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

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 (DL(CO)) and membrane diffusing capacity (DM(CO)) were calculated by a finite element method. DL(CO) calculated at two levels of alveolar PO₂ were used to estimate DM(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 DM(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 DM(CO) caused by a biased sampling distribution of random linear intercepts; errors are exaggerated at a low capillary hematocrit.