The importance of a source term in modeling multibreath inert gas washout

The single path model (SPM) of airway gas transport with a distributed blood source term was used to simulate multiple breath inert lung gas washout of N2, He, and SF6 after total body equilibration with these gases. Normalized phase III inert gas washout slopes were computed for each breath and compared with published experimental data obtained under similar conditions on human subjects. The model predicts a normalized slope asymptote which agrees with experimental results within two standard deviations or less of the mean, depending on the lengths and diameters assumed in the acinar airways of the SPM. In the model and in the human subject data, the asymptote represents the development of a quasi-steady state in which the volume of inert gas exhaled at the mouth is equal to the volume transported into the acinar airways by the pulmonary blood during each breath. The present study indicates that at least in the steady state, airway inhomogeneity is not essential to model lung washout data, and that a distributed blood source term in the SPM yields good agreement with experiment.