8 - Dysnatremias Flashcards
Edelman’s equation
[serum Na] = 1.11 ([exchangeable Na] + [exchangeable K])/TBW - 25.6
4 common causes of hypernatremia
dec access to water
defective thirst mechanism
impaired ADH release/response (DI)
dec urinary concentrating ability (CRI)
how does brain adapt in hypernatremia?
initial cell shrinkage
maintain volume due to accumulation of electrolytes
followed by organic osmolytes accumulation and extrusion of electrolytes
why (mechanistically) do you have to correct hypernatremia slowly?
dissipation of organic solutes from brain during correction is slow
if you do it fast, water will shift to brain > edema
causes of nephrogenic DI
mutation in V2 receptor or AQP2 Li therapy hypokalemia hypercalcemia obstruction sickle cell anemia chronic renal disease
tx of DI
central - give ddAVP
nephrogenic - tx underlying cause
chlorpopamide, clofibrate, carbamazepine, HCTZ
how do you calc water deficit
initial Na * initial TBW = current Na * current TBW
use current Na and 1/2 of current weight for TBW, then figure out how much TBW was when Na was 140 and the difference is the deficit
what fluid should you use to correct hypernatremia?
water when possible
D5W is second choice, but must watch for hyperglycemia
what happens if you correct hypernatremia faster than 0.5 mEq/h?
brain edema
central pontine myelinolysis
when does acute hyponatremia occur?
almost always hospital acquired
generally from excess hypotonic fluids
may be assoc w/ brain edema and dec cerebral blood flow
stimuli for ADH release
effective blood volume *** plasma osmolality nausea pain stress hormones
3 clinical settings assoc w/ acute hyponatremia
post op
inappropriate pain management
oxytocin tx
clinical settings of chronic (>48hrs) hyponatremia
diuretics (thiazide) SIADH compulsive water drinking uncompensated psychosis drugs (chlorpropamide, clofibrate, carbamazepine)
features of pontine myelinolysis
UMN disorders spastic quadreparesis pseudobulbar palsy confusion coma
which group of people tends to have more serious complications from post op hyponatremia?
women (esp menstruant) more likely to have encephalopathy, hypoxia/resp arrest, permanent brain damage, death
what factor makes women more vulnerable to complications from hyponatremia?
inhibition of Na-K ATPase by estrogen and progesterone impairs volume regulation in brain and lungs
how fast can you correct Na in hyponatremia to avoid myelinolysis?
<18 meq/48h
factors making myelinolysis w/ chronic hyponatremia more likely
hypokalemia (dec brain perfusion) liver dz (inc female hormones, dec brain inositol pool) malnutrition burns hypoxemia female gender
osmotically active organs/cells
kidney medulla brain liver heart - sorta monocytes endothelial cells
2 important organic solutes used as osmolytes
taurine and inositol
during correction of hyponatremia, the brain becomes ___, so you must go slowly in order to give time for ____
dehydrated
reclamation of organic solutes
correction of hyponatremia
restore volume w/ normal saline if hypovolemic
fluid restriction if eu or hyper volemic
if symptomatic,
saline initially until euvolemic
then 3% NaCl +/- diuretics
A 25 year old motorcycle enthusiast undergoes orthopedic surgery for repair of multiple femoral fractures. Post operatively he is given IV morphine for pain. He is slightly edematous from saline infusion during surgery. Which statement about ADH is true.
A. ADH level is probably high because of the saline infusion.
B. ADH level is high in part due to narcotic administration.
C. ADH level is probably low due to the stress of surgery.
D. ADH level is probably high due to elevation in renin.
b
An asymptomatic patient has diabetes insipidus. Which statement about his serum electrolytes is/are true?
A. He has adapted to chronic hypernatremia by generating idiogenic osmoles
B. He is chronically thirsty, and serum sodium is normal.
C. His chronic urinary sodium losses will result in hypokalemia.
D. His urinary chloride will be elevated.
B
An elderly lady is started on a diuretic for hypertension. She returns two weeks later with altered mental status and severe hypotonic hyponatremia with hypokalemia. Which statement is true.
A. Caused by loop diuretic overdosage
B. Potassium administration raises serum sodium
C. Rapid correction to normal sodium value prevents neurological damage
D. Thiazide diuretic and normal saline correct hyponatremia
b
A patient has congestive heart failure and volume overload. Which physical finding or lab value is not compatible with volume overload.
A. Low urinary fractional excretion of sodium.
B. Elevated hematocrit
C. Hyponatremia
D. Hypokalemia
b
Which statement regarding the sodium channel found on the luminal membrane of principal cells is false?
A. Aldosterone decreases sodium flux through channel, from lumen to cell interior.
B. Increased delivery of sodium to the collecting duct (e.g. an increase in lumen sodium concentration) increases flux of sodium through channel into the principal cell.
C. Flux of sodium through channel makes the tubule lumen negatively charged.
D. Blockade of sodium flux through the channel by triamterene (e.g. a potassium sparing diuretic) makes the tubule lumen less negatively charged
a
Which of the following mechanisms of action explains why some diuretics are potassium sparing?
A. Carbonic anhydrase inhibition
B. Block the sodium-potassium-2 chloride exchanger
C. Decrease sodium flux through principal cell sodium channel
D. Block aquaporin channels in collecting duct
c
