Airway cross section strongly influences alveolar plateau slope of capnograms for smaller tidal volumes

We compared the predictions of the single path convection-diffusion model ( SPM) and the well-mixed single acinus model (SAM) with the normalized slope s (NS) of experimentally measured volumetric capnograms in which VT was var ied in three healthy spontaneously breathing adults. For values of VT great er than 15 ml/kg, the tidal volume penetrates deep into the acinar airways, mixing by molecular diffusion is rapid and the predictions of the SAM and SPM both agree with the experiment. The SPM however shows much better agree ment with the experimental NS data than does the SAM for values of VT less than 10 ml/kg. The explanation for the departure of the SAM from the observ ed experimental data, at small VT, is that it represents the limiting case (of well-mixed alveolar gas) for the SPM, only at large VT, where the assum ption of rapid mixing is most accurate. We conclude that in general, gas ph ase diffusivity and total acinar airway cross sectional area variation with cumulative volume into the lung are essential to realistically model airwa y gas exchange between VT and FRC and to obtain agreement with experimental data under the widest range of breathing conditions. (C) 2000 Elsevier Sci ence B.V. All rights reserved.