GI - Antiemetics Flashcards
Where is the chemoreceptor trigger zone?
Bottom of the fourth ventricle in the medulla
State four inputs to the CTZ
- Sensory afferents via the midbrain (see, smell, taste something gross.
- Vestibular nuclei (motion sickness)
- Visceral afferents from the gut (stretched, inflamed)
- Direct triggers (drugs, toxins)
List two drug classes that act on the vestibular nuclei
Muscarinic receptor antagonists
H1 receptor antagonists
List two drug classes that act on the visceral afferents from the gut
5HT3 antagonists
D2 receptor antagonists
List seven dug classes that act directly on the CTZ
D2 receptor antagonists 5HT3 receptor antagonists Muscarinic receptor antagonists H1 receptor antagonists Corticosteroids Cannabinoids NK1 receptor antagonists
Name a muscarinic receptor antagonist
Hyoscine hydrobromide (hyoscine - high sea .·. motion sickness)
What is the MoA of muscarinic receptor antagonists?
Competitive blocking of ACh receptors in the Psymp NS in the vestibular nuclei and CTZ
State two uses of muscarinic receptor antagonists
- People who can’t take tablets (available as a patch)
- Motion sickness
- Bowel obstruction
- Palliative care
State two side effects of muscarinic receptor antagonists
Sedation Memory problems Glaucoma Dry mouth Constipation
State two examples of H1 receptor antagonists
Cyclizine
Levomepromazine
Promethazine
What is the MoA of H1 receptor antagonists?
Inhibit histaminergic signals from the vestibular system to the CTZ
State two uses for H1 receptor antagonists
Motion sickness for long journeys when you want to sleep
(Promethazine) morning sickness in pregnancy
GI pathology e.g. Crohn’s, appendicitis, UC
Why shouldn’t cyclizine be used for little old ladies and children?
Sedation or excitation effects
State two side effects of H1 receptor antagonists
Sedation or excitation
Dry mouth
Constipation
Long QT interval
Name a 5HT3 receptor antagonists
Ondansetron
Granesitron
What is the MoA of 5HT3 receptor antagonists?
Antagonising serotinin produced by enterochromaffin cells leads to:
- Reduces smooth muscle cell contraction
- Decreases gut secretions
Also inhibs the CTZ
What are 5HT3 receptor antagonists used for?
Almost everyone - 1st line most of the time
Chemo when medium risk
Second line in hyperemesis gravidarum
GI pathology
State two side effects of 5HT3 receptor antagonists
Constipation Headache Long QT syndrome Elevated liver enzymes Parkinsonism
Name two D2 receptor antagonists that act on the visceral afferents in the gut and the CTZ
Metacloperamide
Domperidone
Name two other D2 receptor antagonists that only act on the CTZ
Chlorpromazine
Haloperidol
Levomepromazine
Prochlorperazine
(Cross over with H1 receptor antagonists - stuff can do two things)
What is a MoA of D2 receptor antagonists?
Acts on the CTZ
Acts as a muscarinic and 5HT3 antagonist
Promotes gastric emptying
- Increased tone at the LOS
- Increased tone and amplitude of gastric contractions
- Decreased tone at the pylorus
Inreased peristalsis in the gut
State two uses of the D2 receptor antagonists that act on the visceral afferents in the gut
GORD
Ileus
Second line in hyperemesis gravidarum
Rescue medication in chemo
State two uses of D2 receptor antagonists that act on the CTZ
Motion sickness
Vertigo
Prochlorperazine first line in pregnancy
Haloperidol for chemo and palliation
State two side effects of D2 receptor antagonists
Galactorrhoea
Parkinsonism
Sedation
Hypotension
