Understanding the effects of body mass index and inhaler mouthpiece size on human upper airway geometry

Background: Understanding the geometries of the human upper airway and their impact on drug particle transport is important. This study investigates the impact of inhaler mouthpiece size on extrathoracic airway geometry across a diverse body mass index (BMI) range. 

Methods and Materials: Magnetic resonance imaging (MRI) was performed on 28 participants. Subjects were asked to bite on replicas of inhaler mouthpieces with incisal openings of 13.5 mm and 27 mm, and the effects of mouthpiece size on upper airway geometry were analysed. 

Results: Mouthpiece size affects the oral space, and the results show a significant linear correlation between oral space and BMI, with a reduction in the cross-sectional area (CSA) between the soft palate and the tongue of approximately 0.08 cm² per BMI unit increase. This trend contrasts with an increase in anterior-posterior (AP) length across three other pharyngeal regions of interest, with the largest increase observed in the oropharynx, where AP length increased by approximately 0.042 cm/(kg·m²). While the impact of device size was significant in the oral space across all participants, its effect in the oropharynx was significant only in the high BMI cohort. This group exhibited an expansion in the oropharynx when using the larger mouthpiece compared to the smaller one. 

Conclusion: These findings may have implications for inhaler design and prescription. The knowledge presented has the potential to improve inhaler efficacy in high BMI populations when accounting for mouthpiece size.