Dextrose 5% in Water, commonly known as D5W, is a widely used intravenous solution. This article clarifies D5W’s properties and how it acts within the human body, addressing its classification and role in fluid management.
Understanding Solution Tonicity
Tonicity describes the concentration of solutes in a solution relative to another, typically the fluid inside a cell. This property dictates how water moves across a semipermeable membrane via osmosis, from an area of lower solute concentration to higher.
Solutions are classified into three types based on their tonicity. An isotonic solution has a solute concentration similar to body cells, meaning no net water movement, allowing them to maintain their normal shape. A hypotonic solution has a lower solute concentration than the cell’s interior, causing water to move into the cell and potentially leading to swelling. Conversely, a hypertonic solution has a higher solute concentration, drawing water out of the cell and causing it to shrink.
What D5W Contains
D5W consists of 5% dextrose, a form of glucose, dissolved in sterile water. When in its container, D5W has an osmolarity typically ranging from 252 mOsm/L to 278 mOsm/L.
This initial osmolarity is close to the normal osmolarity of human blood plasma (approximately 280 to 300 mOsm/L). Therefore, in its original state, D5W is considered an isotonic solution or, in some contexts, slightly hypotonic, as it exists in the bag prior to administration.
How D5W Acts in the Body
While D5W is initially isotonic or slightly hypotonic in the bag, its behavior changes significantly once infused into the body. The dextrose, a simple sugar, is rapidly metabolized by the body’s cells for energy. This metabolic process is quite quick, typically occurring within 30 to 60 minutes after administration.
Once the dextrose is consumed, what remains is essentially free water. This free water then behaves as a physiologically hypotonic solution within the body. Consequently, this water moves from the bloodstream into the body’s cells and other fluid compartments, expanding total body water.
The Significance of D5W’s Action
The way D5W acts within the body has important implications for its medical applications. Because it becomes physiologically hypotonic after dextrose metabolism, D5W serves as a source of “free water” for rehydration. This is useful for correcting conditions with a deficit of water relative to solutes, such as hypernatremia.
D5W also provides minimal calories from the dextrose. This caloric contribution is not substantial enough for nutritional support over extended periods. It is not considered a volume expander because the water distributes widely across fluid compartments.