Understanding Osmolarity in 5% Glucose (D5W)
Osmolarity is a measure of the concentration of dissolved particles in a solution, expressed as milliosmoles per liter (mOsm/L). It helps predict the movement of water across a semipermeable membrane, like a cell wall. To calculate the osmolarity of 5% glucose, one must first understand its composition.
Calculating the Theoretical Osmolarity
A 5% glucose solution contains 5 grams of glucose in every 100 milliliters of water. This is equivalent to 50 grams of glucose per liter (L) of solution. The molecular weight of glucose ($C6H{12}O_6$) is approximately 180.16 g/mol.
To calculate the osmolarity, we can use the following formula: $Osmolarity = (Weight\ of\ solute / Molecular\ weight) \times 1000 \times n$
Where:
- Weight of solute: 50 g/L
- Molecular weight: 180.16 g/mol
- $n$ (number of dissociable particles): Glucose is a non-ionic compound, so it does not dissociate in water, meaning $n=1$.
$Osmolarity = (50 / 180.16) \times 1000 \times 1 \approx 277.5\ mOsm/L$
This calculation, along with manufacturer data, typically indicates a theoretical osmolarity in the range of 252 to 278 mOsm/L. This range is significant because it is similar to the normal osmolarity of human blood plasma (275 to 295 mOsm/kg), making the solution appear isotonic in vitro (in a lab setting).
The Critical Distinction: Osmolarity vs. Tonicity
This is where many misconceptions about D5W arise. While its measured osmolarity is close to that of plasma, its physiological effect, known as its tonicity, is very different. Tonicity describes the effective osmolarity across a semipermeable membrane and is influenced by solutes that cannot easily cross that membrane. Glucose, however, is not an 'effective osmole' because it is rapidly transported into cells and metabolized.
The Physiological Hypotonicity of D5W
The crucial point is that D5W does not remain isotonic inside the body. Here's what happens after it is infused intravenously:
- Initial Infusion: When D5W first enters the bloodstream, its high concentration of glucose exerts an osmotic pressure similar to that of plasma, acting as an isotonic fluid.
- Rapid Metabolism: However, the body's cells rapidly absorb the glucose from the solution to use as energy.
- Free Water Remains: This rapid metabolism leaves behind the sterile water from the solution, with no osmotically active particles left in the plasma.
- Fluid Shifts: The extracellular fluid (the fluid outside the cells) becomes diluted and its osmolarity drops. In response to this osmotic gradient, water moves out of the extracellular space and shifts into the intracellular space (inside the cells).
Therefore, once infused, D5W behaves as a physiologically hypotonic solution. This property is what makes it effective for specific medical applications, particularly for providing hydration to the intracellular compartment.
Clinical Applications and Cautions
Uses of 5% Glucose
Physicians use D5W for several key purposes, including:
- Hydration: It provides a source of free water to correct dehydration, especially intracellular dehydration.
- Nutritional Support: The dextrose provides a small amount of calories (approximately 170 kcal per liter) and reduces protein catabolism, particularly in patients who cannot take nutrition orally.
- Medication Diluent: It is a common vehicle for preparing and delivering a wide range of injectable medications intravenously.
- Treating Hypoglycemia: It can help raise blood sugar levels in cases of low blood sugar.
Risks and Precautions
However, the use of D5W is not without risks, especially if not monitored carefully:
- Hyperglycemia: Rapid infusion can lead to a sudden increase in blood sugar, which is particularly dangerous for diabetic patients or those with impaired glucose tolerance.
- Hyponatremia: The influx of free water into the body can dilute sodium concentrations in the plasma, leading to hyponatremia. This risk is higher in children and patients with increased levels of ADH (antidiuretic hormone).
- Cerebral Edema: The fluid shift into cells can be particularly dangerous for brain tissue. Administering D5W to patients with increased intracranial pressure (ICP) can exacerbate cerebral edema and should be avoided.
- Fluid Overload: In patients with renal or cardiac insufficiency, the extra fluid can lead to hypervolemia (fluid overload).
Comparison of Common Intravenous Fluids
| Solution | Calculated Osmolarity (mOsm/L) | Initial Tonicity | Effective Tonicity (In Vivo) |
|---|---|---|---|
| 5% Glucose (D5W) | $\approx$ 252-278 | Isotonic | Hypotonic |
| 0.9% Normal Saline (NS) | 308 | Isotonic | Isotonic |
| Lactated Ringer's (LR) | 272 | Isotonic | Isotonic |
Conclusion
In summary, the osmolarity of 5% glucose, or D5W, is theoretically within the isotonic range, roughly 252-278 mOsm/L. This measurement, however, is a snapshot of the solution in a container. The moment D5W is administered, the rapid metabolic consumption of glucose by the body's cells transforms it into a physiologically hypotonic fluid. This critical shift is what dictates its medical use for providing intracellular hydration rather than simply expanding extracellular fluid volume. Understanding this dual nature is paramount for its safe and effective clinical application. For more information, refer to official FDA documentation for Dextrose Injections.
Sources
- PMC. Intravenous fluids: issues warranting concern. https://pmc.ncbi.nlm.nih.gov/articles/PMC8900995/
- accessdata.fda. 5% DEXTROSE Injection, USP Trade Name® Container Rx only. https://www.accessdata.fda.gov/drugsatfda_docs/label/2005/016367s178lbl.pdf
- PMC. Misunderstandings about Tonicity and Osmolality Can Lead to Hyponatremia. https://pmc.ncbi.nlm.nih.gov/articles/PMC10522352/
- Picmonic. IV Solutions PDF. https://www.picmonic.com/api/v3/picmonics/1579/pdf
- NCBI. Physiology, Plasma Osmolality and Oncotic Pressure. https://www.ncbi.nlm.nih.gov/books/NBK544365/
