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INTRODUCTION AND EPIDEMIOLOGY

Mesenteric ischemia is a vascular occlusive emergency. An expeditious diagnosis and initiation of treatment are keys to preserving viable bowel and decreasing patient mortality. Both the small and large intestine are susceptible to ischemic injury. The term mesenteric ischemia generally refers to small bowel ischemia, whereas the term colonic ischemia generally refers to large bowel ischemia.

Mesenteric ischemia may occur acutely due to occlusion of the mesenteric vessels or due to systemic hypoperfusion injury. The most common cause of acute mesenteric ischemia is arterial embolism. Chronic mesenteric ischemia also may present and is related to partial vascular obstruction of the mesenteric vessels.

Ischemia to the mesentery may be the consequence of underlying cardiovascular disease. Therefore, mesenteric ischemia is rare in the younger individual. As patients develop cardiovascular comorbidities, the rate of mesenteric ischemia rises. By age 75 years, mesenteric ischemia becomes a more common cause of abdominal pain than appendicitis (see Chapter 71, Acute Abdominal Pain).1 The median age of presentation is reported to be 70 years, but the actual rate of mesenteric ischemia is not known due to the varied presentation and paucity of studies.2 The mortality rate has been reported to be as high as 60% to 80%.3

Anatomy

The entire mesentery is fed by three main vessels: the celiac artery, superior mesenteric artery (SMA), and inferior mesenteric artery (IMA). The celiac artery is responsible for the liver, spleen, and proximal duodenum. The SMA supplies the pancreatic head, distal duodenum, and complete small bowel.3 The IMA supplies the transverse colon to the rectum. These three vessels create an overlapping network of collateralized blood flow to both visceral and nonvisceral organs.4,5 Once arterial vessels enter the muscular layer of the intestine, they form a submucosal vascular plexus5 branching into a rich parallel network associated with the intestinal villi. This rich blood supply is the key to nutrient exchange at the level of the intestinal villi.5

PATHOPHYSIOLOGY

The mesentery experiences significant, daily alterations of blood flow. As part of normal homeostasis there is shunting of blood, up to 200%, to the splanchnic bed in the postprandial state for digestion and nutrient transfer. In the fasting state, much of the blood is shunted away from the splanchnic bed. In times of pathophysiologic stress, such as hypovolemia or sepsis, the body will further shunt arterial blood away from the mesentery to other more vital organs.

In acute arterial occlusive ischemia, profound ischemia is due to obstruction of a major arterial vessel (Table 71B-1). Mesenteric ischemia can also result from pathologic shunting of blood away from the mesentery. Vascular autoregulation and intraabdominal pressure all play a role in blood flow to the intestinal mucosa. Endogenous or iatrogenic alterations to baseline function may result in nonocclusive ...

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