sodium fluid balance Flashcards
What is the commonest electrolyte abnormality in hospitalized patients?
hyponatreamia
Serum sodium < 135 mmol/L
What is the underlying pathogenesis of hyponatraemia?
Increased EXTRAcellular water
Which hormone controls water balance?
ADH
how does ADH (vasopressin) control water?
promotes water retention by inserting
-> aquaporin-2 channels
into the
-> collecting duct cells
which receptors does adh act on for fluid balance? and where
V2
collecting duct cells
adh binds to V1 receptors ON _____ to promote ___
vascular smooth muscle
vasoconstriction
what 2 things increase adh secretion?
- Increased Serum osmolality (osmoreceptors)
- Reduced Blood volume/pressure
(baroreceptors in the carotids, atria and aorta)
What is the effect of increased ADH secretion on serum sodium?
hyponatraemia
because increased water retention = less sodium as a proportion in blood
What is the first step in the clinical assessment of a patient with hyponatraemia?
Clinical assessment of volume status
then
Urinary sodium
What are the clinical signs of hypovolaemia?
Low urine Na+ (<20) !
Reduced urine output Tachycardia Postural hypotension Dry mucous membranes Reduced skin turgor Confusion/drowsiness
what is the MOST RELIABLE clinical sign of hypovolaemia?
Low urine Na+ (<20) !
case ; patient on furosemide presents with tachycardia and confusion.
you find they are hypovolaemic.
Urine sodium normal.
what is potentially wrong?
the patient is on diuretics, they will have a high urine sodium regardless of hypovoalemia
Clinical Features of HYPERvolaemia?
○ Raised JVP
○ Bibasal crackles
○ Peripheral oedema
causes of hypo, hyper and euvolaemia broadly?
Hypovolaemics = things causing fluid loss Euvoleamics = Endocrine conditions Hypervolaemia = Organ failures
3 organs involved in hypervolaemia?
Heart
Liver
Kidney
urine sodium is high in which volume status?
euvolaemia
and hpovolaemia IF on diuretics
what osmolality picture characterises SIADH ?
○ Reduced plasma osmolality
○ Increased urine osmolality (> 100)
As a rule of thumb - urine osmolality will be HIGHER than plasma osmolality
because excess fluid retention due to adh secretion
What investigations would you order in a patient with euvolaemic hyponatraemia?
Hypothyroidism: Thyroid function tests
Adrenal insufficiency: Short Synacthen test
SIADH: Plasma & urine osmolality
How would you manage a hypovolaemic patient with hyponatraemia?
Volume replacement with 0.9% saline
How would you manage a euolaemic and
hypervolaemic patient with hyponatraemia?
Fluid restriction
Treat the underlying cause
signs of a Severe Hyponatraemia?
treatment?
- Reduced GCS
- Seizures
how to correct serum sodium?
what happens if corrected too fast?
Serum sodium must NOT be corrected faster than 8-10 mmol/L in the first 24 hours
it can result in osmotic demyelination (central pontine myelinolysis);
driplegia, dysarthria, dysphagia, seizures, coma and death
how to treat SIADH if fluid restriction not enough?
• Demeclocycline
○ Reduces responsiveness of collecting tubule cells to ADH
○ (risk of nephrotoxicity)
• Tolvaptan
○ V2 receptor antagonist
associated with rapid rises in serum sodium
define high blood sodium?
cause?
Defined as serum sodium concentration > 145 mmol/L
• Caused by unreplaced water loss i.e sweating etc
what is diabetes insidious?
Diabetes insipidus (DI) is a condition characterized by large amounts of dilute urine and increased thirst
Investigations for Diabetes Insipidus?
○ Serum glucose (exclude diabetes mellitus)
○ Serum potassium (exclude hypokalaemia)
§ NOTE: hypokalaemia can induce a nephrogenic diabetes insipidus ○ Serum calcium (exclude hypercalcaemia) ○ Plasma and urine osmolality ○ Water deprivation test
result of water deprivation test for Diabetes insipidus?
would still make lots of dilute urine.
give desmopressin and in central DI, osmolality returns to normal - concentrated.
what’s are the 3 steps in treating hypernatraemia?
Correct water deficit
5% dextrose
Correct extracellular fluid volume depletion
0.9% saline
Serial Na+ measurements
Every 4-6 hours
does diabetes cause hyper or hyponatraemia?
BOTH
Hyperglycaemia -> hyponatraemia
Osmotic diuresis in uncontrolled diabetes leads to loss of water and hypernatraemia
what are the Criteria to diagnose SIADH include the following (meeran book)?
• Hyponatraemia <135 mmol/L
- Plasma osmolality <270 mmol/L
- Urine osmolality >100 mmol/L
- High urine sodium >20 mmol/L
- Euvolaemia
- No adrenal, renal or thyroid dysfunction
purpose of urinary sodium in hyponatraemia analysis?
> 20 = Renal cause of hyponatraemia
< 20 = Non-renal
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