Crystalloid electrolyte solutions include isotonic saline, Ringer’s lactate, Ringer’s acetate, and Plasma-Lyte. In the perioperative period these fluids are used to compensate for anesthesia-induced vasodilation, small to moderate blood losses and urinary excretion. Although evaporation consists of electrolyte-free water, such fluid losses are relatively small during short-term surgery.

These fluids expand the plasma volume to a lesser degree than colloid fluids as they hydrate both the plasma and the interstitial fluid space. However, the distribution to the interstitial fluid space takes 25–30 min to be completed, likely due to the restriction of fluid movement by the finer filaments in the interstitial gel. The slow distribution gives crystalloid electrolyte solutions a fairly good plasma volume expanding effect as long as the infusion continues and shortly thereafter.

Isotonic (0.9%) saline is widely used but has an electrolyte composition that deviates from that of the ECF (““unbalanced””). This fluid is best reserved for special indications only. Isotonic saline may also be considered in trauma and in children undergoing surgery. Hypertonic saline might be used in neurotrauma and, possibly, in preoperative emergency care.