INFLUENCE OF PRELOAD AND AFTERLOAD ON GENIOGLOSSUS MUSCLE LENGTH IN AWAKE GOATS

The genioglossus is an upper airway dilator muscle, the length of whic h is directly related to patency in the oropharyngeal region. We hypot hesized that genioglossal length (Lgg) is dynamically influenced by th e afterload exerted by negative upper airway pressure during inspirati on and by the intrinsic length-tension characteristics of the muscle ( preload). Seven awake goats were chronically instrumented with electro des for EMCgg and sonomicrometry for Lgg. We examined the Lgg-EMGgg re lationship during hypercapnia and inspiratory resistive loading (18 cm H2O/L/s). The goats breathed through the upper airway (TC) or airflow was diverted through a tracheostomy (TO). We found that: (1) passive inspiratory lengthening was observed with negative upper airway pressu re (UAP) but not when UAP = 0 (TO breathing), (2) Lgg shortening for a given EMCgg was significantly decreased with negative inspiratory UAP , and (3) phasic Lgg shortening per unit EMCgg was greatest when Lgg w as near optimal length (Lo). We conclude that genioglossal length is s ubstantially influenced by afterload exerted by negative UAP and that genioglossal active shortening may be limited if the muscle operates a t a length significantly greater or less than the optimal length.