The Multiple Indicator Dilution Method and Its Utility in Risk Assessment

Abstract

The multiple-indicator dilution (MID) technique entails the injection of a mixture of labeled indicators into the blood vessel immediately at the entrance of an organ, e.g., the liver, kidney, heart, or lung, and characterization of outflow dilution profiles from timed venous samples. The mathematical basis of the method encompasses linear systems of partial differential equations that are formulated for flow- or barrier-limited transport combined with intracellular metabolism/excretion. The concept can be generalized to include metabolites. MID experiments are useful for determining tissue partition coefficients as well as kinetic parameters such as membrane permeabilities or metabolic/excretory intrinsic clearances, factors that affect the mean residence times or exposure of solutes to the organ. The main utility of the MID method lies in its role in identifying the basic mechanisms of the interaction of organs with vascular components. The concentration dependence in transport and removal is revealed by the rate coefficients upon varying the input concentrations of unlabeled substances into the organ at steady state. The data obtained with MID experiments can be incorporated into physiologically based pharmacokinetic (PBPK) models such as those used for biological risk assessment. This is especially pertinent in the case where diffusional barriers appear within organs. The insight gained from the MID organ approach may be useful for PBPK models with more realistic representation of organ kinetics.