The fate of bronchiole-originated droplets within and beyond airways

Abstract

The outbreak of coronavirus disease 2019 (COVID-19) has affected people worldwide since December 2019. According to the World Health Organization, more than 4.5 million people worldwide had been infected with SARS-CoV-2 and more than 307,000 people had died as of May 17, 2020. Recent studies have demonstrated that small expiratory droplets that contain pathogens may play a significant role in the transmission of respiratory infectious diseases. In this light, the use of particles that originate in the bronchioles as biomarkers has been explored for the noninvasive diagnosis of infectious lung diseases. However, data are scarce regarding the generation of droplets from the bronchioles and their fate within and beyond the human airways during and after exhalation. To predict the air flow field within the lower respiratory system during exhalation, a truncated adult lower-airway model is developed in this study. The lower airways are considered locally symmetric, one of the paired branches is truncated, and the boundary surface at the truncated location is defined by a velocity inlet whose profile is interpolated from the symmetry interior surface. We find that large droplets (i.e., >5 µm) generated from the bronchioles are likely to be blocked by the lower airways. The deposition of large droplets from the bronchioles is limited to the terminal regions, which means that these droplets are unlikely to be released again via possible aerosolization under the shear force acting on the airway walls. To study the deposition of bronchiole-originated expiratory droplets in the upper airway, we reconstruct a realistic upper airway model from the computed tomography scans of an adult. The upper airway model is linked to the truncated lower-airway model starting from the trachea. We find that the complex geometry of the upper airway leads to the complete deposition when the droplet size is larger than the threshold diameter we found (i.e., >6.5 µm for adult exhale at 20 L/min) in the airways, whereas small particles generated from the bronchioles are likely to escape the mouth to the external environment, even during breathing. The evidence garnered from a computational fluid dynamics (CFD) simulation suggests that particle size plays an important role in virus transmission and disease diagnosis. To investigate the infection risk to healthcare workers (HCWs) due to expiratory droplets as they treat patients with respiratory diseases, a CFD model is developed of the air flow around a patient using a nebulizer mask. The CFD model is validated against the results of a previous shadowgraph study. The CFD results show that HCWs or people around patients are likely to be infected when administering nebulizer treatment. To lower the infection risk for HCWs who are treating patients with lower-airway diseases, a patient-specific airway reconstruction method is developed based on 3D convolutional neural networks. The CFD results of specific patients obtained with the reconstructed airway models can be used to determine the size of drug particles that should be administered to reach the infected regions.