A 65-year-old woman with chronic stable angina is on isosorbide dinitrate 10 mg three times daily and metoprolol 100 mg twice daily. She develops frequent episodes of angina (5–6 per week) and reports severe headaches with nitrate use. She is intolerant to calcium channel blockers due to ankle edema. Her blood pressure is 135/85 mmHg and heart rate is 62 bpm. What is the most appropriate next step in management?