1167 A NEUROMUSCULAR MECHANISM PREVENTING AIRWAY OBSTRUCTION

Abstract

To function as a conduit for air, the extrathoracic airway (ETA) must resist closure when intraluminal pressure decreases during inspiration. Previous work (Fed. Proc., in press) suggested that a neuromuscular mechanism opposes ETA closure. Current experiments elucidate this mechanism. The ETA segment most vulnerable to passive closure was determined. Closing pressure (CP) was defined as that negative intraluminal pressure at which pressure changes are no longer transmitted across the ETA. In 5 rabbits, 30-32 days old, post mortem CP of the ETA was -6.0 ± 1.8 cm H₂O; in each case, the nose and trachea remained open at -40 cm H₂O, thus implicating the pharynx or larynx as the site of closure. In anesthetized rabbits, forward inspiratory movement of the hyoid bone and associated soft tissues was observed. Electromyography of muscles which pull the hyoid and tongue forward (mylohyoid, geniohyoid, genioglossus) demonstrated phasic inspiratory activity which increased during asphyxia from airway occlusion. Post mortem, the action of these muscles was simulated by traction on the hyoid. CP of the ETA with hyoid traction was -14.2 ± 3.1 cm H₂O compared to -6.0 ± 1.8 cm H₂O without traction (N=5, p <.01, paired t-test). This difference in CP with and without traction suggests that the mylohyoid, geniohyoid and genioglossus muscles function as accessory muscles of respiration to prevent inspiratory airway obstruction.

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