Mechanical effects of pharyngeal constrictor activation on pharyngeal airway function

The mechanical effects of pharyngeal constrictor (PC) muscle activation on pharyngeal airway function were determined in 20 decerebrate, tracheotomize d cats. In 10 cats, a high-compliance balloon attached to a pressure transd ucer was partially inflated to just occlude the pharyngeal airway. During p rogressive hyperoxic hypercapnia, changes in pharyngeal balloon pressure we re directly related to phasic expiratory hyopharyngeus (middle PC) activity . In two separate protocols in 10 additional cats, the following measuremen ts were obtained with and without bilateral electrical stimulation (0.2-ms duration, threshold voltage) of the distal cut end of the vagus nerve's pha ryngeal branch supplying PC motor output: 1) pressure-volume relationships in an isolated, sealed upper airway at a stimulation frequency of 30 Hz and 2) rostrally directed axial force over a stimulation frequency range of 0- 40 Hz. Airway compliance determined from the pressure-volume relationships decreased with PC stimulation at and below resting airway volume. Compared with the unstimulated condition, PC stimulation increased airway pressure a t airway volumes at and above resting volume. This constrictor effect progr essively diminished as airway volume was brought below resting volume. At r elatively low airway volumes below resting volume, PC stimulation decreased airway pressure compared with that without stimulation. PC stimulation gen erated a rostrally directed axial force that was directly related to stimul ation frequency. The results indicate that PC activation stiffens the phary ngeal airway, exerting both radial and axial effects. The radial effects ar e dependent on airway volume: constriction of the airway at relatively high airway volumes, and dilation of the airway at relatively low airway volume s. The results imply that, under certain conditions, PC muscle activation m ay promote pharyngeal airway patency.