Test of independence between the membrane and the blood components of DLCO

Lung diffusing capacity for carbon monoxide, DLCO, has been interpreted by the Roughton-Forster (RF) equation. The equation decomposes the total resistance for the CO capture into the sum of two in series, the membrane and the blood resistances: 1/DLCO = 1/DMCO + 1/(θCO·Vc) where θCO is the specific conductance for CO transfer in the red cell and Vc is the capillary volume. We have investigated whether this decomposition is valid for interpreting DLCO by testing independence of the two components. DLCO and DMCO were numerically computed for a simple geometry of biconcave cells in a cylindrical capillary (inset of the figure) by solving diffusion-reaction equations with the Finite Element Method. Two arbitrary membrane thicknesses of 0.2 and 3 μm were considered while keeping the rest fixed. For different thicknesses configurations, the blood resistance was then obtained using the RF equation: 1/(θCO·Vc) = 1/DLCO - 1/DMCO. Results: showed that the blood resistance varies with the thickness for all range of CO-hemoglobin reaction times (Figure) while it was supposed to be constant because the thickness is by definition only relevant to DMCO. Meaning that, the two resistances are not independent. Along with the recent study revealing that the RF extrapolation method proposed for the decomposition is erroneous (Kang, M.-Y. et al. Respir Physiol Neurobiol 2017; in print), this finding shows the RF equation has a conceptual defect.