Transport mechanisms in the human bronchial tree: in vitro experiments using Magnetic Resonance Velocimetry

 In All Presenters, Coletti, Filippo


INSTITUTION: Department of Aerospace Engineering and Mechanics, University of Minnesotoa, MN, USA


BACKGROUND/PURPOSE: Furthering our knowledge of respiratory fluid dynamics is greatly beneficial to understand lung diseases and to improve aerosol drug delivery and mechanical ventilatory techniques.

METHOD: I will discuss recent results obtained from an in-vitro platform designed to study detailed flow features in the central human airways. Idealized and realistic replicas of the bronchial tree are 3D printed and inserted in a flow loop circulating aqueous fluid, allowing us to measure volumetric velocity fields at high spatial resolution using Magnetic Resonance Velocimetry. We consider regimes of steady inhalation, steady exhalation, and oscillatory ventilation for a range of physiologically and clinically relevant Reynolds and Womersley numbers, and evaluate the relative importance of different transport mechanisms.

RESULTS: Longitudinal dispersion is found to be higher during inhalation, while lateral dispersion is higher during exhalation. This asymmetry is more evident in the realistic anatomy, which also shows strong heterogeneity in local ventilation. Streamwise vortices arise due to the local curvature of the branches, and constitute one of the main transport mechanisms for relative flow rates. At the higher Reynolds number, inertia induces significant non-locality, and the vortices are transported across successive generation of bronchial branching. Flow reversal (a phenomenon consequential for gas mixing, particle transport and mechano-transduction at the epithelium) is found in both idealized and realistic airways, during steady and oscillatory regimes. The net flow drift during the ventilation cycle (steady streaming) is experimentally evaluated for the first time, and found to be much smaller than the advective flow, although not insignificant for the realistic airway geometry. In the latter, the different impedance of the various bronchial pathways results in asynchronous ventilation between the lower and upper lobes (pendelluft) which is suggested by Eulerian flow fields and directly shown using Lagrangian pathlines.

CONCLUSION: Three-dimensional measurements of the respiratory flow field in realistic models of the bronchial tree are essential to reach a predictive understanding of the transport mechanisms in the central airways.



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