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MESENTERIC ISCHEMIA

ESSENTIAL CONCEPTS

  • Acute mesenteric ischemia (AMI) is a medical and surgical emergency. Delay in diagnosis is associated with high mortality.

  • Patients often present with abdominal pain out of proportion to physical examination findings.

  • Clinical suspicion of AMI necessitates early radiologic evaluation (computed tomographic [CT] angiography, conventional angiography) or exploratory surgery in patients with peritoneal signs.

  • Chronic intestinal ischemia is a clinical diagnosis. Patients report classic symptoms (eg, abdominal angina) and have radiographic findings showing severe stenoses or occlusion of two or more mesenteric arteries.

  • Colonic ischemia is rarely life threatening and usually resolves with supportive care.

1. Etiology & Pathogenesis

Intestinal ischemia is caused by a reduction in intestinal blood flow, most commonly as a result of occlusion, vasospasm, or hypoperfusion of the mesenteric circulation. It is categorized as acute or chronic, depending on the rapidity and the extent to which blood flow is compromised and whether it is episodic or constant, as might occur in chronic mesenteric ischemia. Ischemia can involve the small intestine or colon. Acute small intestinal ischemia is a medical and surgical emergency that requires prompt diagnosis and a coordinated, interdisciplinary approach. By contrast, acute colonic ischemia (ie, ischemic colitis) is rarely an emergent condition. Acute intestinal ischemia can be further categorized as arterial versus venous, embolic versus thrombotic, and occlusive versus nonocclusive. Other causes of bowel ischemia include strangulating obstructions (adhesions, hernias, metastatic malignancy, intussusceptions) and vasculitis (systemic lupus erythematosus, polyarteritis nodosa).

2. Splanchnic Circulation: Anatomy & Physiology

The vascular supply to the intestines includes the celiac artery, the superior mesenteric artery (SMA), and the inferior mesenteric artery (IMA) (Figure 6–1). The celiac axis supplies blood to the stomach and duodenum. The SMA supplies the small bowel from the distal duodenum to the mid-transverse colon. The inferior mesenteric artery supplies the transverse colon to the rectum. Natural anastomoses exist between branches of the major vessels, and if one artery is occluded, some flow may be maintained via a patent collateral vessel.

When a major vessel is occluded, collateral pathways open immediately in response to a fall in arterial pressure distal to the obstruction. The superior and inferior pancreaticoduodenal vessels are collaterals that connect the celiac axis to the SMA. The phrenic artery connects the aorta to the celiac axis. The marginal artery of Drummond and the arc of Riolan are collaterals that connect the SMA and the IMA. The internal iliac arteries provide collaterals to the rectum. Griffith point in the splenic flexure and Sudeck point in the rectosigmoid area are watershed areas within the colonic blood supply and common locations for ischemia.

The splanchnic circulation receives 25% of the cardiac output under basal conditions and 35% or more postprandially. Approximately 70% of splanchnic inflow goes to the mucosa, which is the most metabolically active area of the gut. The villus tips are the ...

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