On-line computer evaluation of dye indicator dilution curves for determinations of capillary diffusion capacities

A colourimetric indicator diffusion method has been modified to obtain more rapid and less elaborate estimations of the permeability surface area product (PS). The venous light absorbance curves of an intravascular reference substance and a permeable tracer substance are collected by a computer, which immediately presents the concentration curves versus time. Collected data are saved on diskettes. The data are normalized with respect to calibration and fractional as well as integrated capillary extraction values are presented. Extraction values can then be freely chosen for calculations of PS. The program is designed to give maximum precision and safety in the handling of calibrations and collected data. Albumin, labelled with cardiogreen, is used as reference dye and CrEDTA (MW = 341) or cyanocobalamine (vitamin B12, MW = 1355) as tracer. All dyes showed linearity between concentration and densitometer output in the concentration interval used. Both spatial and temporal accuracy of colourimetric recordings are superior to radioactivity measurements. However, the technique is so far limited to the use of cell-free perfusates.