Modelling steady state pulmonary elimination of He, SF6 and CO2: effect of morphometry.

We studied the influence of acinar morphometry on the shape of simulated expirograms computed from a single path convection-diffusion model that includes a source term for gas evolution from the blood (Scherer et al., J. Appl. Physiol. 64: 1022-1029, 1988). Acinar structure was obtained from published data of 3 different lung morphometries. The simulations were performed over a range of tidal volumes (VT) and breathing frequencies (f) comparable to those observed in a previously reported human study. Airways dead space (VDaw) increased with VT in all the morphometric models tested and in the experimental data. The increase in VDaw with VT was inversely related to the diffusivity of the evolving gas and to the rate of increase in airway cross-section of the most mouthward (proximal) alveolated generations of the models. Normalized phase III slope for all the gases decreased with increasing VT in all the models as was previously reported for healthy human subjects. In the model simulations, the greatest sensitivity of phase III slope to VT was seen with the least diffusible gas using the airway morphometry with the smallest cross-sectional areas in the proximal alveolated generations. We conclude that both VDaw and phase III slope of an evolving gas are sensitive to the geometry of the proximal acinar airways and that this is manifest by their dependence on tidal volume, breathing frequency, molecular diffusivity and alveolar/blood source emission rate. The model simulations indicate that heterogeneity of gas washout is not required to explain the magnitude of the phase III slope in healthy human subjects.