A 75-year-old male presents to the emergency department with a several-hour history of back pain in the interscapular region. His medical history includes a previous myocardial infarction (MI) several years ago, a history of cigarette smoking until the time of the MI, and hypertension that is well controlled with hydrochlorothiazide and lisinopril (Prinivil, Zestril). The patient appears anxious, but all pulses are intact. His blood pressure is 170/110 mm Hg and his pulse rate is 110 beats/min. An EKG shows evidence of an old inferior wall MI but no acute changes. A chest radiograph shows a widened mediastinum and a normal aortic arch, and CT of the chest shows a dissecting aneurysm of the descending aorta that is distal to the proximal abdominal aorta but does not involve the renal arteries. Which one of the following would be the most appropriate next step in the management of this patient?