D5W is unique in that it may be categorized as both an isotonic and a hypotonic solution. The amount of dextrose in this solution makes its initial tonicity similar to that of intravascular fluid, making it an isotonic solution. But dextrose (in this concentration) is rapidly metabolized by the body, leaving no osmotically active particles in the plasma.6
D5W provides free water: free, unbound water molecules small enough to pass through membrane pores to the intracellular and extracellular spaces. This smaller size allows the molecules to pass more freely between compartments, thus expanding both compartments simultaneously.6 The free water initially dilutes the osmolality of the extracellular fluid; once the cell has used the dextrose, the remaining saline and electrolytes are dispersed as an isotonic electrolyte solution, providing additional hydration for the extracellular fluid compartment. Dextrose solutions also provide free water for the kidneys, aiding renal excretion of solutes. Because it provides free water following metabolism, D5W is also considered a hypotonic solution.6
D5W is basically a sugar water solution that provides 170 calories per liter, but it doesn't replace electrolytes. However, it's appropriate to treat hypernatremia because it dilutes the extra sodium in extracellular fluid.
D5W shouldn't be used in isolation to treat fluid volume deficit because it dilutes plasma electrolyte concentrations. It's also contraindicated in these clinical circumstances:
* for resuscitation, because the solution won't remain in the intravascular space.
* in the early postoperative period, because the body's reaction to the surgical stress may cause an increase in antidiuretic hormone secretion.2
* in patients with known or suspected increased intracranial pressure (ICP) due to its hypotonic properties following metabolism.
Although it supplies some calories, D5W doesn't provide enough nutrition for prolonged use.