Diffusion limitation and limitation by chemical reactions during alveolar-capillary transfer of oxygen-labeled CO 2

Abstract

A major difficulty in the measurement of carbon dioxide diffusing capacity is the development of significant CO 2 back pressure within the capillary. The use of oxygen-labeled CO 2 limits this back pressure due to rapid dilution of the label into the water pool by isotopic exchange. We demonstrated the use of C 16O 18O to measure D CO 2 (Schuster, 1985). A major question from that study is whether the isotope measures a true membrane diffusing capacity or is limited by back reaction. In this study we examine the diffusing capacity of doubly 18O-labeled carbon dioxide, C 18O 2, a species in which the kinetics of isotopic exchange in pulmonary blood is higher than that of C 16O 18O. Eighteen single breath experiments were performed on two resting male subjects whose CO 2-transfer-kinetics was previously studied with C 16O 18O (Schuster, 1985). Following expiration to residual volume the subjects inspired a gas mixture containing 20% O 2 and 1.5–2.6% C 18O 2. After holding their breath for 0.4–17 sec they exhaled into a tube and the end-expired gas was analysed by mass spectrometry. The time course of the C 18O 2 disappearance from alveolar gas showed a biexponential characteristic as that which had been measured with C 16O 18O, but C 18O 2 disappeared faster. The mean value of the diffusing capacity of C 18O 2 amounts to 1102ml mmHg −1·min −1. It is 228 ± 163 ml mmHg −1·min −1 greater than that of C 16O 18O. This significant difference suggests that: (1) D C 16O 18O is limited in part by isotopic exchange reactions; (2) the observed D C 18O 2 may be taken as a new lower limit for D m CO 2 , higher than values established from other techniques; (3) the decarboxylation of bicarbonate in red cells is not a rate limiting step for CO 2 exchange.