Chemical Pathology - Sodium and Fluid Balance Flashcards
define hyponatraemia
serum sodium < 135 mmol/L
what is the underlying pathogenesis of hyponatraemia?
increased extracellular water
what is water balance controlled by?
ADH
promotes water retention by inserting aquaporin-2 channels into collecting duct cells
where does ADH act?
- V2 receptors on collecting duct (insertion of aquaporin-2)
- V1 on VSMC (causes vasopressin, ADH known as vasopressin here)
what are the 2 main stimuli for ADH secretion?
- inc in serum osmolality (mediated by hypothalamic osmoreceptors)
- decrease blood volume/ pressure (mediated by baroreceptors in carotids, atria, aorta)
what is the first step in the management of a patient with hyponatraemia?
- assess volume status
what are the clinical features of hypovolaemia?
- tachycardia
- postural hypotension
- dry mucous membrane
- reduced skun turgor
- confusion/drowsiness
- dec urine outpur
- low urine Na (<20)
what is the most reliable clinical sign of hypovolaemia?
- low urine Na
if pt on diuretics, they will have high urine Na
what are the clinical features of hypervolaemia?
- raised JVP
- bibasal crackles
- peripheral oedema
causes of hypovolaemic hyponatraemia?
- diarrhoea
- vomiting
- diuretics
- salt losing nephropathy
causes of euvolaemic hyponatraemia?
- hypothyroidism
- adrenal insufficiency
- SIADH
causes of hypervolaemic hyponatraemia?
- cardiac failure
- cirrhosis
- nephrotic syndrome
explain how hypovolaemic hyponatraemia works
- hypovolaemic pt still have excess water
- if you have D&V, you will lose a lot of salt and water
- get a low perfusion pressure and inc in ADH release
- pt then reabsorb more water than salt
= hyponatraemia
how does cirrhosis cause hyponatraemia?
leads to release of various vasodilators
leads to a drop in perfusion pressure
causes of SIADH
- CNS pathology
- Lung pathology
- Drugs (SSRI, TCA, opiates, PPIs, carbamazepine)
- Tumours
- Surgery
how do you make a diagnosis of SIADH?
- no hypovolaemia
- no hypothyroidism
- no adrenal insufficiency
- reduced plasma osmolality
- increased urine osmolality (>100)
how should you investigate hypovolaemic hyponatraemia?
- clinically hypovolaemic?
how should you investigate euvolaemic hyponatraemia?
- TFTs
- short synacthen test
- plasma and urine osmolality
how should you investigate hypervolaemic hyponatraemia?
- ? Fluid overload
Tx of hypovolaemic hyponatraemia
- volume replacement with 0.9% saline
will replenish circulating fluid volume to normal levels, switch off stimulus for ADH release
Tx of hypervolaemic hyponatraemia
- fluid restriction
- treat underlying cause
Tx of euvolaemic hyponatraemia
- fluid restriction
- treat underlying cause
symptoms of severe hyponatraemia
- reduced GCS
- seizures
- seek expert help (treat with 3% hypertonic saline)
what is an important consideration when treating hyponatraemia?
- serum sodium must NOT be corrected faster than 8-10mmol/L in first 24 hours
- if corrected too rapidly = osmotic demyelination (central pontine myelinolysis)
how does central pontine myelinolysis present?
- quadraplegia
- dysarthria
- dysphagia
- seizures
- coma
- death
when are drugs used to treat SIADH?
if fluid restriction is insufficient
which drugs can be used?
- Demeoclocycline (reduces responsiveness of collecting tubule cells to ADH, monitor U&Es as risk of nephrotoxicity)
- Tolvaptan (V2 receptor antagonist)
define hypernatraemia
serum Na concentration >145 mmol/L
what is hypernatraemia caused by?
unreplaced water loss
- GI losses
- sweat losses
- renal losses (osmotic diuresis, diabetes insipidus)
who mostly gets hypernatraemia?
patients who tend not to drink when they are dehydrated (e.g. elderly and children)
what are the investigations for diabetes insipidus?
- serum glucose (exclude DM)
- serum potassium (exclude hypokalaemia as can induce nephrogenic DI)
- serum Ca (exclude hypercalcaemia)
- plasma and urine osmolality
- water deprivation test
how do you treat hypernatraemia?
- fluid replacement (give dextrose because this will replace fluid without giving excess salt)
- treat underlying cause
how would you treat someone who was hypovolaemic and hypernatraemic?
- initially give 0.9% saline to treat hypovolaemia
- followed by dextrose for hypernatraemia
how does diabetes mellitus affect serum sodium>
- hyperglycaemia will draw water out of cells = hyponatraemia
- osmotic diuresis in uncontrolled diabetes leads to loss of water = hypernatraemia
