8 - Dysnatremias

Question 1

Edelman’s equation

Answer 1

[serum Na] = 1.11 ([exchangeable Na] + [exchangeable K])/TBW - 25.6

Question 2

4 common causes of hypernatremia

Answer 2

dec access to water
defective thirst mechanism
impaired ADH release/response (DI)
dec urinary concentrating ability (CRI)

Question 3

how does brain adapt in hypernatremia?

Answer 3

initial cell shrinkage
maintain volume due to accumulation of electrolytes
followed by organic osmolytes accumulation and extrusion of electrolytes

Question 4

why (mechanistically) do you have to correct hypernatremia slowly?

Answer 4

dissipation of organic solutes from brain during correction is slow
if you do it fast, water will shift to brain > edema

Question 5

causes of nephrogenic DI

Answer 5

mutation in V2 receptor or AQP2
Li therapy
hypokalemia
hypercalcemia
obstruction
sickle cell anemia
chronic renal disease

Question 6

tx of DI

Answer 6

central - give ddAVP
nephrogenic - tx underlying cause

chlorpopamide, clofibrate, carbamazepine, HCTZ

Question 7

how do you calc water deficit

Answer 7

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

Question 8

what fluid should you use to correct hypernatremia?

Answer 8

water when possible

D5W is second choice, but must watch for hyperglycemia

Question 9

what happens if you correct hypernatremia faster than 0.5 mEq/h?

Answer 9

brain edema

central pontine myelinolysis

Question 10

when does acute hyponatremia occur?

Answer 10

almost always hospital acquired
generally from excess hypotonic fluids
may be assoc w/ brain edema and dec cerebral blood flow

Question 11

stimuli for ADH release

Answer 11

effective blood volume ***
plasma osmolality
nausea
pain
stress
hormones

Question 12

3 clinical settings assoc w/ acute hyponatremia

Answer 12

post op
inappropriate pain management
oxytocin tx

Question 13

clinical settings of chronic (>48hrs) hyponatremia

Answer 13

diuretics (thiazide)
SIADH
compulsive water drinking
uncompensated psychosis
drugs (chlorpropamide, clofibrate, carbamazepine)

Question 14

features of pontine myelinolysis

Answer 14

UMN disorders
spastic quadreparesis
pseudobulbar palsy
confusion
coma

Question 15

which group of people tends to have more serious complications from post op hyponatremia?

Answer 15

women (esp menstruant) more likely to have encephalopathy, hypoxia/resp arrest, permanent brain damage, death

Question 16

what factor makes women more vulnerable to complications from hyponatremia?

Answer 16

inhibition of Na-K ATPase by estrogen and progesterone impairs volume regulation in brain and lungs

Question 17

how fast can you correct Na in hyponatremia to avoid myelinolysis?

Answer 17

<18 meq/48h

Question 18

factors making myelinolysis w/ chronic hyponatremia more likely

Answer 18

hypokalemia (dec brain perfusion)
liver dz (inc female hormones, dec brain inositol pool)
malnutrition
burns
hypoxemia
female gender

Question 19

osmotically active organs/cells

Answer 19

kidney medulla
brain
liver
heart - sorta
monocytes
endothelial cells

Question 20

2 important organic solutes used as osmolytes

Answer 20

taurine and inositol

Question 21

during correction of hyponatremia, the brain becomes ___, so you must go slowly in order to give time for ____

Answer 21

dehydrated

reclamation of organic solutes

Question 22

correction of hyponatremia

Answer 22

restore volume w/ normal saline if hypovolemic
fluid restriction if eu or hyper volemic

if symptomatic,
saline initially until euvolemic
then 3% NaCl +/- diuretics

Question 23

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.

Answer 23

b

Question 24

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.

Answer 24

B

Question 25

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

Answer 25

b

Question 26

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

Answer 26

b

Question 27

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

Answer 27

a

Question 28

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

Answer 28

c