CRITIQUE OF CONCEPTUAL BASIS OF DIFFUSING-CAPACITY ESTIMATES - A FINITE-ELEMENT ANALYSIS

We present a simple geometric model of a pulmonary capillary segment c ontaining a variable number of red blood cells. The pattern of CO tran sfer from alveolar air to capillary red blood cells in this model is a ccurately computed by a finite element method and used to explore conc eptual flaws in the Roughton-Forster (RF) and morphometric methods of estimating pulmonary diffusing capacity for CO. The CO uptakes calcula ted by the finite element method at two alveolar O-2 tensions are intr oduced into the RF model to determine whether the anatomically defined membrane component of diffusing capacity for CO (Dm(CO)) and pulmonar y capillary blood volume (Vc) are recovered. The same capillary model is also subjected to standard morphometric analysis. Results are compa red at different levels of capillary hematocrit (Hct). The RF method a ccurately recovers Dm(CO) and Vc at a low Hct but modestly overestimat es Dm(CO) and underestimates Vc at higher Hct; errors arise because co nductance of the tissue-plasma membrane for CO varies with alveolar O- 2 tension. The morphometric method seriously overestimates Dm(CO) beca use the true tissue-plasma resistance to diffusion is underestimated a nd the effective membrane utilized for diffusion is overestimated; the se errors are accentuated by a low Hct.