Abstract
The site of deposition of pulmonary delivered aerosols is dependent on the aerosol׳s droplet size distribution, which may change during inhalation. The aim of this study was to develop a freely accessible and adaptable model that describes the growth (due to condensation) and shrinkage (due to evaporation) of inhaled droplets as a function of the distance from the airway wall during various inhalation conditions, for a laminar flow scenario. This was achieved by developing a model with which the evaporation of water from a droplet surface or condensation of water onto the droplet surface can be calculated. This model was then applied to a second model that describes the heat and mass transfer from the airway wall to the inhaled aerosol. The latter was based on the Weibel model. It was found that the growth and shrinkage of inhaled droplets markedly differs, depending on the distance from the airway wall. Droplets near the wall start to grow immediately due to fast water vapor transfer from the wall to the cold inhaled air. This growth continues until the air reaches body temperature and is fully saturated. However, droplets in the center of the airway first evaporate partly, due to a delay in water vapor transfer from the airway wall, before they start to grow. Depending on the conditions during inhalation, the droplet size distribution can widen considerably, which may affect the lung deposition and thereby the efficacy of the inhalation therapy. In conclusion, the model was able to show the effect of the conditions in the respiratory tract on the growth and shrinkage of inhaled droplets during standard inhalation conditions. Future developments can be aimed at expanding the model to include turbulent flow and hygroscopic growth, to improve the accuracy of the model and make it applicable to both droplets of solutions and dry particles.
Highlights
During a standard inhalation procedure, the largest particles are deposited in the mouth and the back of the throat, meaning their effect is essentially lost
The rest of the particles are deposited throughout the respiratory tract, where the spread of deposition is mainly dependent on the particle size distribution of the liquid aerosol or dry powder
If it would be possible to tell whether, and if so, how much the aerodynamic diameter of inhaled particles is changed while traversing the respiratory tract, it could aid in optimizing formulations for inhalation
Summary
During a standard inhalation procedure, the largest particles (either dry powder or droplets, in this paper only aqueous droplets will be considered) are deposited in the mouth and the back of the throat, meaning their effect is essentially lost. A lot is known about the conditions to which inhaled particles and droplets are subjected when inhaled (Ferron, 1977; Olson, Sudlow, Horsfield & Filley, 1973; Tian, Longest, Su & Hindle, 2011) Most of these studies revolve around the use of computational fluid dynamics (CFD), which is an excellent method to accurately model the complex flow profiles that exist inside the airways. The aim of this study is to investigate the behavior, i.e. growth and shrinkage, of inhaled particles in the respiratory tract with a theoretical model based on standard heat and mass balances This will be done by primarily focusing on aqueous particles. The clinical relevance of the results of this study towards inhalation practices will be discussed
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