A 68-year-old man is admitted to the coronary care unit with an acute myocardial infarction. His postinfarction course is marked by congestive hea failure and intermittent hypotension. On the fouh day in hospital, he develops severe midabdominal pain. On physical examination, blood pressure is 90/60 mm Hg and pulse is 110 beats per minute and regular; the abdomen is soft with mild generalized tenderness and distention. Bowel sounds are hypoactive; stool Hematest is positive. Which of the following is the most appropriate next step in this patient's management?