Note: The following has three sections. Section One has definitions and basic concepts. Read through it quickly, line by line, without skipping - it was written to be read that way. Section Two is a table containing the constituents of popularly used IV fluids. Section Three is a brief account of the crystalloids.

I wrote the first section. The second and third sections are lifted from various websites ;)

Definitions and basic concepts

Mole
One mole is the amount of a substance which contains an Avogadro number of its fundamental particles. This is applicable to both elements and compounds.

The gram atomic weight/gram molecular weight is the standard for the definition of the mole ( and the Avogadro number).

The atomic number of an element is the number of protons contained in its nucleus. The gram atomic weight of an element is the same number of grams of the element as its atomic number.

For example, C-12 has twelve protons. So its atomic number is 12 and 12g of carbon = one gram atomic weight

The concept of gram molecular weight is similar - the algebraic sum of its constituent elements' atomic numbers.
For eg: for water...

One gram molecular weight = H+H+O = 1+1+16 = 18g.

And 18g of water contains 6.023 x 10²³ number of molecules of water.

Equivalent weight
Equivalence = number of moles X valence

Valence corresponds to total electrically dissociated chemical reactants.
For example: one mole of Na⁺ = one equivalent of Na⁺ (because the valence of sodium is one)
One mole of CaCl₂ = 4 eq of CaCl₂

Albumin's valence at pH 7.4 is 18

=> one molar solution of albumin (one mole of albumin (69,000g) dissolved in 1L of water) = 18 eq/L of Albumin

Since body fluids are usually much more dilute, mEq/L and mmol/L are used as measuring units

mEq/L = mmol/L x v

Osmolarity
A mole of a solute that dissolves to give 'n' discrete particles in solution is equal to 'n' osmoles of solute

For example: 1 mol of Na+ = 1 osmol of Na+
1 mol of CaCl₂ = 3 osmol of CaCl₂ (because one molecule of CaCl₂ will completely dissociate to give three osmotically active particles)

1 mol of glucose = 1 osmol of glucose

Osmolality
1 osmolal solution = 1 mole of solute/1 kg of water (or solute)

Osmolality is defined as such and used because - it is independent of the temperature of the solution
                                                                        - it is independent of the volume occupiied by the solutes in the solution

Again...

mosmol/ Kg H₂O  = mmol/Liter x n 
(where n is the number of discrete particles a molecule of the solute gives rise to, upon dissolution)

Principle: All the fluids in the major compartments of the body are isoosmolar to each other

Now...

Osmolality of plasma = P_osm = Osmolality of electrolytes + osmolality of glucose + osmolality of urea

Osmolality of electrolytes:
Since Na+ forms the major electrolyte in plasma, only Na+ is considered. And since Na+ is univalent - it gives rise to two osmotically active particles.
So:
osmolality of electrolytes (mosmol/kg H2O) ~ 2P_Na (mmol/liter) (for example: 2 x 138 mmol/L)

Osmolality of glucose:
Osmolality of glucose (mosmol/kg H₂O) = P_G (mmol/liter)
Since clinical laboratories measure glucose concentrations in milligrams per deciliter (converting mmol/L of glucose into mg/dL)

one mole of glucose = 180 g of glucose
1 mmol of glucose = 18 mg

=> osmolality of glucose ( mosmol/kg H₂O) = P_G/18 (mg/dL)

Similarly for urea:
 since labs calculate not urea, but BUN, again in mg/dL

=> osmolality of urea (mosmol/kg H₂0) = P_UN/2.8 (mg/dL)

So the final equation becomes:

Plasma osmolality = 2P_Na (mmol/liter) + P_G/18 (mg/dL) + P_UN/2.8 (mg/dL)

Note the different units employed in the formula. And also note that this formula just converts for milligram/deciliter units - and if the mmol/L values are known, the first formula can be directly employed.

Constituents of IV fluids

Crystalloids

Conventional crystalloids are fluids that contain a combination of water and electrolytes. They are divided into "balanced" salt solutions (e.g., Ringer's lactate) and hypotonic solutions. Either their electrolyte composition approximates that of plasma, or they have a total calculated osmolality that is similar to that of plasma.

