A 68-year-old male with a 12-year history of coronary artery disease (CAD) and angina pectoris was admitted due to severe retrosternal pain lasting 8 hours after emotional agitation. No history of hypertension (HTN). On admission, blood pressure (BP) was 150/90 mmHg, diagnosed with acute inferior wall myocardial infarction. The patient developed sinus bradycardia with a heart rate of 40 bpm. The most appropriate therapeutic drug is: