Role of Alveolar Topology on Acinar Flows and Convective Mixing

Abstract

Due to experimental challenges, computational simulations are often sought to quantify inhaled aerosol transport in the pulmonary acinus. Commonly, these are performed using generic alveolar topologies, including spheres, toroids, and polyhedra, to mimic the complex acinar morphology. Yet, local acinar flows and ensuing particle transport are anticipated to be influenced by the specific morphological structures. We have assessed a range of acinar models under self-similar breathing conditions with respect to alveolar flow patterns, convective flow mixing, and deposition of fine particles (1.3 μm diameter). By tracking passive tracers over cumulative breathing cycles, we find that irreversible flow mixing correlates with the location and strength of the recirculating vortex inside the cavity. Such effects are strongest in proximal acinar generations where the ratio of alveolar to ductal flow rates is low and interalveolar disparities are most apparent. Our results for multi-alveolated acinar ducts highlight that fine 1 μm inhaled particles subject to alveolar flows are sensitive to the alveolar topology, underlining interalveolar disparities in particle deposition patterns. Despite the simplicity of the acinar models investigated, our findings suggest that alveolar topologies influence more significantly local flow patterns and deposition sites of fine particles for upper generations emphasizing the importance of the selected acinar model. In distal acinar generations, however, the alveolar geometry primarily needs to mimic the space-filling alveolar arrangement dictated by lung morphology.

Image‐based computational fluid dynamics in the lung: virtual reality or new clinical practice?

“…They also indicated that the exact shape of alveoli may have less influence on the presence of alveolar recirculation. Many other works have also shown this convective mixing effect both qualitatively and quantitatively . Hofemeir and Sznitman showed that alveolar geometric topology influences localized flow characteristics and deposition sites for finer particles.…”

“…Nevertheless, understanding of airflow mechanics and particle transport in the acinar region has significantly advanced due to CFD. One important question that has been investigated is the role of proximal and distal acinar airways as mixing chambers . Existence of mixing in these submillimeter alveolar units has applications in both pharmacological and clinical sectors.…”

“…One important question that has been investigated is the role of proximal and distal acinar airways as mixing chambers. 65,[69][70][71][72][73] Existence of mixing in these submillimeter alveolar units has applications in both pharmacological and clinical sectors. Further reviews on computational models to simulate transport can be found elsewhere.…”

Biomimetics of fetal alveolar flow phenomena using microfluidics

“…3(b) and 3(c)); this latter flow characteristic is a well-known property of low-Re separated cavity flow phenomena 34,41,53,54 and has been previously shown in the context of adult air-filled lungs both experimentally 41,55,56 and numerically. 33,51,57,58 To deliver a comprehensive rendering of the different flow regimes anticipated across the developmental stages of fetal lungs, our numerical and experimental flow results are summarized in the phase map of Fig. 4.…”

“…A constant pressure of 100 000 Pa was arbitrarily set at the domain outlet. 33,50,51 Mesh refinement studies were performed to ensure that flow solutions were grid independent where changes in the average flow velocity magnitude and average shear rate over the alveolar cavity opening were within 1% upon further mesh refinement, as previously established. 41 All numerical solutions were found to converge with residuals below 10 À7 , measuring the imbalance in conservation of mass.…”