Impact of bronchial asymmetry on microbubble propagation: a numerical study

Microbubbles generated during mechanical ventilation can enter the bronchioles of the human pulmonary airway. The pressure gradients and shear stresses associated with microbubble movement in the bronchioles may cause epithelial cell injury. Bronchioles are small airways that branch asymmetrically, with daughter bronchioles diverging from a parent bronchiole. This asymmetric branching can influence microbubble propagation dynamics in these airways. This study examines the effect of this asymmetry—quantified by the ratio of the diameter of the larger daughter bronchiole to that of the smaller one ( \\(\\phi = D_\\textrm{LD}/D_\\textrm{SL}\\) )—on microbubble dynamics. A two-dimensional asymmetric bifurcating airway model is used for this purpose. The governing equations are solved using the finite element method, with the interface tracked via the arbitrary Lagrangian–Eulerian method. The influence of \\(\\phi \\) on microbubble propagation, shear stress, and pressure gradient is examined across different Reynolds and capillary numbers. Results indicate that \\(\\phi \\) strongly affects microbubble propagation. Even at \\(\\phi = 1.2\\) , the microbubble preferentially enters the larger daughter bronchiole rather than splitting at the carina. This altered trajectory modifies the distribution of pressure gradients and shear stress along the bronchiole walls, with the smaller daughter bronchiole experiencing lower pressure gradient and shear stress at higher values of \\(\\phi \\) . However, comparison across different \\(\\phi \\) values indicates that the capillary number remains the decisive factor in determining shear stress and pressure gradient, with \\(\\phi \\) having a negligible effect on amplifying or diminishing the influence of the capillary number.