Effect of upper airway on tracheobronchial fluid dynamics

The upper airways play a significant role in the tracheal flow dynamics. Despite many previous studies, however, the effect of the upper airways on the ventilation distribution in distal airways has remained a challenge. The aim of this study is to experimentally and computationally investigate the dynamic behaviour in the intratracheal flow induced by the upper respiratory tract and to assess its influence on the subsequent tributaries. Patient‐specific images from 2 different modalities (magnetic resonance imaging of the upper airways and computed tomography of the lower airways) were segmented and combined. An experimental phantom of patient‐specific airways (including the oral cavity, larynx, trachea, down to generations 6‐8) was generated using 3D printing. The flow velocities in this phantom model were measured by the flow‐sensitised phase contrast magnetic resonance imaging technique and compared with the computational fluid dynamics simulations. Both experimental and computational results show a good agreement in the time‐averaged velocity fields as well as fluctuating velocity. The flows in the proximal trachea were complex and unsteady under both lower‐ and higher‐flow rate conditions. Computational fluid dynamics simulations were also performed with an airways model without the upper airways. Although the flow near the carina remained unstable only when the inflow rate was high, the influence of the upper airways caused notable changes in distal flow distributions when the 2 airways models were compared with and without the upper airways. The results suggest that the influence of the upper airways should be included in the respiratory flow assessment as the upper airways extensively affect the flows in distal airways and consequent ventilation distribution in the lungs.