Longitudinal mixing in pulmonary airways--normal subjects respiring at a constant flow

We have measured the impulse response of helium and sulfur hexafluoride in the airways of five normal human subjects at a respiratory flow of 400 ml/s. The longitudinal mixing of the inert gases was characterized by the increased volume variance of the expired concentration response. This parameter was measured over the largest possible range of airway penetrations, 30-290 ml. Employing a symmetrical model of the airway geometry, we have computed the values of a mean mixing coefficient from the volume variance data. This mixing coefficient is largest in the large airways and decreases rapidly with increasing penetration; it may be as much as 4,000 times greater than the molecular diffusivity; and it is relatively independent of the inert gas tested, at least up to an airway penetration of 180 ml. These observations are consistent with several preivously proposed mixing mechanisms including axial streaming, turbulent dispersion, and mixing by geometric asymmetry. However, the latter observation appears to rule out the importance of laminar dispersion since mixing by this mechanism is inversely dependent on the molecular diffusivity.