Question 17

Define the osmolality and tonicity of an intravenous fluid (20% of marks).

Compare and contrast the pharmacology of intravenous Normal Saline 0.9% and 5% Dextrose (80% of marks).

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College Answer

Most candidates gave an adequate definition of osmolality and tonicity. A single concise sentence for each attracted full marks. Some candidates drew diagrams & equations, which added few marks. Some candidates  Confused osmolarity (mOsm/L) and osmolality (mOsm/kg). Tonicity was best defined as the number of ‘effective’ osmols (those that cannot cross the cell membrane) in a solution relative to plasma. The use of a table greatly facilitated the comparison of 0.9% saline and 5% dextrose solutions. Values for composition, osmolarity and osmolality were poorly done. Some manufacturers state calculated values and some approximate values on the bags – both were accepted. No candidate correctly pointed out the fluids respectively have 9g NaCl and 50g dextrose per litre.

Discussion

  • Osmolality is the number of osmoles of solute per kilogram of solvent
  • Tonicity is the osmotic pressure between two compartments, and is related to the difference in the concentration of "effective" osmoles between them
Name Normal saline 5% dextrose
Class Crystalloid fluid Crystalloid fluid
Chemistry Monovalent cation salt Isotonic disaccharide solution
Routes of administration IV, subcutaneously, orally, or as a neb (plus multople others) IV only, though theoretically also orally
Absorption 100% oral bioavailability; well absorbed 100% oral bioavailability; well absorbed
Solubility pKa 3.09; good water solubility pKa = 12.9; good water solubility; in solution the dextrose is in an non-ionised state
Distribution VOD=0.2L/kg, basically confined to the extracellular fluid
(thus: 25% remains intravascular, 75% becomes interstitial)
VOD =0.6L/kg, distributed widely into total body water
(thus: 8% remains intravascular, 26% becomes interstitial fluid and 66% becomes intracellular)
Target receptor As a resuscitation fluid, you could say that the target receptor is the baroreceptor GLUT family of glucose transporter proteins
Metabolism Not metabolised Metabolised extensively by all body tissues, but especially by the liver
Elimination Elininated renally, where specific reabsorption mechanisms in the renal tubule regulate the rate of sodium and chloride excretion Metabolites are water and CO2, which are eliminated by the kidneys and lungs, respectively
Time course of action Half life is 20-40 minutes in healthy volunteers, longer in shock states and in mechanically ventilated patients (up to 8 hours) "Half-life" of the volume expansion effect is perhaps 15-20 minutes in healthy volunteers. Dextrose itself
Mechanism of action Expands the extracellular fluid volume and changes the biochemistry of the body fluids Expands the extracellular fluid volume and changes the biochemistry of the body fluids
Clinical effects Volume expansion:
- by 25% of the infused volume, after 25-30 minutes
- below the circulatory reflex activation threshold
- effect is greater during the infusion (prior to redistribution)
Change in osmolality:
- minimal; unnoticed by osmoreceptors
Change in biochemistry:
- trivial sodium elevation (~0.5-.0 mmol/L)
- nontrivial chloride elevation (up to 3 mmol/L)
- decrease in bicarbonate and base excess (also up to 3 mmol/L)
Volume expansion:
- by 8% of the infused volume, after 15-20 minutes
- below the circulatory reflex activation threshold
Change in osmolality:
- approximately 2.5%, i.e. same as from the ingestion of tap water
- sensed by OVLT osmosensor; leads to decreased vasopressin release and subsequent diuresis
Change in biochemistry:
- dilutional sodium drop (~4 mmol/L)
- this will be noticed by sodium conservation systems such as the angiotensin and aldosterone systems
- decrease in bicarbonate and base excess (also up to 3 mmol/L) because this is a fluid with zero SID
Metabolic changes:
- the dextrose will be metabolised to produce enrgy, with a total gain of 198 calories
 
Single best reference for further information Griffel and Kaufman (1992) Griffel and Kaufman (1992)

References

IUPAC. "Compendium of chemical terminology." the “Gold Book” (1997).

Siggaard-Andersen, O., R. A. Durst, and A. H. J. Maas. "Physicochemical quantities and units in clinical chemistry with special emphasis on activities and activity coefficients (Provisional)." Pure and applied chemistry 53.8 (1981): 1605-1643.

Caon, Martin. "Osmoles, osmolality and osmotic pressure: clarifying the puzzle of solution concentration." Contemporary nurse 29.1 (2008): 92-99.

Hahn, Robert G., and David S. Warner. "Volume kinetics for infusion fluids." The Journal of the American Society of Anesthesiologists 113.2 (2010): 470-481.

Reddi, Benjamin AJ. "Why is saline so acidic (and does it really matter?)." International journal of medical sciences 10.6 (2013): 747.

Hahn, Robert G. "Clinical pharmacology of infusion fluids." Acta medica Lituanica 19.3 (2012): 210-212.