When a patient receives an intravenous (IV) fluid, a common solution is D5, which stands for 5% Dextrose in Water. This solution contains water and dextrose (glucose), but no sodium or other electrolytes. D5 is often administered to replace water loss and provide minimal calories. Its infusion frequently raises the question of whether it affects the body’s electrolyte balance, specifically serum sodium concentration. The answer lies in how the body processes the dextrose, which leads to a dilutional effect on sodium levels.
The Composition and Fate of D5 in the Body
D5 is manufactured to be isotonic, meaning it initially has an osmolality similar to human blood plasma while still inside the IV bag. The solution consists of 5 grams of dextrose dissolved in 100 milliliters of sterile water. This composition provides approximately 170 kilocalories per liter and contains no sodium. The dextrose gives the fluid an initial osmolality of about 252 mOsmol/L, which is close to the body’s normal range.
The properties of D5 change rapidly once the fluid is infused into the bloodstream. Cells throughout the body quickly take up the dextrose component to be metabolized, with the sugar being oxidized to carbon dioxide and water for energy. In a healthy individual, the plasma half-life of a dextrose bolus is typically no more than 15 minutes as it is cleared from circulation. This rapid metabolism means that the dextrose is removed from the blood quickly, leaving behind only the pure water that was the solvent.
The infused fluid effectively transforms into electrolyte-free water after the dextrose is metabolized. Because the original dextrose is no longer contributing to the concentration of particles in the blood, the solution shifts from being isotonic to functioning as a hypotonic fluid in the body. This newly available free water then moves between the body’s fluid compartments, a process that is central to understanding the fluid’s ultimate effect on sodium.
Understanding the Dilution Effect on Sodium Levels
The free water left after dextrose metabolism does not stay contained within the blood vessels for long. Water moves freely across cell membranes based on solute concentration, a process known as osmosis. This newly introduced pure water distributes itself throughout the total body water, including both intracellular and extracellular fluid. The distribution follows natural proportions: about two-thirds moves into the cells, with the remaining one-third staying in the extracellular space.
Introducing this free water dilutes the existing solutes present in the body’s fluid, including sodium. Sodium is the primary electrolyte that determines the osmolality of the extracellular fluid, so its concentration is closely monitored by the body. When a significant volume of water that contains no sodium is added to the system, the overall concentration of sodium ions per unit of volume decreases. This physiological process is the direct mechanism by which D5 can lower the serum sodium level.
The resulting drop in serum sodium is known as dilutional hyponatremia. This effect is why D5 is often used intentionally to correct conditions where serum sodium is too high, such as hypernatremia. Studies show that for every liter of D5 administered, the serum sodium can decrease by an estimated mean of about 2.25 mEq/L in patients with high sodium.
Therapeutic Uses and Patient Monitoring
D5 is administered for several therapeutic reasons, primarily as a source of water to replace fluid losses and to provide minimal carbohydrate calories. For patients who cannot eat or drink, the dextrose component provides a small amount of glucose, helping to minimize the breakdown of body protein and prevent ketosis. It is also widely used as a vehicle to dilute and deliver other injectable medications.
The fluid’s capacity to act as free water makes it useful for correcting certain electrolyte imbalances. D5 is the preferred fluid for treating hypernatremia, a condition of elevated serum sodium, because it provides the necessary free water to dilute the excess sodium. However, this dilutional property necessitates caution and monitoring in other clinical situations. D5 is generally used selectively and is not recommended as a routine maintenance fluid.
Risks and Monitoring
Due to its sodium-lowering effect, D5 is often contraindicated or used with caution in patients who already have hyponatremia, as it can worsen the condition. It is also used carefully in patients at risk for cerebral edema because the rapid shift of water into cells can exacerbate brain swelling. Careful patient monitoring is essential during D5 administration to ensure safety. Providers must frequently check serum electrolyte levels, especially sodium, and monitor fluid input and output. The rate of sodium correction must be carefully controlled, generally not exceeding a decrease of 8 mEq/L per 24 hours, to prevent serious neurological complications.