Use of Extrathoracic Deposition Models for Patient-Specific Dose Estimation during Inhaler Design

The lung dose of inhaled pharmaceutical aerosol that an individual will receive from an inhaler can now be more accurately estimated in light of recent extrathoracic deposition modeling that has correlated characteristic airway dimensions with deposition. This paper first summarizes the current state of extrathoracic deposition models, including recent developments that have quantified the effects of aerosol electrostatics and inhaler mouthpiece diameter on deposition. A generalized equation for predicting extrathoracic deposition in different subjects is then developed and average characteristic airway dimensions representative of different age groups are indicated. A methodology is then presented to predict the lung dose per unit body surface area individuals will receive from an inhaler. A sample calculation shows that a typical 10-year-old child subject would receive a lower lung dose per unit body surface area than an adult subject inhaling through the same inhaler at the same 90 L min-1 flow rate, due to greater extrathoracic deposition in the child. In order to provide an equivalent lung dose per unit body surface area to the child as to the adult, an inhaler particle size adjustment is specified. Finally, the use of idealized geometries for developing inhaler-specific empirical correlations and improving upon inhaler design is outlined.