Pressure-volume properties of the upper airway of rabbits

In 10 anesthetized rabbits the upper airway cephalad of the vocal cords was isolated from the distal airway and sealed. Static deflation pressure-volume data were recorded from the isolated upper airway. The relationship between pressure and volume in the upper airway was a straight line; the correlation coefficient (r) ranged from 0.97 to 1.00. The following quantities were derived from the data: the pressure-volume ratio (upper airway elastance, cmH2O/ml), the pressure in the airway at airway closure (closing pressure, cmH2O), and the airway volume at zero airway pressure (reserve volume, ml). Mean upper airway elastance was 8.13 +/- 1.45 [95% confidence intervals (CI)] cmH2O/ml, closing pressure was -6.93 +/- 1.53 (95% CI) cmH2O, and reserve volume was 0.74 +/- 0.15 (95% CI) ml. There was no significant correlation between elastance and closing pressure (r = 0.47, P greater than 0.1), but closing pressure and reserve volume were significantly correlated (r = 0.77, P less than 0.01). Pressure-volume data recorded from newly dead animals exhibited the same linear relationship between pressure and volume observed in living animals. It is concluded that the pressure-volume properties of the isolated upper airway of the rabbit can be expressed as a single value for airway elastance, that estimation of pressure-volume properties over part of the volume range is representative of the whole volume range, and that pressure-volume properties are determined by passive elastic properties of the airway tissues. It appears that the resistance of the upper airway to collapse by negative intraluminal pressure is more dependent on the initial size of the airway than on its elastance.