Critique of the conceptual basis of diffusing capacity estimates: a
finite element analysis.
Hsia, C. C. W., C. J. C. Chuong, and R. L. Johnson, Jr.
Dept. of Medicine, University of Texas Southwestern Medical Center,
Dallas, TX 75235-9034 and Dept. of Biomedical Engineering, University
of Texas at Arlington, Arlington, TX
APStracts 2:0178A, 1995.
We present a simple geometric model of a pulmonary capillary segment
containing a variable number of red blood cells (RBC). The pattern of
CO transfer from alveolar air to capillary RBC in this model is
accurately computed by a finite element method (FEM) and used to
explore conceptual flaws in the Roughton-Forster (RF) and
morphometric methods of estimating pulmonary diffusing capacity for
carbon monoxide (DLCO). The CO uptake calculated by FEM at two
alveolar O2 tensions are introduced into the RF model to determine if
the anatomically defined DMCO and Vc are recovered. The same
capillary model is also subjected to standard morphometric analysis.
Results are compared at different levels of capillary hematocrit
(Hct). The RF method accurately recovers DMCO and Vc at a low Hct,
but modestly over-estimates DMCO and under-estimates Vc at higher
Hct; errors arise because conductance of the tissue-plasma membrane
for CO varies with alveolar O2 tension. The morphometric method
seriously over-estimates DMCO because the true tissue-plasma
resistance to diffusion is under-estimated and the effective membrane
utilized for diffusion is over-estimated; these errors are
accentuated by a low Hct.
Received 29 November 1994; accepted in final form 19 April 1995.
APS Manuscript Number A1212-4.
Article publication pending Journal of Applied Physiology.
ISSN 1080-4757 Copyright 1995 The American Physiological Society.
Published in APStracts on 2 May 1995.