Non-muscular factors in upper airway patency in the rabbit.

The hypothesis tested in these experiments was that factors other than contraction of upper airway muscles influence the resistance of the upper airway to collapse. The intra-luminal pressures required to close and re-open the upper airway were measured in the isolated upper airways of anesthetised rabbits. The level of activity in upper airway muscles manipulated by ventilation with 100% O2 or 7% CO2 and by muscle paralysis with gallamine. During ventilation with 100% O2 closing pressure was -10.34 +/- 0.53 cm H2O (mean +/- 95% c.i., n = 23) and re-opening pressure was -3.15 +/- 0.51 cm H2O. Ventilation with 7% CO2 changed the closing pressure to -11.63 +/- 0.67 cm H2O (P less than 0.05) and re-opening pressure to -3.81 +/- 0.67 cm H2O (NS). In 10 animals muscle paralysis with gallamine (2 mg/kg i.v.) did not significantly alter closing or re-opening pressures during ventilation with 100% O2, and did not abolish the ability of ventilation with 7% CO2 to augment collapse resistance. In 6 animals death was followed by a fall in closing and re-opening pressures to 30-60% of the values recorded in paralysed animals. We conclude that in this preparation active muscle contraction is not the main source of resistance to airway closure or of the proclivity of the closed airway to re-open.