Soft‑mist inhaler spray dynamics and laryngeal deposition in upper airway models

Targeted laryngeal drug delivery is clinically valuable for procedures requiring rapid topical anesthesia. This study evaluated how the mist from a soft-mist inhaler (Softbreezer) interacts with realistic upper-airway anatomy to influence lidocaine deposition.

An in vitro experimental setup was used to assess deposition from the soft mist inhaler across four upper-airway geometries: two simplified models and two subject-specific models. Each model was tested at inhalation flow rates of 15 and 30 L min^-1. Deposition was quantified across five anatomical regions. High-speed imaging (6100 fps, 20 μs exposure) characterized plume velocity and cone angle.

Droplet size distributions were consistent across flow rates (Dv${ }_{50}\approx$ 16 μm). The subject-specific realistic oral inhalation model achieved the highest laryngeal deposition at 5.3% of the delivered dose at 30 L min^-1, compared with 2.1% in the simplified Virginia Commonwealth University model and 1.6% in the subject-specific artificially opened mouth model. The United States Pharmacopoeia model underestimated upper-airway deposition. High-speed imaging confirmed a slow, coherent plume with minimal turbulence.

Physiologically realistic airway geometry captured during oral inhalation should more accurately represent the flow transitions governing laryngeal deposition.