Ileus and Mechanical Bowel Obstruction

Published on 26/03/2015 by admin

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106 Ileus and Mechanical Bowel Obstruction

Rajeev Dhupar, Juan B. Ochoa

image Definition

Ileus is defined as the absence of physiologic motility of the bowel leading to a disturbance in the progression of bowel contents through the gastrointestinal (GI) tract. Ileus must be distinguished from mechanical bowel obstruction, which is defined as the presence of anatomic barriers, either extrinsic or intrinsic, that prevent the normal progression of bowel contents through the GI tract.

image Pathophysiology

Normal Gastrointestinal Motility

Coordinated contraction of the GI tract can be measured by evaluating its electrical and motor activity. During fasting states, the coordinated contractions are called migrating motor complexes (MMC) and are divided into three phases: resting phase, intermittent contractions of moderate amplitude, and high-pressure waves.1 When a food bolus is introduced into the intestine, the organized MMC disappear, and digested food (chyme) is propelled through the GI tract by spikes in the contraction of smooth muscle in the wall of the gut. Longitudinal progression of intestinal contents (made up by food and secretions) occurs through the coordinated response of several systems. These are:

1 Autonomic nervous system. Activation of the sympathetic nervous system decreases GI motility. Activation of the parasympathetic nervous system increases GI motility.2
2 Interstitial cells of Cajal (ICC). ICC are distributed throughout the tunica muscularis and are electrically coupled with one another. These cells, which are mesenchymal in origin, are responsible for the pacemaker activity of the GI tract.1
3 Myenteric and submucosal nerve plexi. These plexi integrate with the autonomic nervous system. Nitric oxide produced by neuronal nitric oxide synthase (nNOS) induces smooth muscle relaxation.
4 Endocrine system. Multiple endocrine substances affect GI motility. Some of these substances, including motilin, gastrin, and cholecystokinin, increase GI motility. Other hormones such as somatostatin and glucagon decrease GI motility.
5 Smooth muscle. Although there are differences in the muscular layers of the stomach, small bowel, and colon, intestinal motility depends on the coordinated contraction of an outer longitudinal layer and an inner circular layer.
6 Immune system. Activation of the innate immune system can produce profound alterations in GI motility. This appears to be especially evident after surgical manipulation of the small bowel and colon. Inflammatory mediators such as nitric oxide, cytokines, prostaglandins, and oxygen free radicals have direct inhibitory effects on normal contractile activity and may play an important role in the development of ileus due to sepsis and/or after abdominal operations.3,4

Integration of the aforementioned processes results in coordinated muscular contractions in the wall of the stomach and intestine that move fluids in the GI tract in an aboral direction. Additionally, this activity helps to ensure that food is adequately mixed with GI secretions and digested. When motility is normal, there is adequate contact time between the absorptive surfaces of the bowel and chyme to permit absorption. Normal motility ultimately leads to the evacuation of undigested food as fecal matter.

image Clinical Consequences of Ileus

Ileus results in the inability to tolerate enteral feeding, nausea, vomiting, constipation, and obstipation. Accumulation of fluid and air in the bowel results in abdominal distention. Symptoms and consequences of ileus can range from minimal to life threatening. Serious consequences of ileus can include electrolyte abnormalities, intestinal ischemia, intestinal perforation, and abdominal compartment syndrome. Intolerance of enteral nutrition compromises the ability to provide adequate nutritional support to critically ill patients.

image Diagnosis

Tools to aid clinicians in identifying and diagnosing GI dysfunction are poorly developed. Ileus is diagnosed by clinical evaluation of the following signs and symptoms:

1 Abdominal distention
2 Nausea, vomiting, or high output through a nasogastric (Salem sump) tube
3 Reflux of enteral tube feedings
4 Abdominal pain and discomfort (either spontaneous or elicited by palpation of the abdomen)
5 Decreased or absent bowel sounds
6 Constipation and obstipation
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