#34 - Water balance Flashcards
what is total body water, how is it divided?
total body water= 60% of body weight.
65% is intracellular fluid,
35% extracellular fluid (28% interstitial, 7% plasma)
which 3 systems work together to maintain plasma osmolality within a tight range?
- ADH
- thirst (access to water)
- renal function
hypernatremia and hyponatremia are disorders of __/
water homeostasis.
Algorithm for assessing hyponatremia
1 - look at plasma osmolality!!
T/F - Osmolality throughout the body (in all compartments) is generally the same.
True. This is because, outside the kidney, water moves freely across all body fluid compartments.
What are the primary determinants of plasma osmolality?
- glucose
- sodium
- BUN
normal plasma osmolality
280-290
what is the plasma osmolality at which ADH starts to be secreted?
280 and above
what is the basic equation for serum Na
serum Na = total body Na/ total body H20
what is the equation for Posm?
2*Na + glu/18 + BUN/ 2.8
notice that Na is multiplied while the other factors are divided.
Case:
- hyponatremia
- high plasma osmolality
likely cause
hypertonic hyponatremia - -> common cause = diabetes high glucose pulls water out of cells, diluting serum Na. But glucose is part of osmolality, so osmolality remains high.
symptoms of low osmolality
cerebral edema –> neurologic symptoms!
- mental status change
- seizure
- brain herniation due to swelling
case:
- hyponatremia
- low Posm
Next step?
- proceed down the algorithm:
- determine if it is ADH mediated or ADH independent
- if ADH mediated, what is the volume status?
describe the algorithm for hyponatremia
hyponatremic patient
- order plasma osmolality (should be low)
- determine if ADH mediated or ADH independent
-if ADH mediated, determine the volume status.
Causes of ADH independent hyponatremia
- low solute diet
- excessive water drinking (intake exceeds kidney’s ability to make dilute urine)
- renal failure (no urine output)
cause of ADH dependent natremia
kidney is unable to make a dilute urine, due to ADH. Makes small concentrated urine at the wrong time.
3 triggers of ADH release
- high osmolality
- decrease in blood pressure/ volume (perceived or real)
-nonphysiologic (SIADH = syndrome of inappropriate ADH)
What happens to the ADH set point in volume depleted / low BP states?
ADH set point is lowered, and the curve is steeper.
ie, it will start to be released starting at Posm =270 instead of 280, and more will be released quicker.
Define SIADH (diagnostic criteria)
- low serum osmolality (+hyponatremia)
- inappropriately concentrated urine
- -euvolemia
What causes SIADH? - 3 categories
Numerous causes, but 3 big ones are:
- pulmonary disease
- cancer
- medications
How do you determine if hyponatremia is ADH dependent or ADH independent?
look at urine osmolality!!
-ADH independent will have low urine osmolality
-ADH dependent will have (inappropriately) high urine osmolality.
How do you assess volume status?
- difficult
- with physical exam
- tachycardia, orthostatic hypotension
treatment for hyponatremia: who gets treated?
if symptomatic (altered mental status), treat aggressively.
if asymptomatic, just limit fluids.
3 hyponatremia causes that are likely to be acute and need treatment
- water intoxication
- post-operative
- thiazide diuretics
