Chempath 12: Sodium And Fluid Balance Flashcards
What is the most common pathogenesis of hyponatraemia ?
Increased extracellular water
How does ADH increase water reabsorption ?
ADH binds to V2 receptors and causes increased Aquaporin 2 insertion into the collecting duct
Which receptors are found in smooth muscle and cause vasoconstriction ?
A) V1 receptors
B) V2 receptors
A) V1 receptors
They bind Vasopressin and cause vasoconstriction
V2 are found in collecting ducts and respond to ADH
What are the 2 main stimuli for ADH secretion ?
Osmoreceptors- detects high osmolality
Baroreceptors- detect low blood volume/pressure
Where are Osmoreceptors found in the body ?
Hypothalamus
List 3 locations where Baroreceptors are found in the body ?
Carotid
Aorta
atria
What is the most reliable clinical sign of hypovolaemia ?
Low urine Na+
Doesn’t work if on diuretics
List 4 clinical signs of hypovolaemia?
Tachycardia Postural hypotension Reduced skin turgor Dry mucous membranes Sunken eyes
List 4 causes of Hypovolaemic hyponatraemia ?
Diarrhoea
Vomiting
Diuretics
Salt losing nephropathy
List 3 causes of euvolaemic hyponatraemia ?
Hypothyroidism
Adrenal insufficiency
SIADH
List 3 causes of hypervolaemic Hyponatraemia ?
Cardiac failure
Nephrotic syndrome
Cirrhosis
How does liver cirrhosis cause hypervolaemic hyponatraemia ?
Cirrhosis causes release of Nitric oxide which causes vasodilation
This causes reduced blood pressure > sensed by baroreceptors
ADH release is increased.
More water is reabsorbed
How does hypothyroidism cause a Euvolaemic hyponatraemia ?
Hypothyroidism causes reduced cardiac contractility
This causes reduced blood pressure
This causes increased ADH release
More water is reabsorbed than Na+
List 3 causes of SIADH ?
CNS tumour
Small cell lung cancer
Meningitis/ subarachnoid haemorrhage
Drugs (psych drugs: SSRIs, Carbamazapine, Amitryptiline)
How do you treat hypovolaemic hyponatraemia ?
Volume replacement with 0.9% saline
If volume is returned to normal, the stimulus for ADH release is stopped so allows recovery.
How do you treat hypervolaemic and euvolaemic hyponatraemia ?
Fluid restriction
Treat underlying cause
Name the complication that can occur as a result of increasing serum sodium faster than 8-10 mol/L in the first 24 hours ?
Central pontine myelinolysis
Name 2 drugs used to treat SIADH ?
Tolvaptan - ADH receptor antagonist
Demeclocycline - reduces responsiveness of collecting duct cells to ADH
What level of serum sodium defines hypernatraemia ?
> 145 mmol/L
What level of serum sodium defines hyponatraemia ?
<135 mmol/L
Give 3 causes of Hypernatraemia ?
Diarrhoea and vomiting
Diabetes mellitus
Diabetes insipidus
3Ds of hypernatraemia
Name 2 tests used to diagnose diabetes insipidus ?
Serum and urine osmolality
Water deprivation test
Describe the possible outcomes of the water deprivation test ?
Normal: urine becomes very concentrated (sodium >800)
Cranial Diabetes insipidus: unable to concentrate urine (sodium <300)
Nephrogenic Diabetes insipidus : unable to concentrate urine (sodium <300)
Polydipsia: slightly able to concentrate urine, but physiological function has been damaged overtime (sodium <500)
What is the main treatment for hypernatraemia ?
5% Dextrose