F6. Nausea, vomiting and electrolyte balance Flashcards
Central control of nausea and vomiting?
one note
Histamine H1 Receptor Antagonists (e.g. promethazine) action?
- Motion sickness.
- Morning sickness of pregnancy.
- Space motion sickness (by NASA).
- Anti-muscarinic actions.
- Produce drowsiness and sedation.
Muscarinic M1 Receptor Antagonists (e.g. hyoscine) action:
- Prophylaxis and treatment of motion sickness.
- Also reduce gastric motility.
- Anti-muscarinic side effects (e.g. dry mouth).
- Produce drowsiness (less sedation).
- Naturally occurring found in Atropa belladonna.
Dopamine D2 Receptor Antagonists (e.g. metoclopramide, domperidone, prochlorperazine (D2 and muscarinic antagonist)) action?
- Act in CTZ but has unwanted CNS effects.
- D2 receptor involved in movement.
- Antagonism leads to Parkinson’s like symptoms.
- Effective against chemotherapy-induced emesis.
- Also stimulates gastric emptying and reduce nausea.
5-Hydroxytryptamine 5-HT Receptor Antagonists (e.g. ondansetron) action?
- Blocks 5-HT at 5-HT3-receptors in gut and CNS. Cytotoxic drug
- Particularly effective against chemotherapy-induced emesis. Circulating 5-HT
- Not effective forgmut otion sickness.
Neurokinin NK1 Receptor Antagonists (e.g. aprepitant) action?
- NK1 Receptor activated by Substance P.
- NK1 receptor antagonists suppress nausea & vomiting.
- Used in chemotherapy-induced N&V in combination with 5-HT receptor antagonist.
Steroids (e.g. dexamethasone) action?
- Used in chemotherapy-induced N&V.
- Mechanism unknown.
Describe chemotherapy induced nausea and vomiting and dosing
- 5-Hydroxytryptamine Receptor 3 (5-HT3) Antagonist +
- Steroid +
- Neurokinin Receptor (NK1) Antagonist.
Dosing:
* 5-HT3 Antagonist: 8mg 1 hour before treatment, then 8mg every 12 hours for up to 5 days.
* NK1 Antagonist: 125mg 1 hour before treatment, then 80mg daily for next 2 days.
vomiting causes loss of what? and what are the consequences?
- Losses in H+, K+ + H2O.
- Consequences:
-Acid/base imbalance (metabolic alkalosis);
-Electrolyte imbalance;
-impaired Nerve + Muscle function.
Diarrhoea causes losses of what and what are the consequences?
-Losses in Na+ + H2O, result in dehydration.
-Consequences:
-Reduction in blood pressure (fluid loss);
-Muscle weakness/cramps;
-Reduced level of consciousness;
-Heart failure;
-Convulsions.
Urine causes loss in what?
- Variable fluid loss;
- High K+;
- Low Na+;
- Drugs can alter electrolyte secretion.
Describe electrolytes and fluid imbalance
-Effects on nerve impulses and conduction of electrical impulses across heart and result in irregular heart rate.
-Low Na+ can cause lethargy, seizures & respiratory problems.
ONE NOTE
Describe sodium (Na+)
- Most abundant extracellular cation;
- Osmotic activity of ECF depends on relative proportion of Na+ and H2O;
- Na+ and H2O excretion in kidney regulated by vasopressin and aldosterone.
Describe Sodium (Na+) Depletion (hyponatraemia):
- Extracellular fluid, normally 145 mmol/l;
- <135 mmol/l = hyponatraemia;
- > 145 mmol/l = hypernatraemia;
- Hyponatraemia caused by fluid retention, e.g. renal failure, heart failure.
Describe Sodium (Na+) Depletion (hyponatraemia) and ecstasy:
- MDMA- ecstasy;
- Thought need to drink more water;
- Also, MDMA stimulates vasopressin secretion and water retention;
- Result in hyponatraemia.
What does Sodium (Na+) Depletion (hyponatraemia) lead to? treatment?
-Hyponatraemia leads to H2O into cells ->cerebral oedema ->nausea & vomiting ->coma ->death.
-Treatment: H2O restriction and increase salt intake.
Describe sodium (Na+) Excess (hypernatraemia)?
- Due to decrease H2O. Rare as thirst stimulates drinking;
- Diabetes; increase urination, increasing H2O loss
- H2O moves out of cells, causing decrease in cell volume and decrease in brain size causing coma.
Describe Potassium depletion and excess
Depletion = Hypokalaemia;
Excess = Hyperkalaemia.
Describe potassium
-K+ mostly inside cells;
-Hyperkalaemia extracellular > 5.5 mmol/l:
-Muscleweakness;
-ECGchanges,arrythmias.
-Hypokalaemia < 2.5 mmol/l:
-Muscleweakness;
-ECGchanges.
-Alteration in Na+/K+ balance across cell membrane;
-Alterations in membrane potential, affects AP, calcium channels, etc.
Describe Potassium (K+), Effect of pH:
-pH decrease (Acidosis): H+ into cells, K+ out of cells = hyperkalaemia (& decreased losses in urine).
-pH increase (Alkalosis): Increase entry of K+ into cells = hypokalaemia (& increased losses in urine).
ONE NOTE
Hypokalaemia Main Causes?
Decreased intake, alkalosis, increased losses (vomiting, urinary losses
(diuretics)), impaired renal function.
Hyperkalaemia Main Causes?
Decreased excretion (renal failure), acidosis, medications like ACEi especially with K+ supplements.
