A Theoretical Analysis of the Single Breath Diffusing Capacity for Carbon Monoxide

We used a computerized lung model to examine methods of measuring the single breath diffusing capacity for carbon monoxide (DLCOSB). Although the single breath maneuver consists of inhalation, breath holding, and exhalation, current methods of measuring DLCOSB use one equation that is accurate only for breath holding, and they attempt to correct for the effects of inhalation and exhalation using a rigidly standardized procedure. Using a uniform lung model, we verified that variations in performing the standarized maneuver and variations in alveolar gas sampling, which frequently occur in practice, cause wide variability in the measured DLCOSB. We developed a new method of calculating DLCOSB based on three equations to describe diffusion in the lung during inhalation, breath holding, and exhalation. In computer simulations of the single breath maneuver, this method yielded accurate measurements of DLCOSB despite variations in flow rates, breath hold times, or the size and timing of the collected sample of exhaled alveolar gas. Furthermore, using a lung model in which the diffusing capacity was nonuniformly distributed, we found that this method gave an accurate estimate of the overall diffusing capacity of the lung from the collection of the entire exhaled alveolar gas sample, while previously accepted methods overestimated DLCOSB. We predict that a more precise calculation of the overall diffusing capacity using the entire alveolar gas sample can minimize errors in the test introduced by variations in the maneuver or variations in the distribution of diffusion.