The pharynx is the site of upper airway obstruction during sleep. As a
collapsible tube, pharyngeal patency is determined by transmural pres
sure and the compliance of the pharyngeal wall. Thus, several factors
may influence upper airway patency including the activity of upper air
way dilating muscles, the magnitude of caudal traction generated by th
oracic inspiratory activity, vascular tone and mucosal surface forces.
Changing ventilatory motor output influences upper airway patency pri
marily by altering dilating muscle activity or caudal traction. Increa
sed ventilatory motor output enhances upper airway patency, Isolated r
eduction of ventilatory motor output has no significant effect on uppe
r airway patency. However, upper airway narrowing or occlusion occur a
t the nadir of ventilatory drive during induced periodic breathing and
during central apnea. The latter indicates that negative intraluminal
pressure is not required for upper airway obstruction during sleep. T
herefore, upper airway occlusion during sleep may be due to: (1) passi
ve collapse of a compliant upper airway by gravitational factors or (2
) active closure generated by the contraction of the pharyngeal constr
ictors.