Physiology of Vomiting

Describe the control of gastrointestinal motility, including sphincter function

Vomiting is the active, forceful expulsion of gastric contents from the stomach. It is different from regurgitation which is a passive process.

It is a mechanism to expel toxic substances from the GIT.

Stimulation

Stimulants to vomiting can act centrally, or directly in the bowel:

• Central stimulation
  Central stimuli may act directly on the vomiting centre. Others act via the CTZ, which is part of the area postrema located outside of the blood-brain barrier, and so it can be stimulated by circulating substances. Central vomiting stimuli include:
  • Direct:
    • Emotion
    • Pain
    • Olfactory
    • Visual
  • Via the CTZ:
    • Vestibular acting on:
      • H₁
      • ACh
    • Drugs/Toxins acting on:
      • 5-HT₃
      • D₂
      • μ-opioid receptors

• GIT stimulation
  GIT stimuli travel SNS and PNS afferents to the vomiting centre. The CTZ is not involved and so anti-emetics which act here are not useful in this type of vomiting.

  GIT vomiting stimuli include distension and toxins. Neurotransmitters include:

  • 5-HT₃ in mucosal stretch receptors
  • ACh in NTS afferents
  • H₁ in NTS afferents

Postoperative Nausea and Vomiting

Central structures involved include:

• Chemoreceptor trigger zone
• NTS
• Multiple pathways exist (similar to those described above), and neurotransmitters involved include:
  • 5-HT₃
  • D₂
  • NK₁
  • H₁
  • mACh
• Risk factors
  • Patient factors
    • Female
    • Non-Smoker
    • Young age
    • History of PONV or motion sickness
  • Anaesthetic factors
    • Volatile use
    • Nitrous oxide use
      Relative risk of 1.4.
    • Opioid use
    • Anaesthesia duration
  • Surgical factors
    • Gynaecological surgery
      Likely not an independent risk factor, and simply confounded by female gender.
    • Strabismus surgery in children

Process of vomiting

Vomiting consists of a set of processes coordinated by the vomit centre in the medulla oblongata, and is divided into three phases:

• Pre-ejection phase
  • Prodromal nausea
  • Salivation
  • Retrograde intestinal contraction which forces intestinal contents into the stomach
• Retching Phase
  • Deep inspiration and breath-holding to splint the chest
  • Epiglottic closure
  • Elevation of the soft palate (prevents nasal soiling)
• Expulsive phase
  • Relaxation of oesophageal sphincters
  • Pyloric contraction
  • Violent contraction of the diaphragm and abdominal muscles

References

1. Brandis K. The Physiology Viva: Questions & Answers. 2003.
2. Kam P, Power I. Principles of Physiology for the Anaesthetist. 3rd Ed. Hodder Education. 2012.
3. Petkov V. Essential Pharmacology For The ANZCA Primary Examination. Vesselin Petkov. 2012.
4. Pierre S, Whelan R. Nausea and vomiting after surgery. Continuing Education in Anaesthesia Critical Care & Pain, Volume 13, Issue 1, 1 February 2013, Pages 28–32.