A 75-year-old woman without a history of liver disease or anticoagulant use is admitted to the intensive care unit with lower gastrointestinal bleeding. Findings of computed tomography angiography of the abdomen and pelvis and upper endoscopy are negative. Colonoscopy shows blood throughout the colon, but was unable to localize a definite source. A colonoscopy report from 3 years ago shows a 5-mm ascending colon tubular adenoma and sigmoid diverticulosis. In the past 48 hours, she has required transfusion of 6 units of packed red blood cells for ongoing bleeding. Digital examination and anoscopy show no significant anorectal pathology. The patient is hypotensive, requiring 2 vasopressors, and with tachycardia to 115 beats/min. What is the best next step in management?