INFLUENCE OF DIFFUSION ON ESTIMATIONS OF PROTEIN REFLECTION COEFFICIENT BY DOUBLE-INDICATOR METHOD

In isolated perfused organs, vascular protein reflection coefficients (sigma) can be calculated from the changes in hematocrit and perfusate protein concentration (C(P)) that occur during edema formation. This technique requires the assumption that transvascular protein flux by d iffusion is negligible. To assess diffusion-induced errors in calculat ions of sigma, we derived an expression for C(P) that includes determi nants of diffusive protein flux: protein permeability-surface area pro duct (PS), transvascular fluid flux (J), true sigma, and transvascular protein concentration. We used this expression to obtain values of C( P) under various experimental conditions and then calculated values of sigma (measured sigma) for those conditions. Diffusion causes measure d sigma to be lower than true sigma. The diffusion-induced error is la rger and potentially substantial when J/PS is low and when true sigma is high. Diffusion-induced error is also larger when the amount of ede ma formation is greater. In recent isolated canine lung experiments wh ere J/PS was approximately 2.7, diffusion-induced errors in measured s igma for albumin would have been approximately 0.06 (at true sigma = 0 .5) and approximately 0.18 (at true sigma = 0.9). When J/PS was higher , the potential for diffusion-induced errors was much smaller. We conc lude that diffusion causes underestimation of true sigma and that the error in measured sigma may be substantial when J/PS is <5 and when tr ue sigma is >0.5.