Studies of the human oropharyngeal airspaces using magnetic resonance imaging. III. The effects of device resistance with forced maneuver and tidal breathing on upper airway geometry.

The anatomical geometry of the upper airways of five healthy volunteers were studied using three-dimensional inhalation-gated magnetic resonance imaging (MRI) for two dummy inhalation devices of varying airflow resistances (0.049(0.5) and 0.004(0.5) kPa(0.5)(L/min)(1)) using a forced maneuver (sharp inhalation) and tidal breathing. The range of maximum inspiratory pressures (MIP) was 0.02-6 kPa. Significant airway expansion occurred for the lower resistance device with forced maneuver compared with tidal breathing. The mean upper airway volume for forced maneuver through a low-resistance device was 60 (SD 15) cm(3) compared with 38 (SD 9) cm(3) for tidal breathing through the same device. Regionally, these increases occurred in the oropharynx (factor of three increase) and the laryngo-pharynx (factor of two increase). Significant changes did not occur for the buccal and laryngeal regions. No significant airway volume change occurred between the two breathing modes using the high-resistance device. In contrast to an earlier study using tidal breathing, device-related volume changes occurred with forced maneuvers in the oropharynx and laryngopharynx. For the low resistance device, significant differences for the minimum airway equivalent radius at the epiglottis occurred between forced maneuver (8.0 mm) and tidal breathing (5.2 mm), and for the maximum airway diameter in the laryngo-pharynx (12.4 mm vs 9.1 mm). For forced maneuvers, significant differences in maximum airway diameter in the oropharynx were also apparent between the devices (12.6 mm low vs. 9.1 mm high) and also in the laryngopharynx (12.4 mm low vs. 9.4 mm high). The minimum radius at the epiglottis varied significantly between devices under forced maneuver breathing (8.0 mm low vs. 8.3 mm high). There was no correlation between airway expansion or contraction and MIP. A linear relationship was found between airway volume changes and maximum calculated volumetric airflow. This work has implication for the modeling of inhaled aerosol dynamics and in vitro particle impaction studies.