Interpretation of volume kinetics in terms of pharmacokinetic principles.

Volume kinetics is pharmacokinetics for infusion fluids and describes how an infused volume is distributed and eliminated. Generally, pharmacokinetic parameters can be estimated by measuring the concentration of a drug. However, it is almost impossible to directly measure the concentration of fluids. Therefore, in volume kinetics, the disposition of fluids is indirectly quantified by measuring hemoglobin concentration under the premise that there is no loss of hemoglobin. That is, if the hemoglobin concentration is repeatedly measured while administering the fluids, the degree of dilution of the plasma volume for each corresponding hemoglobin concentration can be obtained. The degree of plasma volume dilution involves the same concept as plasma volume expansion. The method of quantifying the drugs disposition with compartmental analysis has been equally applied to volume kinetics. The transfer of fluids between compartments is explained by the first-order kinetics, and it is assumed that fluid is only removed from the central compartment. Population analysis can also be used to identify covariates that can account for inter-individual variability in volume kinetic parameters. Body weight and mean blood pressure are well known as representative covariates of volume kinetic parameters. Using volume kinetic parameters, the volume expansion effects of crystalloid and colloid solutions can be better understood, which can be helpful for appropriate fluid therapy. Although volume kinetics has limitations, its implications are important for clinicians when administering fluids.