A 62-year-old woman from Mumbai with a 5-year history of stable angina is currently on atenolol 50 mg daily and isosorbide dinitrate 10 mg thrice daily. She reports that her angina is well-controlled, occurring only once or twice per week with exertion. However, she now presents with a 2-week history of persistent headache, dizziness, and mild syncope on standing. BP sitting: 110/70 mmHg; BP standing: 88/55 mmHg. HR sitting: 58/min; HR standing: 62/min (blunted response). She denies chest pain currently. Which of the following is the most appropriate next step in management?