Chemical Pathology 11 - Sodium and fluid balance Flashcards
What is the definition of hyponatraemia?
Serum sodium <135
Upon which receptors does ADH act?
V1 (collecting duct) and V2 (on VSMCs)
What are the 2 stimuli for ADH secretion?
Serum osmolality (detected by hypothalamic osmoreceptors)
Blood volume/ pressure (mediated by baroreceptors in carotids, atria and aorta)
What are the clinical signs of hypovolaemia?
Tachycardia Postual hypotension Dry mucous membranes Reduce skin turgor Confusion/drowsiness Reduced urine output KEY: LOW URINE Na+ (<20)
Recall 4 causes of hypovolaemic hyponatraemia
Diarrhoea
Vomiting
Diuretics
Salt losing nephropathy
Recall 3 causes of euvolaemic hyponatraemia
Hypothyroidism
Adrenal insufficiency
SIADH
(Euvolaemic = Endocrine - 2 ‘E’s)
Recall 3 causes of hypervolaemic hyponatraemia
Cardiac failure
Cirrhosis
Nephrotic syndrome
In which patients can you not use urine sodium as a reliable test result?
Patients on diuretics
How does hypothyroidism cause euvolaemic hyponatraemia?
Hypothyroidism –> Reduced cardiac contractility –> detected by baroreceptors –> more ADH –> increased water resorption –> low plasma Osm secondary to dilution –> less water excreted in urine –> high urinary Osm
How does adrenal insufficiency cause euvolaemic hyponatraemia?
Adrenal insufficiency –> low aldosterone and cortisol
Aldosterone is necessary for sodium and water resorption, cortisol is necessary for water clearance, therefore you get excess ADH
What are the 5 main causes of SIADH?
CNS pathology Lung pathology Drugs (SSRI, PPI, opiates) Tumours Surgery
What 3 tests should be done in euvolaemic hyponatraemia?
TFTs for hypothyroidism
Short SynACTHen test for adrenal insufficiency
Plasma and urine osmolality for SIADH
Why is urine sodium low in cardiac failure (hypervolaemic hyponatraemia)
Hyperaldosteronism –> retention of sodium
What will be the urine and plasma and urine osmolality in SIADH
Plasma = low (because it’s hyponatraemia!)
Urine = high (>100)
What will urine sodium be in cardiac failure?
low
Why do you get hyperaldosteronism in cardiac failure?
Activation of RAAS
How do you manage a patient with hypovolaemic hyponatraemia?
Fluid replacement with 0.9% saline