Disorders of Water Balance

Question 1

maintenance of water balance

Answer 1

osmotic release of ADH, thirst

Question 2

stimuli for ADH release

Answer 2

high osmolality (most sensitive)

low ECF, low BP, SIADH
*pressure trumps osmolality

Question 3

total osmolality equation

Answer 3

2Na+ glucose/18 + BUN/2.8 + EtOH/3.7

Question 4

tonicity equation

Answer 4

2Na + glucose/18

Question 5

what value determines ICF volume

Answer 5

the inverse of tonicity (higher tonicity= less ICF)

water balance determined by Na concentration

Question 6

ECF estimated by..

Answer 6

physical exam! signs of weight change, edema, crackles, JVD, BP, pulse increase (low ECF)

Na balance determined by volume

Question 7

estimate total body Na

Answer 7

changes as ECF volume changes EXCEPT SIADH and pure water loss (higher ECF but stimulates naturesis)

Question 8

describe electrolyte chart

Answer 8

left to right: Na, Cl, BUN
K, bicarb, Cr

glucose far right

Question 9

define hyponatremia

Answer 9

low Na concentration- below 135

Question 10

how to determine cause of hypotonic hyponatremia?

Answer 10

low Uosm: means no ADH, UNa will be low and this is from polydipsia or low solute intake

high Uosm: ADH is active

1. hypovolemic- w/ true hypovolemia, urine Na will be low; from vomiting, diarrhea, burns
2. hypervolemic- low EABV w/ low UNa; from CHF, cirrhosis, nephrosis
3. euvolemic (from exam): high/normal UNa (no RAAS), likely SIADH

Question 11

list broad causes of SIADH

Answer 11

• pulm disorders (ARDS, infections)
• tumors
• exercise
• drugs
• CNS disorders
• HIV
• pain

Question 12

cause of pseudohyponatremia

Answer 12

artifact of flame flow cytometer, misjudges total volume as higher and perceives low Na concentration

Question 13

Dx of pseduohyponatremia

Answer 13

normal Sosm, osmolal gap, asymptomatic, normal ECF, more proteins (myeloma)

Question 14

causes of hypertonic hyponatremia

Answer 14

hyperglycemia, mannitol, glycine

increased tonicity causes water to flow out of cell, perceived as low Na concentration

Question 15

tx for hypovolemic hyponatremia

Answer 15

correct volume, give isotonic saline

Question 16

tx for SIADH

Answer 16

underlying cause, fluid restriction, salt tablets, loop diuretics, V2 antagonist, 3% saline

Question 17

what is the risk of rapid hyponatremia correction

Answer 17

osmotic demyelination syndrome- body compensates for hyponatremia, making things more osmotic too quickly would cause rapid flow of water out of CNS cells

Question 18

3 causes of hypernatremia

Answer 18

low ECF volume- hypotonic fluid loss

normal ECF- pure water loss

high ECF- hypertonic fluid gain

Question 19

how to determine cause of low ECF hypernatremia

Answer 19

extrarenal: low urine volume, high osmolarity but low Na

renal loss: polyuria, dilute urine
-from osmotic diuresis or diuretics

Question 20

find cause of normal ECF hypernatremia

Answer 20

renal loss: polyuria of dilute fluid, think DI

extrarenal: from loss at skin or lungs, low urine volume high Osm

Question 21

causes of high ECF hypernatremia

Answer 21

hypertonic fluid gain, either:

high Na intake, see polyuria w/ higher osm

mineralcorticoid excess: see HTN and higher urine Na (pressure naturesis)

Question 22

how does Uosm help w/ polyuria

Answer 22

w/ dilute urine: water diuresis from polydipsia or DI

w/ concentrated urine: osmotic or solute diuresis

Question 23

determine cause of polyuria b/w polydipsia and DI

Answer 23

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

Question 24

risk w/ rapid correction of hypernatremia

Answer 24

cerebral edema- to much hypotonicity could cause ICF swelling in the brain