With the two-volume model, the fractional expansion of V ^{t}can be plotted,44 which is not possible by other methods (fig. 5A).
Calculations help to analyze how infused fluid is distributed at any given time.17 For this purpose, the rate of elimination is given by Cl (v ^{c}−V ^{c})/V ^{c}. The volume expansion of V ^{c}and V ^{t}can be generated by multiplying the fractional expansion (i.e. , the dilution) of V ^{c}and V ^{t}by their respective baseline volumes44 (fig. 4). The distribution and elimination can also be highlighted by computer-generated plots (figs. 6 and 7).
Fig. 6. Computer simulation of the percentage of the amount of infused Ringer's solution that still remains in the plasma, calculated as (v ^{c}−V ^{c}) · 100/infused volume, based on typical kinetic data for a brisk 30-min infusion in volunteers (A ) and a much slower infusion during 60 min in perioperative patients (B ). The light lines show what the fraction would have been if distribution from the plasma to the interstitial fluid space was immediate. Cl = clearance; Cl ^{d}= distribution clearance; R ^{o}= rate of infusion; V ^{c}and V ^{t}= size of central and peripheral fluid spaces, respectively, which are termed v ^{c}and v ^{t}when expanded.
Fig. 7. Computer simulation of how rapidly acetated Ringer's solution leaves the plasma to enter the interstital fluid space (V ^{t}, light line ) or is excreted as urine (dark line ). The infusion is given at a rate of 50 ml/min during 30 min. Kinetic data derived from preeclamptic women (A , Ref. 19) and surgical patients (B , from the analysis made in fig. 3).
Simulations may be used to predict the outcome of infusions that have not been performed22 (fig. 8). This requires that parameter values derived from several infusion volumes and rates have yielded similar plasma dilution-time curves (model linearity).13,14,32,34
Fig. 8. Comparison of the potency of two infusion fluids. Volume kinetic analysis was first obtained by infusing the fluids (0.9% saline and 7.5% saline in 6% dextran, HSD) in six ewes on separate days. The average parameter values were then used to simulate how much of each fluid was required to dilute the plasma by 10%, 20%, and 30%, when infused at four different rates. The marks show the ratio between the infusion rates needed to reach the target dilution. The potency of HSD relative to 0.9% saline apparently increases with the infusion time and not with the target dilution. Reprinted with permission from Anesth Analg 2002; 95:1547–56.22
Glucose 2.5% solution has been most carefully validated in this respect. In one study, six volunteers received 10 and 15 ml/kg glucose 2.5% solution over 30 min and also 15 and 25 ml/kg over 60 min.34 The bias (median residual error) associated with simulating plasma dilution in the 24 experiments averaged −0.009 dilution units. Two thirds of this error was due to inability of the glucose kinetics to account for rebound hypoglycemia. The inaccuracy (median absolute residual error) was 0.026 dilution units.
It is used as an aid when designing experiments. For example, it is virtually impossible to have two infusion fluids create the same plasma volume expansion over time without using volume kinetics. Adjusting the infusion rates is an essential challenge when testing whether one of the two fluids exerts an “intrinsic” effect, similar to the case in studies of colloid fluids as well as with artificial blood during shock.
Robert G. Hahn; Volume Kinetics for Infusion Fluids. Anesthesiology 2010;113(2):470-481. doi: https://doi.org/10.1097/ALN.0b013e3181dcd88f.
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