Computational modeling of aerosol transport, dispersion, and deposition in rhythmically expanding and contracting terminal alveoli

Inhalation dosimetry in the alveolar region is critical in assessing health risks of inhaled toxicants and therapeutic outcomes of systemic pulmonary drug delivery. Most previous studies of acinar deposition focused on respiratory bronchioles or alveolated ducts with either steady unidirectional or tidal flow through the duct. The precise nature of particle dynamics in response to different wall motions of the blind-ended terminal alveoli is unclear. The objective of this study is to numerically investigate the flow and particle behaviors in terminal alveoli which rhythmically expand and contract at varying modes, frequencies, and amplitudes. Parametric study of physiological factors that affect alveolar deposition will be undertaken to gain a better understanding of relevant deposition mechanisms. Particle dispersion and rhythmic wall effect on alveolar deposition will be examined in the absence of gravity. An idealized terminal alveolus model was developed with rhythmically alveolar moving boundary conditions. The dynamic wall expansion mode and magnitude were based on experimentally measured chest wall motions and tidal volumes. Dimensional analysis was used to develop an alveolar deposition correlation. Results of this study show radical discrepancies in airflow kinetics in comparison to the alveolated duct. Recirculation zones that characterize the airflow in the alveolated duct are absent in the terminal alveolus. This study also demonstrates that the alveolar deposition is sensitive to the particle diameter, alveolar orientation, and breathing frequency, but is relatively insensitive to the breathing depth. The periodic alveolar wall motion leads to incremental particle dispersion, which was found to have a net effect in reducing particle deposition in the alveolus. A correlation was proposed for particle deposition in terminal alveoli that captured the separate contributions from the gravitational sedimentation and rhythmic wall motion.