Kinetics of isotonic and hypertonic plasma volume expanders

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.

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.

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 on 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.

The relative efficiency of crystalloid infusion fluids differs depending on 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.