Sodium and Fluid balance Flashcards
What is normal range for Na?
135-145
What is hyponatraemia range?
Na <135
What is the pathogenesis of hyponatraemia?
The problem is not salt loss
It is EXCESS WATER compared go the salt
As such you must always treat for the excess water, not the salt
What hormone controls water balance in the kidneys?
ADH
What is the function of ADH
It promotes water retention by inserting Aquaporin 2 channels into the collecting duct cells
What are physiological triggers to ADH secretion?
high urine osmolality (salt)
low blood volume)
What is the first step in the management of hyponatraemic patients?
ASSESS VOLUME STATUS
What are clinical features of hypovolaemia?
Tachycardia, postural hypotension Dry mucous membranes Reduced skin turgor Confusion, drowsiness Reduced urine output Low urine Na
What must you be aware of as urine findings if the patient is on diuretics?
They will have HIGH Na regardless
What are clinical features of hypervolaemia?
Raised JVP
Bibasal crackles
Peripheral oedema
What are causes of hypovolaemic hyponatraemia?
Diarrhoea, vomiting
Diuretics
Salt losing nephropahthy
How does hypovolaemic hyponatraemia occur?
With D&V > excess water loss > low perfusion pressure
This is detected by baroceptors > increase ADH production > increased water reabsorption
So more water is reabsorbed, and there is more water compared to salt
What are causes of euvolaemic hyponatraemia?
Hypothyroidism
Adrenal insufficiency
SIADH
How does hypothyroidism cause hyponatraemia?
Reduced cardiac congtractility > reduced BP detected by baroceptosr > more ADH > more water
What are causes of hypervolaemic hyponatraemia?
Cirrhosis
Cardiac failure
Renal failure
Summarise urinary sodium in the three hyponatraemias
Hypo/hypervolaemic: LOW
euvolaemic: high
What is SIADH
Syndrome of inappropriate ADH
caused by inappropriately released ADH e.g. CNS pathology, lung pathology,drugs, tumours, surgery,
What is plasma and urine osmolality in SIADH?
Plasma osmolality: low
Urine osmolality: HIGH
How do you treat hypovolaemic hyponatraemia?
Volume replacement with 0.9% SODIUM CHLORIDE
How do you treat euvolaemic hyponatraemia?
Fluid restrict + treat underlying cause
How do you treat hypervolaemic hyponatraemia?
Fluid restrict + treat underlying cause
What is clinical presentation of severe hyponatraemia’
Reduced GCS
Seizures
Seek expert help (to treat with 3% hypertonic saline)
What must you be aware of when correcting hyponatraemia?
CENTRAL PONTINE MYELINOLYSIS
So do not correct serum sodium too rapidly
what are symptoms of CENTRAL PONTINE MYELINOLYSIS
quadriplegia dysarthria dysphagia seizures coma death
What drugs do you use to treat SIADH, if fluid restriction is insufficient?
Demeclocycline
Tolvaptan
What serum level defines hypernatraemia?
Serum Na >145
What is hypernatraemia caused by?
Unreplaced water LOSS - should not happen as patient should feel thirsty and drink to compensate. So only occurs in elderly/fasting/can’t keep up with losses
- GI losses (nausea, vomiting)
- Sweat losses
- Renal losses (osmotic diuresis, diabetes insipidus, Conn’s)
How do you treat hypernatraemia?
5% DEXTROSE (fluid replacement)
Treat underlying cause
How do you treat someone with hypovolaemic hypernatraemia?
0.9% saline (for hypovolaemia)
5% dextrose (for hypernatraemia)
what is the difference between osmolaRity and osmolaLity
Osmolarity = calculated from blood test using FORMULA Osmolality = measurement of particles in solution, using MACHINE
What is the formula for osmolarity??
2(Na + K) + urea + glucose
what occurs in Gilbert’s syndrome?
Reduction in UDP glucuronyl transferase activity to 30%
what occurs in Crigler Najjar syndrome?
Complete deficiency of UDP glucuronyl transferase
What are causes of SIADH
CNS pathology (/meningitis, encephalitis, absecess)
Lung pathology (pneumonia, TB)
Drugs (SSRI, TCA; opiate, PPI, carbamazepine)
Tumour (non small cell lung cancer, breast cancxer)
Surgery
What does Conn’s do
Tumour causing excess production of Aldosterone > excess salt retention and K+ excretion
also low renin levels (as it is suppressed by aldosterone)
How do you test for conn’s
Aldosterone to renin ratio
How do you test for diabetes insipidus
water deprivation test
+ add desmopressin
what must you look at to check if it is TRUE hyponatraemia
serum OSMOLALITY
What do high/normala/low serum osmolality indicate in the context of hyponatraemia?
High osmolality: glucose/mannitol infusion
Normal osmolality: spurious, drip arm sample, pseudohyponatraeemia
Low osmolality: true hyponatraemia
what electrolyte imbalance can occur following TURP^
HYPONATREMIA
from irrigation absorbed through the damaged prostatte
How do glucose/mannitol in blood cause hyponatraemia?
Glucose and mannitol are osmotically active
they can draw water from the cells into the plasma
this dilutes down the sodium
what are causes of hypontraemia post surgery?
overhydration with hypotonic IIV fluids
Transient increase in ADH due to stress of surgeyr
what are the different neurological impacts of HYPOnatraemia vs HYPERnatraemia rapid correction
HYPOnatraemia: cerebral pontine myelinolysis aka locked in syndrome
HYPERNATRAEMIA: cerebral oedema
explain symptoms of diabetes insipidus
hypernatraemia (lethargy, thirst, irritaability, confusion, coma, fits)
polyuria, polydipsia
what are causes of cranial diabetes insipidus
surgery, trauma, tumours
LACK OF ADH
what are causes of nephrogeniuc diabetes insipidus
receptor defect (insensitivity to ADH)
thiazide diuretic
inherited channelopathies
lithium, demeclocyclinee, hypercalcaemia
Explain T4 renal tubular acidosis
ALDOSTERONE DEFICIENCY/RESISTANCE
causes acidosis and hyperkalaemia
