Chemical Pathology 11 - Sodium and fluid balance Flashcards
What is the definition of hyponatraemia (commonest electrolyte abnormality)?
What is the underlying pathogenesis of hyponatraemia?
Serum sodium <135
Increased EXTRACELLULAR water
Upon which receptors does ADH act?
V2 (collecting duct - leads to insertion of aquaporins-2) and V1 (on vascular SMCs - leads to vasoconstriction at higher concentrations)
What are the 2 stimuli for ADH secretion?
Increased serum osmolality (detected by hypothalamic osmoreceptors)
Reduced blood volume/ pressure (mediated by baroreceptors in carotids, atria and aorta)
Vomiting - loss of water AND salt - causes hyponatraemia because reduced blood volume -> ADH secretion -> ONLY water reabsorption into the blood
What are the clinical signs of hypovolaemia?
Hypovolaemia: Tachycardia Postual hypotension Dry mucous membranes Reduce skin turgor Confusion/drowsiness Reduced urine output
MOST RELIABLE INDICATOR: LOW URINE Na+ (<20) - if you are hypovolaemic, you need to hold onto sodium so urine sodium will be low
Always remember to send off this test
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 - because they will be peeing out salts due to the medication
Urine sodium can also be low in very hypervolaemic patients -> high urine sodium points more towards euvolaemic hyponatraemia
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 plasma and urine osmolality in SIADH
Plasma = low (because it’s hyponatraemia!)
Urine = high (>100)
Diagnosis of SIADH can only be made if there is no hypovolaemia, no hypothyroidism and no adrenal insufficiency
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 - you’re giving fluid to shut down the issue that’s causing ADH secretion
Be careful - shouldn’t be given if they are NOT hypovolaemic as they will hang onto the water and exacerbate the issue (i.e. an ADH-secreting tumour will still continue to secrete ADH)
How do you manage a patient with hypervolaemic hyponatraemia?
Fluid restriction
Treat the underlying cause
How do you manage a patient with euvolaemic hyponatraemia?
Fluid restriction
Treat the underlying cause
If you give fluids, this will exacerbate the hyponatraemia
What are the symptoms of SEVERE hyponatraemia?
Reduced GCS
Seizures
What is the max rate of serum Na+ correction in hyponatraemia and why?
No more that 8-10mmol/L in 1st 24 hours
Risk of osmotic demyelination (central pontine myelinolysis - happens a few days later with quadraplegia, dysarthria, coma, death)
If sodium is corrected too quickly, reverse it with dextrose and desmopressin
How is SIADH treated?
Water restriction
PLUS (but both used rarely)
- Demeclocycline (reduces responsiveness of collecting tubule cells to ADH - but caution because nephrotoxic) OR
- Tolvaptan (V2 receptor agonist) - use cautiously as they work rapidly
Why does SIADH cause euvolaemia?
SIADH –> hypervolaemia (due to water retention) –> natiuretic peptide released from heart –> increased sodium excretion to try and pull water into urine –> euvolaemia (as you have now lost the water, but have also lost sodium in order to do so)
What are the main causes of hypernatraemia?
Unreplaced water loss
Due to GI losses or renal losses (eg diabetes insipidus)
Diarrhoea and vomiting usually causes hyponatraemia but can sometimes cause hypernatraemia
