Nausea, vomiting + electrolyte imbalance Flashcards
What stimulates N+V?
Toxins
Smells
Motion sickness
Migraine
Pregnancy
What drugs stimulate N+V?
Chemotherapy
SSRIs
Digoxin / theophylline
Iron salts
Erythromycin
Levodopa
Opioids e.g. morphine
What happens physiologically to the body when someone is about to vomit?
- Discomfort, dry mouth, salivary inhibition
- Yawning
- Reappearance of saliva
- Pyloric sphincter closes
- Tone of stomach decreases
- deep breath
- Contraction of abdominal muscles to force food out
- Forced expiration to prevent inhalation
How do we centrally regulate vomiting in the body?
Central regulation of vomiting occurs in the vomiting centre + chemoreceptor trigger zone (CTZ)
CTZ is sensitive to chemical stimuli - main site of anti-emetic drugs
Output from the CTZ stimulate the vomiting centre leading to vomiting
Neuronal signals from GIT feed in to CTZ + vomiting centre
What hormones/neurotransmitters are involved in N+V?
Histamine
Acetylcholine
Dopamine
5-HT
(Blocking these receptors in the brain prevents N+V)
How does chemotherapy induce N+V?
Chemotherapy agents stimulate release of 5-HT from enterochromaffin cells in GI Tract
Types of anti-emetic drugs
H1-Histamine receptor antagonists
- e.g. promethazine
- effective in motion sickness
- produce drowsiness
Anti-muscarinic agents
- e.g. hyoscine
- effective in motion sickness
- reduce gastric motility
- anti-muscarinic side effects (dry mouth)
- produce drowsiness
Dopamine antagonist
- e.g. domperidone + metoclopramide
- act in CTZ, has unwanted CNS effects
- stimulates gastric emptying = reduce nausea
5-hydroxytryptamine antagonist
- e.g. ondansetron
- blocks 5-HT at 5-HT3-receptors in gut + CNS
- effective against anti-cancer drugs
- not effective for motion sickness
Cannabinoid receptor agonist
- action unknown
Neurokinin receptor antagonist
- NK1 receptors activated by substance P
- NK1 receptor antagonists suppress nausea + vomiting
- e.g. aprepitant
- used in cancer chemotherapy
Steroids
- e.g. dexamethasone
- used in chemotherapy
- action unknown
What is the standard prescription for cancer chemotherapy nausea + vomiting?
5-HT3 receptor antagonist
Steroid
NK1 receptor antagonist
Vomiting causes electrolyte imbalance.
What are the consequences of this?
Vomiting = loss in H+, K+ + H2O
Consequences
- acid/base imbalance
- electrolyte imbalance
- nerve/muscle function
How does diarrhoea affect electrolyte imbalance? And what are the consequences?
Losses in Na+ + H20
Dehydration leads to:
- low blood pressure
- muscle weakness/cramps
- reduced level of consciousness
- heart failure
- convulsions
How does urine affect electrolyte imbalance?
Variable fluid loss
High K+, low Na+
What is the most abundant extracellular cation?
What regulates H20 + its excretion in kidneys?
Sodium
Osmotic activity of extracellular fluid depends on relative proportion of Na+ + H20
Na+ + H20 excretion in kidney regulated by vasopressin + aldosterone
What is Hyponatraemia?
Sodium depletion
Caused by fluid retention e.g. renal failure, heart failure
Extracellular fluid should have 145mmol/l of sodium
Hyponatraemia = < 135mmol/l of sodium
Hypernatraemia = > 145 mmol/l
Leads to:
- water into cells
- cerebral oedema (brain swelling)
- nausea/vomiting
- coma
- death
How does MDMA affect electrolyte imbalance?
MDMA = ecstasy
Makes you drink more water
Stimulates vasopressin secretion (water retention)
Causes hyponatraemia
What is the treatment of Hyponatraemia?
Water restriction
Increase salt intake
