16-Anti-emetics Flashcards
What happens in vomiting?
Stomach, oesophagus and related sphincters are relaxed
Tension in gastric and oesophageal muscles triggers afferent nerve impulse
Contraction of upper small intestine, pyloric sphincter and pyloric region of stomach
Contents of upper jejunum, duodenum and pyloric stomach move into the body and fundus of stomach
What are the consequences of vomiting and why do they occur?
Dehydration
Hypochloraemic metabolic alkalosis: Loss of gastric H+ and Cl- raises pH -> alkalosis
Hypochloraemia=loss of H+ ions=alkalosis, so kidney keeps H+ and transiently excretes K+ and HCO3- to lower the pH to normal.
K+ moves into cells to replace lost H+ to maintain neutrality -> hypokalaemia
How do emetic stimuli trigger vomiting?
4 ways:
1) Stimuli from peripheral organs to vomiting centre
Stimulates mechano/chemo-receptors or gut entochromaffin cells, releasing 5-HT.
2 ways to reach vomiting centre: Visceral afferents release mAch/histamine onto Nucleus of Solitary Tract, which feeds into vomiting centre, OR 5HT released onto CTZ which activates the vomiting centre
2) Endogenous drugs/toxins
Release emetogenic agents (5HT, prostanoids, dopamine, free radicals) onto the CTZ, activating D2 receptors on the CTZ
3) Motion sickness
AchM/histamine released from labyrinth onto vestibular nuclei
Chemical signal activates CTZ
4) Pain, repulsive stimuli/emotions
mAch/histamine released onto higher centres
Name the 5 classes of anti-emetic drug
1) Mixed receptor antagonist (promethazine)
2) D2 receptor antagonists (metoclopramide/domperidone)
3) mAchR antagonists - Hyoscine
4) Serotonin receptor antagonists - Ondensatron
5) Endocannabinoids
