Studies of the human oropharyngeal airspaces using magnetic resonance imaging. II. The use of three-dimensional gated MRI to determine the influence of mouthpiece diameter and resistance of inhalation devices on the oropharyngeal airspace geometry.

The anatomical geometry of the upper airways of 20 volunteers has been studied using three-dimensional (3D)inhalation-gated magnetic resonance imaging (MRI)for four separate dummy inhalation devices of varying diameters and resistances. The only anatomical parameters showing a consistent dependence upon device characteristics were the total airway and buccal volumes between large and small mouthpieces and the distance from the back of the teeth to the first solid obstacle encountered. Individual subjects showed varied device dependent changes: 45% having an increase in regional airway volumes, particularly in the nasopharynx (+46% volume increase)and laryngo-pharynx (+36% volume increase)for the high-resistance devices compared with the low-resistance ones. However, 30% of subjects showed the opposite behavior, a reduction in naso-pharynx volume (-17%), laryngo-pharynx volume (-17%), and laryngeal cavity (-11%). 25% showed no significant difference in airway volume between high- and low-resistance devices. There was a correlation between maximum inspiratory pressure (MIP)and change in airway volume for high-resistance devices, with those exhibiting expansion having generally lower MIP than the group showing contraction (with the non-responders intermediate). Mean airway minimum and maximum cross-sectional areas and radii were not influenced by device. The geometric mean radius at the epiglottis was 5.1 mm (standard deviation [SD] 1.1) and 5.7 mm (SD 1.1) at the vocal cords. Significant differences observed between males and females included MIP for high-resistance devices, the volume and minimum and maximum radii of the laryngeal and laryngeo-pharynx regions, and total airway volume. A cadaver cast exhibited a number of striking differences in comparison with the in vivo data, most notably a significantly greater total volume.