Kinematically irreversible flow and aerosol transport in the pulmonary acinus: a departure from classical dispersive transport

Current theories describe aerosol transport in the lung as a dispersive (diffusion-like) process, characterized by an effective diffusion coefficient, D eff , in the context of reversible alveolar flow. Our recent experimental data, however, question the validity of these basic assumptions. In this study, we described the behavior of fluid particles (or bolus) in a realistic numerical alveolated duct model with rhythmically expanding walls. We found acinar flow exhibiting multiple saddle points –characteristic of chaotic flow–, resulting in substantial flow irreversibility. Computations of axial variance of bolus spreading indicate that the growth of the variance with respect to time is faster than linear, a finding inconsistent with dispersion theory. Lateral behavior of the bolus shows fine scale stretch-and-fold striations, exhibiting fractal-like patterns with a fractal dimension of 1.2, which compares well with the fractal dimension of 1.1 observed in our experimental studies performed with rat lungs. We conclude that kinematic irreversibility of acinar flow due to chaotic flow may be the dominant mechanism of aerosol transport deep in the lungs.