3. Measurement of dynamic change of airspace: The probe will be tilted upward and downward gradually to locate Retropalatal (RP) and Retroglossal (RG) in pharynx, defined as air column at the highest plane near the uvula before it was obscured by soft palate & at the plane just above hyoid bone, respectively. The transverse diameters of RP and RG are determined by outer margin of air-column, where they were measured at Muller's maneuver, inspiration and expiration.