a 78-year-old man is brought to the emergency department with a 12-hour history of abdominal pain, diarrhea, and vomiting. he has a history of atrial fibrillation and was previously treated for congestive heart failure with digoxin. physical examination reveals a distended abdomen with significant guarding. rectal examination reveals guaiac positive stool in the vault. white blood cell count is 24, 000/ml. abdominal x-ray reveals edema of the bowel wall. what is the most appropriate treatment for this patient?