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In 75% of patients, acute intestinal obstruction results from adhesive bands or internal hernias secondary to previous abdominal surgery or from external hernias. The incidence of acute intestinal obstruction requiring hospital admission within the first few postoperative weeks is 5–25%, and 10–50% of these patients will require surgical intervention. The incidence of postoperative intestinal obstruction may be lower following laparoscopic surgery than open procedures. However, the laparoscopic gastric bypass procedure may be associated with an unexpected high rate of intestinal obstruction, with a higher reoperative rate. Other causes of intestinal obstruction not related to previous abdominal surgery include lesions intrinsic to the wall of the intestine, e.g., diverticulitis, carcinoma, and regional enteritis; and luminal obstruction, e.g., gallstone obstruction, intussusception.

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Two other conditions that must be differentiated from acute intestinal obstruction include adynamic ileus and primary intestinal pseudo-obstruction. Adynamic ileus is mediated via the hormonal component of the sympathoadrenal system and may occur after any peritoneal insult; its severity and duration will be dependent to some degree on the type of peritoneal injury. Hydrochloric acid, colonic contents, and pancreatic enzymes are among the most irritating to the peritoneum, whereas blood and urine are less so. Adynamic ileus occurs to some degree after any abdominal operation. Retroperitoneal hematoma, particularly associated with vertebral fracture, may cause severe adynamic ileus, and the latter may occur with other retroperitoneal conditions, such as ureteral calculus or severe pyelonephritis. Thoracic diseases, including lower-lobe pneumonia, fractured ribs, and myocardial infarction, frequently produce adynamic ileus, as do electrolyte disturbances, particularly potassium depletion. Finally, intestinal ischemia, whether from vascular occlusion or intestinal distention itself, may perpetuate an adynamic ileus. Intestinal pseudo-obstruction is a chronic motility disorder that frequently mimics mechanical obstruction. This condition is often exacerbated by narcotic use.

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Pathophysiology

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Distention of the intestine is caused by the accumulation of gas and fluid proximal to and within the obstructed segment. Between 70 and 80% of intestinal gas consists of swallowed air, and because this is composed mainly of nitrogen, which is poorly absorbed from the intestinal lumen, removal of air by continuous gastric suction is a useful adjunct in the treatment of intestinal distention. The accumulation of fluid proximal to the obstructing mechanism results not only from ingested fluid, swallowed saliva, gastric juice, and biliary and pancreatic secretions but also from interference with normal sodium and water transport. During the first 12–24 h of obstruction, a marked depression of flux from lumen to blood of sodium and water occurs in the distended proximal intestine. After 24 h, sodium and water move into the lumen, contributing further to the distention and fluid losses. Intraluminal pressure rises from a normal of 2–4 cmH2O to 8–10 cmH2O. The loss of fluids and electrolytes may be extreme, and unless replacement is prompt, hypovolemia, renal insufficiency, and shock may result. Vomiting, accumulation of fluids within the lumen, and the sequestration of fluid into the edematous intestinal wall ...

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