The patency of the pharyngeal airway depends on pharyngeal cross-sectional area, compliance and shape. The extrinsic tongue muscles and the muscles of the pharyngeal wall play a major role in modulating these mechanical properties. The velopharynx appears to be the region most susceptible to collapse, and recent data suggest that compliance and the critical collapsing pressure (Pcrit) vary along the rostral-caudal extent of the velopharynx in humans and lower mammals. The rat pharynx will be isolated and pharyngeal pressure will be changed systematically while magnetic resonance images are obtained. Up to 15, 1.0 mm axial slices through the velopharynx and caudal oropharynx will be obtained, and the cross-sectional area of each slice computed. The pressure: area relation will then be obtained for each slice, which will allow derivation of regional pharyngeal compliance and Pcrit. The influence of pharyngeal muscle activity on these variables is evaluated by comparing control images with those obtained while the tongue or pharyngeal wall muscles are stimulated electrically. Geometric analyses will allow us to describe the ratio of the lateral to the anterior-posterior diameters of each axial slice (i.e., the """"""""ellipticity"""""""" of the airway), and the location of the flow-limiting segment (FLS) of the pharynx. The hypotheses are: 1) compliance is regionally dependent, with the highest values in the velopharynx; 2) the FLS is located in the velopharynx; 3) the shape of the velopharynx is most elliptical at the FLS; 4) activation of tongue or pharyngeal constrictor muscles will decrease regional compliance and Pcrit, and make the airway less elliptical (i.e., will """"""""stiffen"""""""" the airway). These experiments will provide unique information on the geometry and intrinsic mechanical properties of the pharynx, and the changes that occur with pharyngeal muscle contraction. The data may help to guide treatment strategies that are aimed at using airway pressure changes and/or implantable pharyngeal muscle nerve stimulators to alleviate airway obstruction during sleep.
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