Expansion
Four inputs converge on the vomiting centre in the medulla
Expansion
Four afferent routes to the vomiting centre in the medulla:
- Chemoreceptor trigger zone (area postrema, outside the blood-brain barrier): drugs, toxins, uraemia, hypercalcaemia. Rich in D2 and 5-HT3 receptors
- Vagal and splanchnic afferents from the gut: distension, irritation, obstruction. Rich in 5-HT3 receptors, released from enterochromaffin cells, which is why chemotherapy causes vomiting
- Vestibular system: motion sickness. H1 and muscarinic receptors
- Higher centres: sight, smell, pain, anxiety, raised intracranial pressure
Matching drug to mechanism:
- Ondansetron (5-HT3): chemotherapy, postoperative, gut causes
- Metoclopramide and domperidone (D2, prokinetic): gastric stasis
- Cyclizine (H1) and hyoscine (antimuscarinic): motion and vestibular causes
- Haloperidol and levomepromazine (D2): metabolic and drug causes
- Dexamethasone and aprepitant (NK1): chemotherapy
Prolonged vomiting causes loss of hydrogen, chloride and potassium, giving a hypochloraemic hypokalaemic metabolic alkalosis with paradoxical aciduria.