Control of Gastric Emptying

Describe the control of gastrointestinal motility, including sphincter function.

Gastric emptying is a neurally and hormonally mediated process which aims to present food to the small bowel in a controlled manner. Different drugs, hormones, and physiological states can either encourage or inhibit gastric emptying.

Determinants of Gastric Emptying

Rate of gastric emptying is a function of:

• Antral pressure
  Main determinant as pyloric resistance tends to be low, and is affected by:
  • Stomach
  • Duodenum
  • Systemic factors
  • Drugs
• Pyloric resistance

Stomach

• Gastric distension
  Vagal excitation from gastric stretch causes release of gastrin, increasing peristaltic frequency.
• Composition of chyme:
  • Liquids empty faster than solids
    • Liquids have a half-time of ~20 minutes, and empty in an exponential fashion
    • Solids have a half-time of ~ 2 hours, with a dwell time of ~30 minutes, and empty in a linear fashion
  • Protein independently stimulates gastrin release

Duodenum

The duodenum has hormonal mechanisms which have a negative feedback on gastric emptying. These include:

• Duodenal distension
• Hypoosmolar and hyperosmolar chyme
• Acidic chyme
  In response to acid the duodenum releases secretin and somatostatin:
  • Secretin directly inhibits gastric smooth muscle
  • Somatostatin inhibits gastrin release
• Fat and protein
  Fat and protein breakdown products stimulate release of cholecystokinin, which inhibits gastrin.
  • Carbohydrate-rich meals empty faster than protein, which empty faster than fat.

Systemic

• Motilin released by the small bowel enhances the strength of the migrating motor complex, a peristaltic wave of contraction through the whole GIT which occurs every 60-90 minutes
• Sympathetic input from the coeliac plexus inhibits gastric emptying
• Pregnancy has a number of effects on gastric emptying:
  • Progesterone relaxes smooth muscle and inhibits gastric smooth muscle response to ACh and gastrin, as well as creating incompetence of the LoS leading to GORD
  • Gastrin production increases
    Some gastrin is produced by the placenta.
  • Gastric acid production is increased during the the third trimester
• Parasympathetic input enhances gastric motility

Effect of drugs

Drugs which increase gastric emptying include:

• Metoclopramide
• Erythromycin

Drugs which inhibit gastric emptying include:

• Opioids
• Alcohol
• Anticholinergic agents

References

1. CICM July/September 2007
2. Kam P, Power I. Principles of Physiology for the Anaesthetist. 3rd Ed. Hodder Education. 2012.