Mechanics and gas mixing in constricted lungs

Lung elastance, resistance, and the concentration of He in expired gas were measured simultaneously during He washin maneuvers in 5 normal subjects with increasing doses of methacholine delivered by aerosol. A model for the mechanics of constricted airways was tested against these data. The model predicts two states for the constricted airway, one nearly open and one nearly closed. With increasing smooth muscle activation, the fraction of the peripheral airways in the nearly closed state increases. As a result, elastance and resistance increase and ventilation becomes more nonuniform. The predictions of the model agree well with both the mechanics and the gas mixing data.