Disorders of Water Balance Flashcards
maintenance of water balance
osmotic release of ADH, thirst
stimuli for ADH release
high osmolality (most sensitive)
low ECF, low BP, SIADH
*pressure trumps osmolality
total osmolality equation
2Na+ glucose/18 + BUN/2.8 + EtOH/3.7
tonicity equation
2Na + glucose/18
what value determines ICF volume
the inverse of tonicity (higher tonicity= less ICF)
water balance determined by Na concentration
ECF estimated by..
physical exam! signs of weight change, edema, crackles, JVD, BP, pulse increase (low ECF)
Na balance determined by volume
estimate total body Na
changes as ECF volume changes EXCEPT SIADH and pure water loss (higher ECF but stimulates naturesis)
describe electrolyte chart
left to right: Na, Cl, BUN
K, bicarb, Cr
glucose far right
define hyponatremia
low Na concentration- below 135
how to determine cause of hypotonic hyponatremia?
low Uosm: means no ADH, UNa will be low and this is from polydipsia or low solute intake
high Uosm: ADH is active
- hypovolemic- w/ true hypovolemia, urine Na will be low; from vomiting, diarrhea, burns
- hypervolemic- low EABV w/ low UNa; from CHF, cirrhosis, nephrosis
- euvolemic (from exam): high/normal UNa (no RAAS), likely SIADH
list broad causes of SIADH
- pulm disorders (ARDS, infections)
- tumors
- exercise
- drugs
- CNS disorders
- HIV
- pain
cause of pseudohyponatremia
artifact of flame flow cytometer, misjudges total volume as higher and perceives low Na concentration
Dx of pseduohyponatremia
normal Sosm, osmolal gap, asymptomatic, normal ECF, more proteins (myeloma)
causes of hypertonic hyponatremia
hyperglycemia, mannitol, glycine
increased tonicity causes water to flow out of cell, perceived as low Na concentration
tx for hypovolemic hyponatremia
correct volume, give isotonic saline
tx for SIADH
underlying cause, fluid restriction, salt tablets, loop diuretics, V2 antagonist, 3% saline
what is the risk of rapid hyponatremia correction
osmotic demyelination syndrome- body compensates for hyponatremia, making things more osmotic too quickly would cause rapid flow of water out of CNS cells
3 causes of hypernatremia
low ECF volume- hypotonic fluid loss
normal ECF- pure water loss
high ECF- hypertonic fluid gain
how to determine cause of low ECF hypernatremia
extrarenal: low urine volume, high osmolarity but low Na
renal loss: polyuria, dilute urine
-from osmotic diuresis or diuretics
find cause of normal ECF hypernatremia
renal loss: polyuria of dilute fluid, think DI
extrarenal: from loss at skin or lungs, low urine volume high Osm
causes of high ECF hypernatremia
hypertonic fluid gain, either:
high Na intake, see polyuria w/ higher osm
mineralcorticoid excess: see HTN and higher urine Na (pressure naturesis)
how does Uosm help w/ polyuria
w/ dilute urine: water diuresis from polydipsia or DI
w/ concentrated urine: osmotic or solute diuresis
determine cause of polyuria b/w polydipsia and DI
water deprivation: if Uosm goes up above 600, this means polydipsia
Uosm still low- means DI
-central DI will respond w/ higher Uosm following desmopressin, nephrogenic will not
risk w/ rapid correction of hypernatremia
cerebral edema- to much hypotonicity could cause ICF swelling in the brain
