A single-model solution for volume kinetic analysis of isotonic fluid infusions

Background: Volume kinetics was developed to analyze the distribution and elimination of intravenously given fluid. However, when groups of patients are being compared, the current approach is limited by the need for several models, which yield parameters that cannot be compared. To meet the requirement to handle all patients in a group individually and without pooling, a new all-encompassing model was designed. The aim of this paper was to test whether the new model could be used to analyze all patients in a group. Methods: The new model consists of 'rate' and 'amount' parameters instead of 'clearance' and 'dilution' parameters. With this change, a redundant parameter can be taken out, but the biexponential nature is retained. The new parameters are the volume of distribution V-1 (ml), the intercompartmental rate constant k(t) (/min) and the elimination rate constant k(r) (/min). The success rates of the new and original models in producing results within a set of pre-determined quality requirements were compared using blood dilution data from 10 volunteers challenged with intravenous lactated Ringer's solution. Results: The new model could be used to analyze all 10 cases within the pre-determined criteria, but the original biexponential model failed in 70% of cases. The residuals improved with the new model. The medians (interquartile ranges) were as follows: V-1, 4931 ml (4239-6149 ml); k(t), 0.0384/min (0.0024-0.1140/min); k(r), 0.0140/min (0.0015-0.0043/min). Conclusion: The new model was suited to the analysis of all cases, and is therefore a better approach to study how clinical conditions change the distribution and elimination of infused fluid.