Airway secretions influence upper airway patency in the rabbit

The hypothesis tested in these experiments was that the properties of the upper airway mucosal surface may be important in reopening of the closed airway, and that mucosal surface properties may depend on airway secretions. The intraluminal pressures required to close and reopen the upper airways were measured in the isolated upper airways of anesthetized rabbits. Atropine (0.1 mg/kg i.v.), given to reduce the volume of upper airway secretions, had no effect on closing or on reopening pressures. Stimulation of upper airway secretions in 6 animals with methacholine (0.2 mg/kg subcutaneously) changed closing pressures from -10.63 +/- 0.57 to -16.91 +/- 0.60 cm H2O (p less than 0.05) but made the airway less likely to reopen, changing reopening pressures from -3.45 +/- 0.48 to -2.12 +/- 0.39 cm H2O (p less than 0.04), and caused frequent failure of the airway to reopen spontaneously. Filling the upper airways with saline to mimic the hydrostatic forces present in the mucus-filled airway caused both closing and reopening pressures to become more negative. We conclude that reopening pressure is influenced by the secretions lining the airway surface, and, therefore, that airway closure and airway reopening may be substantially independent.