Distribution of ventilation and of diffusing capacity to perfusion in the lung

We developed a method for estimating the distribution of ventilation V̇A) and of diffusing capacity (G) to perfusion (Q̇) in the lungs. We used O₂, CO₂ and CO together with six inert gases of widely differing solubility and assumed that mass transfer efficiency of each gas in a gas exchange unit is limited by both V̇A/Q̇ and G/Q̇ ratios. The underlying lung model comprised 20 units along both the V̇AQ̇ and G/Q̇ axes. Using numerical analysis, we transformed the data into a virtually continuous distribution of Q̇ in the V̇A/Q̇-G/Q̇ field. We tested the precision of the numerical procedure by examining the recovery of various artificial distributions, and found that distributions with up to two modes could be recovered with reasonable accuracy. Analytical results from 15 patients with interstitial pneumonia of unknown etiology (IPF) revealed the following features. (1) In an early disease stage, most of the lung was operating in the range of normal V̇A/Q̇, without a significant contribution of diffusion limitation. (2) An advanced stage of the disease exhibited a widening of V̇A/Q̇ distribution and either broad unimodal or bimodal distribution of G/Q̇, extending to G/Q̇ below 10^-3 ml (STPD)/(ml·Torr) with diffusion-limited O₂ exchange. (3) Severe diffusion limitation causing disequilibrium of inert gas across the blood-gas barrier was observed in three (far advanced fibrosis; active interstitial inflammation) out of 15 patients. These findings suggest that inhomogeneity of G/Q̇ does exist and may play an appreciable role in causing impairment of gas exchange in patients with interstitial pneumonia.