Tracheal and neck position influence upper airway airflow dynamics by altering airway length

Upper airway obstruction during sleep is characterized by inspiratory airflow limitation and reductions in maximal inspiratory airflow (V̇I(max)). To determine how mechanical factors modulate V̇I(max), we analyzed pressure- flow relationships obtained in the isolated upper airway of paralyzed cats. V̇I(max) and its determinants, the pharyngeal critical pressure (Pcrit) and the nasal resistance (Rn) upstream to the flow-limiting site (FLS), were measured as caudal tracheal displacement, neck position, and airway length were systematically varied. As the proximal tracheal stump was displaced caudally, graded increases in V̇I(max) from 145.3 ± 90.8 (SD) to 285.9 ± 117.5 ml/s (P < 0.02) and decreases 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, significant decreases in V̇I(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 airway length, upper airway length was found to decrease by 1.15 ± 0.14 cm during neck flexion and to lengthen by 0.45 ± 0.12 cm during neck extension. When tracheal displacement and neck position were altered, V̇I(max) and Rn correlated directly and Pcrit correlated inversely with airway length (P < 0.001). We conclude that alterations in airflow mechanics with caudal tracheal displacement and changes in neck position are primarily due to alterations in airway length.