Electrolyte

Question 1

Difference Between Volume and
Water Disorder

Answer 1

Osmolarity = Water Balance

Question 2

Osmole

Answer 2

the amount of a substance that
dissociates in solution to form one mole of
osmotically active particles

Question 3

Osmolarity

Answer 3

the concentration of osmotically active
particles in 1L of solution

Question 4

Osmosis

Answer 4

the passage of water from an area of
high to low water
concentration through a semi-permeable
membrane

Question 5

Osmolarity

Answer 5

• Osmolarity can be directly measured
• can also be calculated

Question 6

How can serum osmolarity be calculated?

Answer 6

2 [Na] + [glucose]/18 + [urea N]/2.8]
Normal= [2(140)]+ [90/18] + [12/2.8] ~ 290

Question 7

Serum Sodium concentration is the main
determinant of Osmolarity

Answer 7

Question 8

Dysnatremias are a Disorder of Water Metabolism?

Answer 8

Question 9

Volume vs. Water

Answer 9

Question 10

Volume vs. Water

Answer 10

Volume = isotonic
– E.g. Normal saline, 0.9% NaCl
Water=hypotonic
– Electrolyte free solution, e.g. D5W I

Question 11

How Much Water, How Much Volume?

Answer 11

Question 12

Fluid Compartments

Answer 12

Question 13

Pseudohyponatremia

Answer 13

Seen when using indirect ion-selective electrode
measurements
(sample is diluted prior)

Question 14

Causes of Pseudohyponatremia

Answer 14

Diseases of lipids/protein
* Severe hyperlipidemia
* Paraproteinemias

Question 15

ISO/Hypertonic Hyponatremia causes

Answer 15

• Most commonly seen with hyperglycemia
• Can also be seen with mannitol, glycine/sorbitol
  (dilutional hyponatremia)

Question 16

ISO/Hypertonic Hyponatremia; correction for hyperglycemia

Answer 16

Correction factor of 1.6 mEq/L for every 100 mg/dL increase in glucose

Question 17

True Hypotonic Hyponatremia

Answer 17

Question 18

Approach

Answer 18

Question 19

Urine Osmolality?

Answer 19

Surrogate for presence and activity of ADH

Question 20

The “appropriateness” of ADH

Answer 20

• Dilute urine signifies suppressed ADH
• Concentrated urine signifies increased ADH

Question 21

Hyponatremia with Low Urine Osmolality

Answer 21

• ADH independent
  – Primary polydipsia
  – Beer potomania
  – Tea and toast diet

Question 22

Hypotonic hyponatremia with elevated urine
osmolality

Answer 22

implies ADH activity

Question 23

How solute Intake Drives Urine Output

Answer 23

Question 24

How solute Intake Drives Urine Output

Answer 24

If ADH is suppressed, maximal urine dilution is
achieved
– 50 mOsm/L in 18L → 900 mOsm excreted

Question 25

Assume only 100 mOsm of dietary solute.

Answer 25

With maximal dilution, only 2L of water can be ingested before it exceeds your maximal urine output

Question 26

Hyponatremia Trivia

Answer 26

• Lowest reported serum Na was a case of
chronic schizophrenic with water intoxication,
lowering her plasma Na to 84 mEq/L (Langgard,
NEJM 1962)
• Effect of excess water in animal studies first
demonstrated in 1926
• First fatal hyponatremia case described in 1935
when a 50 year old woman undergoing
cholecystectomy received 9L of intravenous
hypotonic solution during surgery

Question 27

Urinary Dilution?

Answer 27

Maximal dilution = urine osmolality 50-100

Question 28

suboptimal urine dilution?

Answer 28

Urine osmolality >100

Question 29

Causes of suboptimal urine dilution?

Answer 29

– Impaired ability of renal dilution (diminished GFR, thiazide diuretic)
– Presence of ADH (SIADH, volume depletion, nausea, pain)
– Abnormal ADH receptor in cortical collecting duct

Question 30

Stimuli For ADH?

Answer 30

• ↑ Tonicity – osmoreceptors stimulate release
• Volume depletion (either true or ineffective arterial volume – e.g. CHF, cirrhosis)
• Pain, nausea (marathon runners)
• Medications (antidepressants, antiepileptics, antipsychotics, ecstacy)
• Tumors (paraneoplastic ADH release)

Question 31

Is ADH release Appropriate?

Answer 31

Syndrome of Inappropriate ADH (SIADH)
– Euvolemic
– Elevated urine osmolality (often higher than serum osmolality)
– Low uric acid

Question 32

Consequences of Hyponatremia?

Answer 32

• Cell volume dysregulation
– Osmotic swelling (intracellular →extracellular osmolality)
– Severe hypotonicity will lead to an intracellular influx of water → cell apoptosis
– Gradual hypotonicity will lead to increased intracellular volume
• Brain cell volume is the most sensitive to change

Question 33

Defense Against Water?

Answer 33

Organic osmolytes
– glutamate
– taurine
– myoinositol

Question 34

How is organic osmolytes released?

Answer 34

Released from cells through
– volume-sensitive leak pathways
– specific transporters

Question 35

Importance of organic osmolytes against water?

Answer 35

Allows the cell to lower intracellular solute concentrations to equal the extracellular hypotonic plasma → preserved cell volume

Question 36

Complication of organic osmolyte release?

Answer 36

ODS
– Shift of organic osmolytes takes several days
– Important in development of osmotic demylination syndrome if chronic hyponatremia is rapidly corrected