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).
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.
| Name | Normal saline | 5% dextrose |
| Class | Crystalloid fluid | Crystalloid fluid |
| Chemistry | Monovalent cation salt | Isotonic monosaccharide 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) |
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.