Pathophysiology

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The main components of gastric motility are as follows:

1. relaxation to accommodate food with only a small increase in intraluminal pressure;

2. contractions to mix food with gastric juice and reduce particle size;

3. controlled gastric emptying.

Relaxation of the gastric fundus is mediated mainly through efferent vagal inhibitory neurons with some additional contribution from the action of cholecystokinin. Psychogenic stimuli, swallowing, and afferent impulses from mouth and esophagus initiate gastric relaxation. These factors are bypassed during intragastric tube feeding.

After relaxing to accommodate food, the proximal stomach has low-frequency tonic contractions which pass food towards the distal stomach where gastric smooth muscle cells have an innate electrical activity of approximately 3 cycles/min. Muscle contraction occurs when depolarization exceeds the threshold. Conduction of depolarization from the middle of the body of the stomach through the antrum to the pylorus causes the peristaltic waves that pass distally through the stomach. Contraction in the distal stomach is enhanced by vagal cholinergic nerves and inhibited by the sympathetic innervation. Gastrin increases the frequency of contractions.

Gastric emptying of liquids depends on the pressure gradient across the pylorus and is not regulated by pyloric activity. Solids empty more slowly. As contractions reach the distal antrum, the pylorus closes and solid material is squeezed and ground before being passed back into the body of the stomach. When digestible solids have been reduced to small particles, they pass from the stomach with the liquid material. The effects of different food contents on the rate of gastric emptying are summarized in Table 1. Receptors in the proximal small bowel are sensitive to pH, osmolality, and fatty acid content. Hormonal and local neural pathways then regulate gastric emptying. Motilin and neurotensin increase gastric emptying. Secretin, catecholamines, and b-endorphins may delay gastric emptying. During fasting, undigested solids are propelled from the stomach by the migrating motor complex—a wave of electromechanical activity that normally occurs approximately every 90 min and passes from the stomach distally to the ileum. Motilin is released from the proximal ileum and modulates gastric contraction during the migratory motor complex. Migrating motor complexes occur more frequently during sleep and are inhibited by stress.

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Table 1 The effect of feed content and properties on the comparative rate of gastric emptying

During constant infusion of nasogastric feeds in healthy volunteers, gastric volume reaches a maximum 2.25 h after starting the feed and then decreases to a steady state volume after 6 h. There is little information on the usual pattern of gastric emptying in the critically ill. In a study of 10 patients with burns affecting more than 20 per cent of body surface area no significant difference was found compared with normal volunteers, although others have found gastroparesis in many critically ill patients with development of large nasogastric aspirates when fed into the stomach. Autonomic imbalance and reduced motilin secretion are probably responsible.

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