Background: Major differences in plasma volume expansion between infusion fluids are fairly well known, but there is a lack of methods that express their dynamic properties. Therefore, a closer description enabled by kinetic modeling is presented. Methods: Ten healthy male volunteers received, on different occasions, a constant-rate intravenous infusion over 30 min consisting of 25 ml/kg of 0.9% saline, lactated Ringer's solution, acetated Ringer's solution, 5 ml/kg of 7.5% saline, or 3 ml/kg of 7.5% saline in 6% dextran. One-, two-, and three-volume kinetic models were fitted to the dilution of the total venous hemoglobin concentration over 240 min. Osmotic fluid shifts were considered when hypertonic fluid was infused. Results: All fluids induced plasma dilution, which decreased exponentially after the infusions. The ratio of the area under the dilution-time curve and the infused fluid volume showed the following average plasma-dilution dose-effect (efficiency), using 0.9% saline as the reference (= 1): lactated Ringer's solution, 0.88; acetated Ringer's solution, 0.91; hypertonic saline, 3.97; and hypertonic saline in dextran, 7.22 ("area approach"). Another comparison, based oil kinetic analysis and simulation, showed that the strength of the respective fluids to dilute the plasma by 20% within 30 min was 0.94, 0.97, 4.44, and 6.15 ("target dilution approach"). Between-subject variability was approximately half as high for the latter approach. Conclusions: The relative efficiency of crystalloid infusion fluids differs depending oil whether the entire dilution-time profile or only the maximum dilution is compared. Kinetic analysis and simulation is a useful tool for the study of such differences.
