FEN: Osmolality Flashcards
Define osmolality
Osmolality is a measure of the osmoles of solute per Kg of solvent (Osm/kg)
Define osmolarity
Osmolarity is a measure of osmoles per liter of solution (Osm/L)
What is normal range for plasma osmolality?
275-290 mOsm/kg
How to convert from plasma osmolality to plasma osmolarity?
Plasma osmolarity (mOsm/L) = plasma osmolality x 0.995 (i.e. plasma osmolarity is about 1% lower than plasma osmolality)
List two ways normal plasma osmolality is maintained
- Thirst
2. Secretion of arginine vasopressin (i.e. antidiuretic hormone [ADH[) from the posterior pituitary
List three determinants of plasma osmolality from most to least impactful
- Sodium
- Glucose
- BUN
What is a common equation used to calculate plasma osmolality?
2*Na + Glucose/18 + BUN/2.8
What is the physiologic consequence of an increase in plasma osmolality?
- osmotic shift of fluid into the plasma
2. cellular dehydration and shrinkage
What is the physiologic consequence of a decrease in plasma osmolality?
- osmotic shift of fluid into cells
- cellular overhydration and swelling
- red blood cell swelling can cause cell rupture (i.e. hemolysis)
- Brain cells can swell, causing cerebral edema and herniation. Most likely to occur with acute hyponatremia (occuring in less than 2 days)
Describe isotonic, hypertonic and hypotonic fluids
- Isotonic do not cause fluid shift
- Hypertonic cause fluid shift from IC to the EC (i.e. IS, IV) compartment, causing dehydration and shrinkage
- Hypotonic shift fluid from EC to the IC, causing overhydration and swelling
Define equivalent weight
Equivalent weight = MW divided by valence
What is the MW, valence and equivalent weight of Sodium?
- MW: 23
- Valence: 1
- Equivalent weight: 23 g
What is the MW, valence and equivalent weight of Potassium
- MW: 39
- Valence: 1
- Equivalent weight: 39 g
What is the MW, valence and equivalent weight of Chloride
- MW: 35.5
- Valence: 1
- Equivalent weight: 35.5 g
What is the MW, valence and equivalent weight of Magnesium
- MW: 24
- Valence: 2
- Equivalent weight: 12 g
Define an osmole
Osmole is number of particles in a solution assuming complete dissociation
What is a milliequivalent?
1000 mEq per 1 equivalent
What is a milliosmole?
1000 mOsm per 1 osmole
Convert 23.4% NaCL to mEq/mL
- 23.4 grams per 100 mL
- MW of NaCl = 58.5
- Equivalent = 58.5 grams
(23.4 g/100 mL) *(1 equivalent/58.5 g) * (1000 mEq/1equivalent) = 4 mEq/mL
In general, how do you calculator the osmolarity of a fluid in mOsm/L?
- Convert a substance from grams or equivalents into moles.
2. Convert from moles into osmoles assuming complete dissociation or using osmotic coefficient
Calculate osmolarity of normal saline using the osmotic coefficient
- NS = 0.9 g/100mL
- 1 mol = 58.5 g
- 1 mol NaCl = 2 Osm
- 93% dissociation in fluid
(0.9 g/100 mL)(1 mol/58.5g)(2 Osm/1mol)(1000 mOsm/1 Osm)(1000mL/1L) *0.93 = 287 mOsm/L
Calculate the osmolarity of D5W
- D5W = 5 g/100 mL
- 1 mol = 198 g
- 1 mol = 1 Osm
- complete dissociation
(5 g/100 mL)(1 mol/198g)(1 Osm/1mol)(1000mOsm/1Osm)(1000 mL/1L) = 252.5 mOsm/L
Calculate the osmolarity of D5W/NS using the osmotic coefficient
- Osmolarity of NS w/ coefficient = 287 mOsm/L
- Osmolarity of D5W = 252.g mOsm/L
252.5 m/Osm/L + 287 mOsm/L = 539.5 mOsm/L
Calculate osmolarity of normal saline + potassium chloride 20 mEq/L
- 1 equivalent KCl = 74.5 g
- 1 mol KCl = 2 Osm
- 1 mEq = 0.0745 g; 20 mEq = 1.49 g
- (1.49 g/L)(1 mol/74.5g)(2 Osm/1mol)(1000 mOsm/1Osm)= 40 Osm/L
- Add osmolarity of NS + KCl = 287 mOsm/L + 40 mOsm/L = 327 mOsm/L
