Clinical meaning
Acute mesenteric ischemia (AMI) is a vascular emergency resulting from insufficient blood flow to the intestines, causing rapid progression from mucosal injury to transmural necrosis, perforation, and death if not recognized and treated promptly. Mortality rates remain 60-80% due to delayed diagnosis.
The intestinal blood supply comes from three major mesenteric arteries: the celiac artery (foregut — stomach to proximal duodenum, liver, spleen, pancreas), the superior mesenteric artery (SMA, midgut — distal duodenum to proximal two-thirds of transverse colon), and the inferior mesenteric artery (IMA, hindgut — distal transverse colon to proximal rectum). The SMA supplies the largest territory and is the vessel most commonly involved in acute mesenteric ischemia.
Four etiologic categories: (1) SMA embolism (40-50% of cases) — most commonly from left atrial thrombus in atrial fibrillation, left ventricular thrombus post-MI, or valvular vegetations; emboli typically lodge 3-10 cm distal to the SMA origin (beyond the middle colic artery), sparing the proximal jejunum. (2) SMA thrombosis (20-30%) — occurs at sites of pre-existing atherosclerotic stenosis, usually at the SMA ostium; patients often have a history of chronic mesenteric ischemia (intestinal angina — postprandial abdominal pain, food fear, weight loss). (3) Non-occlusive mesenteric ischemia (NOMI, 20%) — from splanchnic vasoconstriction in low-flow states: cardiogenic shock, sepsis, hemorrhagic shock, high-dose vasopressor therapy (especially norepinephrine and vasopressin); no arterial occlusion is present. (4) Mesenteric venous thrombosis (MVT, 5-10%) — from hypercoagulable states (protein C/S deficiency, Factor V Leiden, antiphospholipid syndrome), portal hypertension, intra-abdominal inflammation, or oral contraceptive use.