A 54-year-old woman has an intermittent jaw, and shoulder pain for the past week. The symptoms occur with activity and are relieved by rest. On physical examination, BP 150/68 mmHg and HR 90/min. There is no jugular venous distention or cardiac murmurs. Lung fields are clear, and extremities show no edema. The troponin I level is elevated. Electrocardiogram shows 1.0-mm ST-segment depression in leads V1 through V4 with T-wave inversions. She is given aspirin, intravenous nitroglycerin. low-molecular-weight heparin, clopidogrel, and atorvastatin. Which of the following is the most appropriate additional immediate treatment for this patient?