Electrolyte Flashcards
Difference Between Volume and
Water Disorder
Osmolarity = Water Balance
Osmole
the amount of a substance that
dissociates in solution to form one mole of
osmotically active particles
Osmolarity
the concentration of osmotically active
particles in 1L of solution
Osmosis
the passage of water from an area of
high to low water
concentration through a semi-permeable
membrane
Osmolarity
- Osmolarity can be directly measured
- can also be calculated
How can serum osmolarity be calculated?
2 [Na] + [glucose]/18 + [urea N]/2.8]
Normal= [2(140)]+ [90/18] + [12/2.8] ~ 290
Serum Sodium concentration is the main
determinant of Osmolarity
Dysnatremias are a Disorder of Water Metabolism?
Volume vs. Water
Volume vs. Water
Volume = isotonic
– E.g. Normal saline, 0.9% NaCl
Water=hypotonic
– Electrolyte free solution, e.g. D5W I
How Much Water, How Much Volume?
Fluid Compartments
Pseudohyponatremia
Seen when using indirect ion-selective electrode
measurements
(sample is diluted prior)
Causes of Pseudohyponatremia
Diseases of lipids/protein
* Severe hyperlipidemia
* Paraproteinemias
ISO/Hypertonic Hyponatremia causes
- Most commonly seen with hyperglycemia
- Can also be seen with mannitol, glycine/sorbitol
(dilutional hyponatremia)
ISO/Hypertonic Hyponatremia; correction for hyperglycemia
Correction factor of 1.6 mEq/L for every 100 mg/dL increase in glucose
True Hypotonic Hyponatremia
Approach
Urine Osmolality?
Surrogate for presence and activity of ADH
The “appropriateness” of ADH
- Dilute urine signifies suppressed ADH
- Concentrated urine signifies increased ADH
Hyponatremia with Low Urine Osmolality
- ADH independent
– Primary polydipsia
– Beer potomania
– Tea and toast diet
Hypotonic hyponatremia with elevated urine
osmolality
implies ADH activity
How solute Intake Drives Urine Output
How solute Intake Drives Urine Output
If ADH is suppressed, maximal urine dilution is
achieved
– 50 mOsm/L in 18L → 900 mOsm excreted
Assume only 100 mOsm of dietary solute.
With maximal dilution, only 2L of water can be ingested before it exceeds your maximal urine output
Hyponatremia Trivia
• 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
Urinary Dilution?
Maximal dilution = urine osmolality 50-100
suboptimal urine dilution?
Urine osmolality >100
Causes of suboptimal urine dilution?
– Impaired ability of renal dilution (diminished GFR, thiazide diuretic)
– Presence of ADH (SIADH, volume depletion, nausea, pain)
– Abnormal ADH receptor in cortical collecting duct
Stimuli For ADH?
• ↑ 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)
Is ADH release Appropriate?
Syndrome of Inappropriate ADH (SIADH)
– Euvolemic
– Elevated urine osmolality (often higher than serum osmolality)
– Low uric acid
Consequences of Hyponatremia?
• 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
Defense Against Water?
Organic osmolytes
– glutamate
– taurine
– myoinositol
How is organic osmolytes released?
Released from cells through
– volume-sensitive leak pathways
– specific transporters
Importance of organic osmolytes against water?
Allows the cell to lower intracellular solute concentrations to equal the extracellular hypotonic plasma → preserved cell volume
Complication of organic osmolyte release?
ODS
– Shift of organic osmolytes takes several days
– Important in development of osmotic demylination syndrome if chronic hyponatremia is rapidly corrected
