Alveolar volume and sequential filling impact the estimation of D/sub LCO/ and D/sub LNO/

This paper presents the development, testing, and application to experimental data of a new model to: (1) estimate the degree of sequential filling during inhalation, and (2) the dependence of the diffusing capacity (D/sub L/) on alveolar volume (V/sub A/). The model assesses sequential filling through a single parameter, k, where k=0 represents complete alveolar mixing, and k=1 represents no alveolar mixing (sequential filling). A linear (a+bV/sub A/) and an exponential (/spl alpha/V/sub A//sup /spl beta//) dependence of D/sub L/ are used to describe the volume dependence of D/sub L/ for carbon monoxide (CO) and nitric oxide (NO), respectively. Application of the model to experimental data from 7 normal humans predicts k=0.51 (0.35), b=0.69 (0.79), and /spl beta/=0.65 (0.19) (mean(SD)). We conclude: (1) D/sub L/ depends on lung volume (strongly for NO, weakly for CO), and (2) the effect of sequential filling must be considered when estimating D/sub L/.