TRACHEAL AND NECK POSITION INFLUENCE UPPER AIRWAY AIR-FLOW DYNAMICS BY ALTERING AIRWAY LENGTH

Upper airway obstruction during sleep is characterized by inspiratory airflow limitation and reductions in maximal inspiratory airflow (VI(m ax)). To determine how mechanical factors modulate VI(max), we analyze d pressure-flow relationships obtained in the isolated upper airway of paralyzed cats. VI(max) and its determinants, the pharyngeal critical pressure (Pcrit) and the nasal resistance (Rn) upstream to the flow-l imiting site (FLS), were measured as caudal tracheal displacement, nec k position, and airway length were systematically varied. As the proxi mal tracheal stump was displaced caudally, graded increases in VI(max) from 145.3 +/- 90.8 (SD) to 285.9 +/- 117.5 ml/s (P < 0.02) and decre ases in Pcrit from -3.0 +/- 3.0 to -9.5 +/- 3.4 cmH2O (P < 0.002) were seen without any significant change in Rn. During neck flexion, signi ficant decreases in VI(max) from 192.1 +/- 68.5 to 87.2 +/- 48.4 ml/s (P = 0.001), increases in Pcrit from -5.3 +/- 2.03 cmH2O to -1.6 +/- 1 .4 cmH2O (P < 0.001), and decreases in Rn from 29.7 +/- 12.2 cmH2O . l -1 . s to 16.2 +/- 8.9 cmH2O . l-1 . s (P < 0.001) were noted compared with the neutral or extended neck position. Relative to the neutral a irway length, upper airway length was found to decrease by 1.15 +/-0.1 4 cm during neck flexion and to lengthen by 0.45 +/- 0.12 cm during ne ck extension. When tracheal displacement and neck position were altere d, VI(max) and Rn correlated directly and Pcrit correlated inversely w ith airway length (P < 0.001). We conclude that alterations in airflow mechanics with caudal tracheal displacement and changes in neck posit ion are primarily due to alterations in airway length.