RED-CELL DISTORTION AND CONCEPTUAL BASIS OF DIFFUSING-CAPACITY ESTIMATES - FINITE-ELEMENT ANALYSIS

To understand the effects of dynamic shape distortion of red blood cel ls (RBCs) as it develops under high-flow conditions on the standard ph ysiological and morphometric methods of estimating pulmonary diffusing capacity, we computed the uptake of CO across a two-dimensional geome tric capillary model containing a variable number of equally spaced RB Cs. RBCs are circular or parachute shaped, with the same perimeter len gth. Total CO diffusing capacity (DLCO) and membrane diffusing capacit y (DMCO) were calculated by a finite element method. DLCO calculated a t two levels of alveolar PO2 were used to estimate DMCO by the Roughto n-Forster (RF) technique. The same capillary model was subjected to mo rphometric analysis by the random Linear intercept method to obtain mo rphometric estimates of DMCO. Results show that shape distortion of RB Cs significantly reduces capillary diffusive gas uptake. Shape distort ion exaggerates the conceptual errors inherent in the RF technique (J. Appl. Physiol. 79: 1039-1047, 1995); errors are exaggerated at a high capillary hematocrit. Shape distortion also introduces additional err or in morphometric estimates of DMCO caused by a biased sampling distr ibution of random linear intercepts; errors are exaggerated at a low c apillary hematocrit.