Describe the physiology of gastric emptying (80% of marks). Outline the gastrointestinal effects of erythromycin (20% of marks).
This question was best answered by using a classification system, or systematic approach to gastric emptying. Receptive relaxation (triggered by movement of food through the pharynx and oesophagus), vagally mediated relaxation of fundus and upper body of stomach, the pyloric pump (being intense peristalsis in lower body of the stomach that results in stomach emptying) and the pyloric sphincter(a circular muscle that allows water and fluids to easily pass through but restricts solids until it is mixed in chime to almost fluid consistency). Candidates were also expected to mention regulatory factors e.g. food volume through myenteric reflexes / gastrin stimulatory motor effects and enhanced pyloric pump, acidity and osmolality of chyme in duodenum, presence of breakdown products of protein and fat through enteric nervous system, sympathetic and parasympathetic nervous systems and hormones such as cholecystokinin, secretin and gastric inhibitory peptide.
Erythromycin is a commonly used prokinetic and some knowledge of effects was expected (e.g. the fact that it
stimulates motilin receptors on GI smooth muscle and promote onset, frequency and duration of migrating motor complex, from stomach and spreading caudally thus increasing gastric emptying).
Regulation of gastric emptying:
|Enhance gastric empyting||Slow gastric emptying|
Acts as a motilin receptor agonist and motilin release enhancer
Increases the automaticity of enteric nervous system motor function
Doses of 250mg (as low as 40mg) are effective, whereas the antimicrobial dose is 1g
Main side effect is QT prolongation
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