Assessment of muscle action on upper airway stability in anesthetized dogs.

Skeletal muscle activation is believed to be important in the maintenance of upper airway patency. To determine where and how muscles affect pharyngeal stability, we assessed in heavily anesthetized, ventilated dogs, the negative pressure required to close the nasopharynx and the passage from the oral to the pharyngeal airway before and after electrical stimulation of six pairs of upper airway muscles: the sternohyoid, sternothyroid, ceratohyoid, thyrohyoid, genioglossus, and geniohyoid. Before muscle stimulation, the pressures required to close the nasopharynx and the oral passage were -9.8 +/- 2.3 (mean +/- SEM) and -4.9 +/- 1.9 cm H2O, respectively. Submaximal electrical stimulation of each of the six muscle pairs caused consistent and substantial improvements in nasopharynx closing pressure. In contrast, the stabilizing actions of these muscles on the oral passage was less consistent and smaller in magnitude. Using pressure in the sealed upper airway as an index of forces acting on the airway, we found a strong association between the effect of muscle stimulation on the closing pressure and the effect of muscle stimulation on producing a dilating force on the airway. We conclude that muscles in the anterior and lateral pharyngeal wall improve nasopharynx stability to a greater extent than oral passage stability and that the action by any of these six muscle pairs improves airway stability through their ability to dilate the upper airway.