Morphometric model for pulmonary diffusing capacity I. Membrane diffusing capacity

The pulmonary diffusing capacity is related to the quantitative design characteristics of the pulmonary gas exchanger. The current model for estimating Dl_O2 from morphometric data breaks the diffusion path for O₂ into four steps, three of which represent the membrane part of Dl_O2. A critique of this model on the basis of newer evidence leads to a modification of the model where the path from the alveolar surface to the erythrocyte membrane is considered as a single step. The structural determinant of this model for Dm_O2 is the ratio of effective diffusion surface to effective total barrier thickness. The effective surface is formulated as a fraction of the alveolar surface area, the most robust measure of lung design, whereas the effective barrier thickness is the harmonic mean distance - or mean proximity - between alveolar surface and erythrocyte surface. The methods for obtaining the morphometric measurements are discussed. The results show that the new morphometric estimates of Dm_O2 are 33% lower than those obtained with the old model, resulting in a reduction of the estimates of Dl_O2 by 10-20%.