A 62-year-old man with hypertension and hyperlipidemia presents to the ED with 45 minutes of substernal chest pressure at rest radiating to the left arm. He is diaphoretic and nauseated. He had similar exertional chest pain over the last week, but this episode is more severe and occurred at rest. ECG shows ST-segment depressions in leads V4–V6. Initial troponin is mildly elevated. Oxygen saturation on room air is 96%. Blood pressure is 148/88 mm Hg and heart rate is 92/min. Which of the following medications should be started early to decrease heart rate and reduce the risk of malignant arrhythmias in this patient’s condition?