The most likely diagnosis is acute mesenteric ischemia. Acute mesenteric ischemia is caused by inadequate blood flow to all or part of the small bowel. Causes include superior mesenteric artery (SMA) embolism, SMA thrombosis, nonocclusive mesenteric ischemia (usually due to splanchnic vasoconstriction), mesenteric venous thrombosis, and focal segmental ischemia. SMA embolism from the left atrium or ventricular mural thrombi is the most common cause. The classic presentation is acute onset of severe abdominal pain; the abdomen is typically soft and less tender than expected based on the patient's symptoms (pain out of proportion to the examination).This patient has many findings that should heighten the suspicion for acute mesenteric ischemia including chronic atrial fibrillation, onset of periumbilical abdominal pain, emesis, hypotension, abdominal pain that is out of proportion to the abdominal examination, and a forceful bowel evacuation. An elevated leukocyte count, metabolic acidosis, and elevated plasma lactate level are common, but their absence should not exclude the diagnosis. Plain abdominal films may be normal early in the course of disease. A high degree of clinical suspicion is necessary to diagnose mesenteric ischemia in time for potentially life-saving interventions to be effective. Immediate surgical consultation and angiography are appropriate for this patient.

Acute pancreatitis is almost always associated with the sudden onset of pain. The pain is generally located in the epigastrium and radiates to the back. Pain is often accompanied by fever, nausea, and repeated vomiting. Although an elevated serum amylase level is usually from a pancreatic source, hyperamylasemia can be seen in a variety of other conditions including intestinal ischemia. This patient's presentation is not typical for acute pancreatitis.

Campylobacter enteritis could account for this patient's abdominal pain, elevated leukocyte count, and hypotension; however, this patient's absence of bloody stools and diarrhea, along with the presence of persistent hypotension, make this diagnosis less likely.

Colonic ischemia comprises 75% of all cases of intestinal ischemia. Typical symptoms are the acute onset of mild, crampy abdominal pain with tenderness on examination over the affected region of colon. Bleeding often occurs within a few days of pain onset. The location of this patient's pain and early vomiting are not compatible with colonic ischemia.