Anatomy and physiology

Relevant blood supply:

• coeliac axis - first portion of the duodenum
• superior mesenteric artery - the remainder of the small bowel and the large bowel up to the splenic flexure
• inferior mesenteric artery - the remainder of the large bowel and part of the rectum
• branches of the internal iliac artery - distal rectum

Potential areas of ischaemia:

• splenic flexure - watershed between the superior and inferior mesenteric arteries
• rectosigmoid region - watershed between the inferior mesenteric artery and the branches from the internal iliac artery

Normally, the intra-abdominal digestive organs receive 25% of the cardiac output. Hypotension causes mesenteric arterial occlusion with shunting of the blood to the heart and brain. The bowel can tolerate a 70% reduction in blood supply without damage. The mucosa is the most vulnerable area; the muscularis propria, the least.

Ischaemia, i.e. an inadequate blood supply, results in tissue hypoxia and the accumulation of toxic waste products, which may cause tissue necrosis, i.e. infarction.

The degree of bowel damage sustained depends upon:

• rapidity of the ischaemia and its duration
• extent of any collaterals
• metabolic requirements of the area affected
• nature of bowel flora - for example, anaerobes such as Clostridia species may produce toxins that accentuate damage