Journal of Applied Physiology, Vol 75, Issue 4 1734-1739, Copyright © 1993 by American Physiological Society
Influence of diffusion on estimations of protein reflection coefficient by double-indicator method
P. White Jr, R. G. Brower, J. T. Sylvester, T. Permutt and S. Permutt
Department of Medicine, Johns Hopkins Medical Institutions, Baltimore 21205.
In isolated perfused organs, vascular protein reflection coefficients
(sigma) can be calculated from the changes in hematocrit and perfusate
protein concentration (CP) that occur during edema formation. This
technique requires the assumption that transvascular protein flux by
diffusion is negligible. To assess diffusion-induced errors in calculations
of sigma, we derived an expression for CP that includes determinants of
diffusive protein flux: protein permeability-surface area product (PS),
transvascular fluid flux (J), true sigma, and transvascular protein
concentration. We used this expression to obtain values of CP under various
experimental conditions and then calculated values of sigma (measured
sigma) for those conditions. Diffusion causes measured sigma to be lower
than true sigma. The diffusion-induced error is larger and potentially
substantial when J/PS is low and when true sigma is high. Diffusion-induced
error is also larger when the amount of edema formation is greater. In
recent isolated canine lung experiments where J/PS was approximately 2.7,
diffusion-induced errors in measured sigma 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 conclude that diffusion causes underestimation
of true sigma and that the error in measured sigma may be substantial when
J/PS is < 5 and when true sigma is > 0.5.
