This chapter is relevant to Section I2(i) of the 2017 CICM Primary Syllabus, which expects the exam candidates to develop or at least fake "an understanding of the pharmacology of colloids and crystalloids". The material being explored here is 5% dextrose, or a litre of water with 50g of glucose in it. This has only appeared in the CICM exam once, in Question 17 from the first paper of 2018, where the candidates were asked to compare it to saline.
Class Crystalloid fluid Chemistry Isotonic disaccharide solution Routes of administration IV only, though theoretically also orally Absorption 100% oral bioavailability; well absorbed Solubility pKa = 12.9; good water solubility; in solution the dextrose is in an non-ionised state Distribution VOD =0.6L/kg, distributed widely into total body water
(thus: 8% remains intravascular, 26% becomes interstitial fluid and 66% becomes intracellular)
Target receptor GLUT family of glucose transporter proteins Metabolism Metabolised extensively by all body tissues, but especially by the liver Elimination Metabolites are water and CO2, which are eliminated by the kidneys and lungs, respectively Time course of action "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 Clinical effects 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
- 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)
In short, this is essentially just free water. No anions, no cations. No added buffer, no antimicrobial agent, no artificial colours or preservatives. For flavor, 278mmol of dextrose is added. If you were trying to use this as a source of nutrient, the whole litre would yield 835 kJ, or 198 calories
The pKa of dextrose is 12.9, so at the pH of the bag this monosaccharide is in a non-ionised state. If you were forced to mix this in some nightmarish situation where you have sterile glucose and sterile water but no actual pre-made Baxter bag, you would throw 50g of glucose into 1 litre of water. That would make a 5g per 100ml (or a 5% by weight) solution.
Concentrated dextrose is still just free water, but syrupy. Extremely syrupy. This sort of stuff should only be given via a central line. Red blood cells, in contact with such hyperosmolar fluid, will shrivel and die. Ditto the fragile venous endothelium. Administration of such things via a peripheral vein will reward one with nothing but thrombosis and phlebitis.
Ever wonder what a glucometer would read when you dribble some 5% dextrose solution onto it?
It will say "HI", stupidly.
Because it cant read such a high value. Indeed even 5% dextrose is too high a value for it to comprehend. It is calibrated to perceive a range of glucose that is compatible with human life, much like the human eye is only calibrated to perceive "visible" light. At a glucose concentration above 30mmol/L one begins to feel quite ill. At around 60mmol/L, one may find oneself in a hyperosmolar coma. Think of it: normally your osmolality doesn't get above 290, so with an addition of 60 osmoles of glucose, you get an osmolality of 350mOsm/L.