Structural differences between human and rat lungs: implications for Monte Carlo modeling of aerosol deposition

The geometrical structure of the lung is one of the main factors governing inhaled particle deposition; structural differences among different species are, therefore, of great importance for extrapolation modeling. A statistical analysis of morphometric data for the human and rat tracheobronchial tree reveals significant interspecies differences in airway branching patterns: compared to the relatively dichotomous and symmetric structure of the human lung, the rat lung displays a more monopodial airway branching pattern. Thus, for the rat lung, we recommend characterizing the properties of a given airway, i.e., its size, physiologic function, and distance from the trachea, by its diameter rather than by a theoretically assigned generation number. A Monte Carlo method is used to construct an airway geometry along each inhaled particle's path by randomly selecting airway parameters from their frequency distributions and the correlations among them. While the airway geometry is selected randomly, particle deposition in individual airways is calculated analytically. These stochastic deposition calculations allow for structural differences between the human and rat lung, and the variability of the airway system within each species.