Helium and sulfur hexafluoride washout in asymmetric lung models

Model calculations were carried out on extensible lung models that are characterized by asymmetries in branching pattern or cross section within the acinus. The calculations contribute to a better understanding of the intrapulmonary mixing of gases poorly soluble in blood. For single-breath washout stimulations of helium (He) and sulfur hexafluoride (SF6), only small differences were found between the results obtained from a lung model with an asymmetric branching pattern using either a local homogeneous or inhomogeneous delta V/V distribution. This indicates that the geometry of this model is the most important factor for generating realistic values for the slope of the alveolar plateau, the Bohr dead space and the end-tidal concentration ratio for SF6 and He. The calculated relation between the decrease in the slope of the alveolar plateau and postinspiratory breath-holding times showed good agreement with experimental findings in healthy adult subjects. Multiple-breath washout simulations enabled us to explain the experimentally observed high dilution of poorly soluble tracer gases in the end-tidal gas for the first few breaths during the washout procedure. Simulations of simultaneous multiple-breath washout experiments of He and SF6 showed that a shift in the crossover point of the normalized end-tidal washout curves is related in particular to cross-section asymmetry of the lung model. These asymmetric lung models are the only ones described up to now that explain many of the features of single- and multiple-breath washout experiments for gases poorly soluble in blood.