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Nephrology Board > Electrolyte > Flashcards

Electrolyte Flashcards

1
Q

Difference Between Volume and
Water Disorder

A

Osmolarity = Water Balance

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2
Q

Osmole

A

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

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3
Q

Osmolarity

A

the concentration of osmotically active
particles in 1L of solution

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4
Q

Osmosis

A

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

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5
Q

Osmolarity

A
  • Osmolarity can be directly measured
  • can also be calculated
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6
Q

How can serum osmolarity be calculated?

A

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

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7
Q

Serum Sodium concentration is the main
determinant of Osmolarity

A
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8
Q

Dysnatremias are a Disorder of Water Metabolism?

A
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9
Q

Volume vs. Water

A
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10
Q

Volume vs. Water

A

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

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11
Q

How Much Water, How Much Volume?

A
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12
Q

Fluid Compartments

A
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13
Q

Pseudohyponatremia

A

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

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14
Q

Causes of Pseudohyponatremia

A

Diseases of lipids/protein
* Severe hyperlipidemia
* Paraproteinemias

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15
Q

ISO/Hypertonic Hyponatremia causes

A
  • Most commonly seen with hyperglycemia
  • Can also be seen with mannitol, glycine/sorbitol
    (dilutional hyponatremia)
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16
Q

ISO/Hypertonic Hyponatremia; correction for hyperglycemia

A

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

17
Q

True Hypotonic Hyponatremia

A
18
Q

Approach

A
19
Q

Urine Osmolality?

A

Surrogate for presence and activity of ADH

20
Q

The “appropriateness” of ADH

A
  • Dilute urine signifies suppressed ADH
  • Concentrated urine signifies increased ADH
21
Q

Hyponatremia with Low Urine Osmolality

A
  • ADH independent
    – Primary polydipsia
    – Beer potomania
    – Tea and toast diet
22
Q

Hypotonic hyponatremia with elevated urine
osmolality

A

implies ADH activity

23
Q

How solute Intake Drives Urine Output

A
24
Q

How solute Intake Drives Urine Output

A

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

25
Q

Assume only 100 mOsm of dietary solute.

A

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

26
Q

Hyponatremia Trivia

A

• 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

27
Q

Urinary Dilution?

A

Maximal dilution = urine osmolality 50-100

28
Q

suboptimal urine dilution?

A

Urine osmolality >100

29
Q

Causes of suboptimal urine dilution?

A

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

30
Q

Stimuli For ADH?

A

• ↑ 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)

31
Q

Is ADH release Appropriate?

A

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

32
Q

Consequences of Hyponatremia?

A

• 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

33
Q

Defense Against Water?

A

Organic osmolytes
– glutamate
– taurine
– myoinositol

34
Q

How is organic osmolytes released?

A

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

35
Q

Importance of organic osmolytes against water?

A

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

36
Q

Complication of organic osmolyte release?

A

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

Nephrology Board (1 decks)

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