Normal saline (0.9%) is actually hypertonic with respect to sodium and especially to chloride, if the osmolality is calculated. However, when normal saline is subjected to a freezing point depression test in an osmometer, its osmolality is approximately 285 mOsm/kg. The calculated value is derived by simple addition of its ionic constituents, whereas the measured value is affected by ionic association or dissociation. Sodium chloride has a relative osmolality of 1 compared with that of sodium and chloride, the value of which is 2. Other balanced electrolyte solutions are slightly hypotonic in vitro (265 mOsm/kg) in comparison with their calculated values and normal plasma. Solutions that contain less than the concentration of electrolytes found in Ringer's lactate solution are not used often intraoperatively.
When an electrolyte-free solution such as D5W is administered, less than 10% stays intravascular. Approximately two thirds are distributed to the intracellular space. Intravascular resuscitation is minimal, and cellular swelling occurs. The administered free water causes a decrease in the serum and interstitial electrolyte concentrations (dilutional effect) and may lead to symptomatic hyponatraemia.

When an electrolyte-free solution such as D5W is administered, less than 10% stays intravascular. Approximately 2/3 is distributed to the intracellular space. Intravascular resuscitation is minimal and cellular swelling occurs. The administration of free water causes a decrease in serum and interstitial electrolytes (dilutional effect) and may lead to symptomatic hyponatraemia.
When solutions such as 0.2% or 0.45% saline are administered, similar, although slightly less pronounced, redistribution occurs. Therefore, a balanced salt solution with a sodium concentration of 130 mmol/L or more is normally chosen when major operative procedures are performed and when excessive blood loss is anticipated. More hypotonic solutions and D5W should be restricted to minor procedures and for some paediatric operations.

Normal saline (0.9% saline solution):

• 9 g of NaCl per litre of water
• 154 mmol/l
• 154 mmol/l
• Osm- 308
• PH= 5.0
• Potential problem = hyperchloraemic metabolic acidosis, more likely with renal insufficiency

Hartmanns solution

• Na+ 131 mmol/l
• Cl-= 111 mmol/l
• Lactate = 29
• K+ 5 mmol/l
• Ca++ 2 mmol/l
• pH = 6.5
• Osm = 279
• Potential problem = potassium may accumulate

Hypertonic Saline Solutions

• intracellular water is drawn out into the extravascular space
• less volume is required
• hypertonicity, hyperosmolarity (over 310, stop),
• hypernatraemia (over 160, stop)

Hypertonic saline solutions include 1.8%, 3%, 5%, 7.5%, and 10% sodium chloride solutions. Other anions such as lactate and acetate may be incorporated. They are sometimes mixed with colloids such as dextran. Because the osmolality of hypertonic solutions exceed that of intracellular water and because sodium and chloride ions can not freely cross cell membranes, the ECF becomes slightly hyperosmolar. A gradient for water to pass from the cells into the extravascular compartment is established, and the extracellular volume is expanded by approximately 2.5 L after administering 1 litre of 3% saline. Because electrolytes freely cross capillary membranes, the fluid is divided between the intravascular and extravascular compartments according to their relative volumes. Although hypertonic saline solutions increase the intravascular volume more than would the same volume of a balanced salt solution, they do so at the expense of a decreased intracellular volume. If large volumes of previously administered balanced electrolyte solutions have already increased intracellular volume, hypertonic saline is therapeutic. If not, cellular dehydration can result.

Potential Complications

The use of hypertonic saline solution has recently increased, due to increased use in intraoperative administration and trauma resuscitation.
A major concern is hypernatraemia. However, hypernatraemic complications have not been reported in the clinical trials. Comprehensive reviews of many of the aspects of hypertonic saline have been published. Hyperchloraemic acidosis may occur owing to the large chloride load. However, substitution of hypertonic sodium acetate, although transiently improving acid-base parameters, has not been found to improve outcome and, curiously, increases lactaemia.
 

Distribution of Crystalloids:

5% Dextrose
5% Dextrose is rapidly lost from the intravascular compartment. The glucose is rapidly taken up by the cells. The fluid is distributed in proportion to their contribution to TBW. Hence the distribution of 1litre 5% dextrose is approximately:

Intracellular:660mls (2/3rds)
Extracellular:340mls (1/3^rd)

Normal Saline
This has a Sodium concentration similar to that of the ECF. This limits the distribution of the fluid to the ECF. Within the ECF, the fluid is distributed between the ISF (3/4) and the intravascular volume (1/4) in proportion to their contribution to ECF volume.

Hence the distribution of 1litre Normal Saline is approximately:

Extracellular:1000mls (ISF 750mls, IVF 250mls)

Sunday, March 30, 2003