20) Anti-Emetics Flashcards
State the 5 major classes of anti-emetic drugs.
Mixed receptor antagonists Dopamine (D2) receptor antagonists Muscarinic receptor antagonists Serotonin (5-HT3) receptor antagonists Cannabinoids
What is nausea and vomiting often preceded by?
Sweating, salivation and an increase in heart rate
Describe the process of vomiting.
Stomach, oesophagus and associated sphincters are relaxed
Contraction of upper small intestine, pyloric sphincter and pyloric region of stomach
Contents of upper jejunum, duodenum and pyloric region of stomach move to the body and fundus of the stomach
Lower and upper oesophageal sphincters and oesophagus relaxes
Retching/vomiting may occur
What are the consequences of severe vomiting?
Dehydration
Hypochloraemic metabolic alkalosis
Contributes to reduction in bicarbonate excretion and increase in bicarbonate reabsorption
Increase in sodium reabsorption and increase in potassium excretion (hypokalaemia)
What types of receptors in the stomach transmit signals to the vomiting centre and chemoreceptor trigger zone?
Chemoreceptors and Mechanoreceptors
What is special about the location of the CTZ and vomiting centre?
It is located in a part of the brain that has a very porous blood brain barrier
So the CTZ and vomiting centre act as an early warning system to protect the brain from toxin damage
Promethazine
mixed receptor antagonist.
Describe the mode of action of this drug.
It is a competitive antagonist for the following receptors:
Histaminergic
Muscarinic
Dopaminergic
Order of potency of antagonist activity: H > M > D
What are the uses of promethazine / MRA ?
Motion sickness Disorders of the labyrinth Hyperemesis gravidarium Pre and post-operatively Relief of allergic symptoms, anaphylactic emergency, night sedation; insomnia
What are the unwanted effects of promethazine?
Dizziness Tinnitus Fatigue Sedation Convulsions
Metoclopramide
Domperidone
dopamine receptor antagonists.
Describe the order of agonist potency of these drugs.
D > H > M
What effect do Dop-recep-antag drugs have on the GI tract?
They have PROKINETIC effects on the GI tract:
Increase smooth muscle motility
Accelerate gastric emptying
Accelerate the transit time of intestinal contents
Why are these drugs poor at treating motion sickness?
The vestibular system has connections to the CTZ and it has direct connections to the vomiting centre
The dopamine antagonists block dopamine receptors in the CTZ but they are not blocking the rest of the signals that are going directly from the vestibular system to the vomiting centre
State some uses of metoclopramide and domperidone // Dopamine Receptor Antagonists
Uraemia (severe renal failure)
Radiation sickness
GI disorders
Cancer chemotherapy