ECFV and Tonicity

Question 1

What are the distributions of body water in different compartments?

Answer 1

body water 60% weight for men, 50% for women
2/3 of body water intracellular, 1/3 extra
1/4 of extra cellular is intravascular

Question 2

What are the uses and limitations of the physical exam in the assessment of effective arterial volume?

Answer 2

increase in pulse or decrease in BP indicate large decreased volume
postural changes in pulse or BP more sensitive = tilt or orthostasis
also look for dry mucous membranes, JVD, sunken eyes, edema, pulm congestion, turgor
not applicable in autonomic neuropathies

Question 3

What are the uses and limitations of the plasma analysis in the assessment of effective arterial volume?

Answer 3

low volume leads to increased BUN and urea and vice versa
must be sure another process not causing this
BUN/creatinine ratio >20 suggest low volume

Question 4

What are the uses and limitations of the urine analysis in the assessment of effective arterial volume?

Answer 4

low fractional excretion of Na and Cl indicate decreased volume - more accurate than spot urine
must be sure low fractional excretion not due to primary Na retention

Question 5

What is the difference in solute composition of the intracellular and extracellular fluid?

Answer 5

no osmotic gradients usually because most cells freely permeable
Na and its salts in EC fluid - when measured indicates total body osmolality so low Na means low K in IC also

Question 6

What is the osmolar gap?

Answer 6

difference between calculated osmolality and measured plasma osmolality - measured >10 more indicates some uncalculated substance

Question 7

What are four (five) substances that can increase the osmolar gap?

Answer 7

ethanol
methanol
ethylene glycol
isopropyl alcohol
(acetone)

Question 8

What compartment do sensing mechanisms determining Na handling sense?

Answer 8

intravascular only = effective arterial volume

Question 9

What are the different sensors for osmoregulation vs. volume regulation?

Answer 9

hypothalamic osmoreceptors
low and high pressure baroreceptors
both affect ADH secretion

Question 10

What are the different effectors in osmoregulation and volume regulation?

Answer 10

AVP and thirst
Aldo, AII, catechols
thirst mechanism kicks in later than AVP

Question 11

What is the difference between osmolality and tonicity?

Answer 11

tonicity only includes particles that can’t penetrate cell membranes and thus osmotically pull water
urea is ineffective

Question 12

What do changes in plasma osmolality or EABV do to ADH secretion?

Answer 12

small increases in plasma osmolality lead to ADH secretion but larger decreases in EABV needed to do this
hypotonicity usually do to low EABV

Question 13

What is the pathophysiology giving rise to impaired renal water excretion?

Answer 13

decreased EABV causes inappropriate ADH secretion - low in patients with volume depletion or edematous states
causes hyponatremia

Question 14

What is psuedohyponatremia?

Answer 14

increases in plasma triglycerides or protein can cause error in measurement of Na
increases in glucose lead to hyponatremia but no changed osmolality - as glucose rises, serum Na falls

Question 15

What is primary/psychogenic polydipsia?

Answer 15

dilution normal - patient drinking more than kidney can excrete
evaluate by checking urine osmolality - should be normally dilute

Question 16

What is SIADH?

Answer 16

EABV normal, body fluids hypoosmolar, concentrated urine and high ADH levels inappropriate
diseases of CNS - those affecting hypothalamus, pulm diseases, carcionmas
also glucocorticoid def, certain central acting drugs, and renal failure

Question 17

What is the approach to a hyponatremic patient?

Answer 17

check plasma osmolality to see if matches Na
urine osmolality to rule out psuedo
urine osmolality high - assess EABV

Question 18

What are the three different types of hypernatremia and examples of each?

Answer 18

hypovolemic - loss of more water than Na -GI (diarrhea, vomiting), skin (burns, fever) kidney (osmotic diuresis)
euvolemic - diabetes insipidus
hypervolemic - hypertonic Na admin or feedings

Question 19

What conditions can lead to enhanced renal water loss (and thus hypernatremia)?

Answer 19

osmotic diuresis, central diabetes insipidus, nephrogenic DI

present as polyuria and polydipsia

Question 20

What is central DI?

Answer 20

conditions where ADH secretion from posterior pituitary is inhibited - trauma, tumors, granulomas, infection, familial

Question 21

What is nephrogenic DI?

Answer 21

ADH levels appropriate but still overly dilute urine - ADH resistance - urinary obstruction, hypokalemia, hypercalcemia, amyloidosis, sickle cell, polycystic KD, drugs, familial

Question 22

How can central and nephrogenic DI and primary polydipsia be distinguished?

Answer 22

response of urine osmolalilty to water deprivation and ADH admin -
PP - urine concentrates with water dep
CDI - urine concentrates with ADH
NDI - no urine concentration

Question 23

What are the clinical observations of a patient with hyponatremia?

Answer 23

acute - cerebral edema and sig. CNS symptoms - seizures or coma
chronic - neurologic symptoms unusual even with large losses

Question 24

What is the pathophysiology behind the clinical observations of hyponatremia?

Answer 24

Water moves into cells in brain causing swelling - cells and brain that transport solutes out to return to normal

Question 25

What can happen if chronic hyponatremia is returned to normal too quickly?

Answer 25

central pontine myelinolysis - cells shrink - quadriparesis, swallowing dysfunction, inability to speak, decreased mental status, psuedobulbar palsy

Question 26

What time period differentiates acute and chronic development of hypo/hypernatremia?

Answer 26

48 hours

Question 27

Why can acute hyponatremia be treated rapidly?

Answer 27

brain cells haven’t had time to transport solutes out yet
IV admin of hypertonic saline
can add furosemide if volume expansion is a problem

Question 28

What can happen if chronic hypernatremia is returned to normal too quickly?

Answer 28

cerebral edema

Question 29

How should hypernatremia be corrected?

Answer 29

water admin at rate leading to half correction in 24 hours