Dependency of upper airway patency on head position: the effect of muscle contraction.

In the present study we examined the effect of flexion and extension of the head on upper airway (UAW) patency in anesthetized dogs, and compared the dilatory and stabilizing effects of electrically stimulated UAW muscles at the different head positions. Flexion of the head increased UAW resistance (Ruaw) and reduced maximal flow (Vmax), but had little effect on the negative pressure at which UAW collapse occurred (Pcrit). Extension of the head, on the other hand, resulted in more negative Pcrit values and increased Vmax without significantly affecting Ruaw. Electrically induced UAW muscle contraction affected the pressure-flow curve and Ruaw, as well as Pcrit. Changing head position had a substantial effect on the dilatory and stabilizing effect of the various UAW muscles. However, independent of head position, genioglossus stimulation was most effective in reducing Ruaw and increasing Pcrit. We conclude that in the anesthetized, supine dog, head position affects the mechanical properties of the UAW and the effects of UAW muscle contraction.