Transport of H2 and SF6 in the lung.

The dead spaces for hydrogen and sulfur hexafluoride are predicted from the solution of a partial differential equation, applied to Weibel's morphometric data of the lung, and including longitudinal convection and diffusion coupled with instantaneous radial diffusion. Traces of H2 and SF6 were washed in and out of the lungs of two normal subjects. Dead spaces for both gases were calculated from the wash-out curves by a least squares analysis. Prediction and experiment agree in the case of H2. The model overestimates the dead space for SF6 particularly for large tidal volumes and for high breathing frequencies. Several factors which can contribute to this disagreement are considered. From simulation experiments it is evident, that the dead space for SF6 is highly sensitive to factors which influence molecular dispersion in the region of respiratory bronchioles. Cardiogenic mixing and some sort of flow-dependent mixing in this zone cannot be ruled out. However, the experimental data can also be explained by choosing another set of morphometric data for the alveolated airways.