocardial infarction (ejection fraction 35%) presents with recurrent episodes of ventricular tachycardia (VT) despite being on metoprolol 100 mg daily. His serum potassium is 3.8 mEq/L, magnesium is 2.0 mg/dL, and QTc interval is 420 ms. He is hemodynamically stable during the VT episode. What is the most appropriate next step in management?