A. Defined as a compromise of intestinal arterial or venous perfusion that may occur acutely or over the course of several months in the setting of chronic ischemia.

B. Decreased blood flow and oxygen to the bowel leads to ischemia, acidosis, leukocytosis, and the eventual development of sepsis and multiple-organ failure.

C. Mortality is in excess of 60% to 80% for acute arterial occlusion, usually due to a delay in diagnosis and the rapid progression of intestinal ischemia to tissue necrosis.

D. Risk factors include advanced age, atrial arrhythmias, history of congestive heart failure or recent myocardial infarct, valvular heart disease, previous cardiac or vascular surgeries, and atherosclerotic disease.

E. Early diagnosis and prompt revascularization are the key factors to a favorable outcome.

A. Acute mesenteric ischemia is usually divided into three categories, with occlusive disease or thromboembolism accounting for 80% of all cases.

1. Mesenteric arterial occlusion from embolism, thrombosis, dissection, vasculitis, or stent placement.

2. Mesenteric venous occlusion secondary to bowel obstruction, thrombosis, or phlebitis.

3. Nonocclusive ischemia as a result of cardiopulmonary bypass, various shock states, and vasoconstrictive medications (i.e., α-adrenergic, digitalis, or vasopressin).

B. Mesenteric arterial embolism or thrombosis involves the superior mesenteric artery (SMA) in 85% of cases. Most individuals have preocclusive atherosclerotic disease in other locations, including the visceral vessels.

C. Emboli from a cardiac source typically lodge at the first branch point of the SMA, the inferior pancreatoduodenal artery. Arterioarterial emboli tend to be smaller and lodge in the more distal mesenteric circulation.

D. Thrombosis usually develops at or near the origin of vessels or areas of concurrent atherosclerotic stenoses. In contrast with embolic occlusion, acute ischemia from thrombosis is usually a late complication of atherosclerotic disease and develops after two of the three mesenteric arteries are completely occluded.

E. Mesenteric venous thrombosis (MVT) is a rare disorder resulting from a variety of acquired and inherited hypercoagulable states. MVT usually involves the superior mesenteric and splenic veins and, less commonly, the inferior mesenteric and portal veins.

A. Acute arterial obstruction will rarely present with acute mesenteric ischemia at normotensive pressures due to the excellent collateral circulation of the gut.

B. Ischemic times as short as 3 hours can produce significant irreversible damage to the intestinal mucosa.

C. Reduction of blood flow initiates a cascade of events, including an acute inflammatory response with the release of cytokines and platelet-activating factor, resulting in the breakdown of the mucosal barrier with bacterial translocation and the ultimate progression of sepsis, multisystem-organ failure, and death.

D. Nonocclusive ischemia is the result of mesenteric vasospasm, usually in the distribution of the SMA.

1. Homeostatic mechanisms attempt to maintain cardiac and cerebral perfusion at the expense of visceral and peripheral organs.