A 28-year-old primigravida at 16 weeks of gestation presents to the antenatal clinic with a 3-month history of poorly controlled hypertension (BP 160/105 mmHg). She was started on enalapril 10 mg daily 6 months ago for essential hypertension. On examination, she appears well; fetal heart rate is 145 bpm. Urine dipstick shows no proteinuria. Which of the following is the most appropriate next step in her antihypertensive management?