Red cell distortion and conceptual basis of diffusing capacity estimates: finite element analysis

C. C. W. Hsia 1 , C. J. C. Chuong 2 , and R. L. Johnson Jr. 1 1 Department of Medicine, University of Texas Southwestern Medical Center, Dallas 75235; and 2 Biomedical Engineering Program, University of Texas at Arlington, Arlington, Texas 76019 Received 31 January 1997; accepted in final form 2 June 1997. Hsia, C. C. W., C. J. C. Chuong, and R. L. Johnson, Jr. Red cell distortion and conceptual basis of diffusing capacity estimates: finite element analysis. J. Appl. Physiol. 83(4): 1397-1404, 1997. To understand the effects of dynamic shape distortion of red blood cells (RBCs) as it develops under high-flow conditions on the standard physiological and morphometric methods of estimating pulmonary diffusing capacity, we computed the uptake of CO across a two-dimensional geometric capillary model containing a variable number of equally spaced RBCs. RBCs are circular or parachute shaped, with the same perimeter length. Total CO diffusing capacity (D L CO ) and membrane diffusing capacity (D M CO ) were calculated by a finite element method. D L CO calculated at two levels of alveolar P O 2 were used to estimate D M CO by the Roughton-Forster (RF) technique. The same capillary model was subjected to morphometric analysis by the random linear intercept method to obtain morphometric estimates of D M CO . Results show that shape distortion of RBCs significantly reduces capillary diffusive gas uptake. Shape distortion 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 error in morphometric estimates of D M CO caused by a biased sampling distribution of random linear intercepts; errors are exaggerated at a low capillary hematocrit. Roughton-Forster technique; morphometry; pulmonary diffusing capacity; membrane diffusing capacity; random linear intercept; capillary model 0161-7567/97 $5.00 Copyright © 1997 the American Physiological Society