Distribution and elimination of crystalloid or colloid solutions during inhalational anesthesia have not been adequately investigated. Hemoglobin dilution and fluid kinetic model have been shown to reveal the distribution and elimination of various kinds of fluids. Therefore, we assessed fluid space changes after Hartmann's solution or hydroxyethyl starch solution (HES) infusion during desflurane anesthesia.

We infused 20 ml/kg of Hartmann's solution, 8.5 ml/kg of Hextend® and 8.5 ml/kg of Voluven® during 20 min, after anesthesia induction and before surgical incision, and measured the hemoglobin changes. We used mass balance equations and a fluid kinetic model to evaluate the changes of fluid space. In the fluid kinetic model, we used one volume model, which allows estimation of the size of the body fluid space expanded by the fluid (V) and the elimination rate constant ( k_r ).

The expanded plasma volume of three different fluids, calculated using mass balance equations, showed a similar degree of expansion during infusion, however, after finishing infusion, the dilution effect of Hartmann's solution decreased rapidly and lasted less than HES. Fluid kinetic model shows the mean size of V of 12.3 ± 5.9 L for Hartmann' solution, 5.2 ± 1.6 L for Hextend, and 4.5 ± 1.6 L for Voluven. Corresponding k_r values were 263.0 ± 161.8, 36.5 ± 31.8, and 34.1 ± 21.3 ml/min, respectively.

The distribution volume of intravenous fluids analyzed by kinetic model showed that crystalloid fluid has a similar volume distribution compared to extracellular fluid and HES distributed to a volume larger than blood volume. Analysis and simulation of plasma volume expansion using this model provide a helpful tool for anesthesiologists planning fluid therapy.