Particle deposition in the acinus region of the lung is a significant area of interest, because particles can potentially travel into the bloodstream through the capillaries in the lung. Drugs, in the form of aerosols, small particulates in a volume of air, may be delivered through the respiratory system. Also, toxic, airborne, particles could enter the body through the pulmonary capillaries in the acinus region of the lung. In order to accurately predict particle deposition, the aspects that influence deposition needs to be understood.
Many physiological features may influence flow and particle deposition in the lung; the geometry of the acinus, expansion and contraction of the alveolar walls due to breathing mechanics, heterogeneities in the lung, breathing flow rate, and the number of breaths. In literature, streamlines and pathlines have been examined, both experimentally and computationally, in models representing the alveolar region of the lung. Some of these studies suggest the presence of irreversible flow, which would significantly influence particle deposition. However, none of these models incorporated all significant features: non-symmetric, three dimensional, expanding geometry. Therefore, flow mechanics, behind particle deposition, in the alveolar region are not well understood. Furthermore, lung disease influences the physiological factors that impact particle deposition. Emphysema physically changes the structure of the alveolar region of the lung. How particle deposition changes with emphysema is not fully understood.
In this work, two different alveolar geometries were examined using Particle Image Velocimetry (PIV). The first model represented a healthy alveolar sac, while the second model represented an emphysematous alveolar sac. The same, realistic flow rate was used for both models, which allowed for the fluid flow to be examined as only a function of geometry. The PIV technique was validated by comparing to CFD results, using a simple balloon geometry. Pathlines were plotted in the models in order to examine the fluid flow with respect to time. The fluid was examined, by use of streamlines and pthalines, at the entrance of the alveolar sacs and in areas of high probability for irreversible flow. It was found that the fluid flow inside both alveolar sac geometries was completely reversible, and therefore no mixing was taking place. The comparison between the healthy and emphysematic alveolar sac models showed that the pathlines in health traveled closer to the alveolar walls. Particle deposition by Brownian Diffusion was estimated for particle diameter range of 0.1 µm to 0.01 µm. For the pathlines that began at the duct entrance, the pathlines came approximately 1.5 times closer to the wall in the healthy case when compared to emphysema. Because the pathline traveled closer to the alveolar walls, the particle diffusion was greater in the healthy then emphysema. In the healthy geometry particles with a diameter less then 0.02 µm were estimated to diffuse to all of the alveolar walls within a 5 second time frame, where in emphysema 7 seconds would be needed. It was also determined that if a particle diffuses off of the original streamline, it will remain in the alveolar sac, therefore allowing it to deposit in later breaths.