In vivo estimation of tracheal distensibility and hysteresis in normal adults.

We used the acoustic reflection technique to measure the cross-sectional area of tracheal and bronchial airway segments of eight healthy adults. We measured airway area during a slow continuous expiration from total lung capacity (TLC) to residual volume (RV) and during inspiration back to TLC. Lung volume and esophageal pressure were monitored continuously during this quasi-static, double vital capacity maneuver. We found that 1) the area of tracheal and bronchial segments increases with increasing lung volume and transpulmonary pressure, 2) the trachea and bronchi exhibit a variable degree of hysteresis, which may be greater or less than that of the lung parenchyma, 3) extrathoracic and intrathoracic tracheal segments behaved as if they were subjected to similar transmural pressure and had similar elastic properties, and 4) specific compliance (means +/- SE) for the intrathoracic and bronchial segments, calculated with the assumption that transmural pressure is equal to the transpulmonary pressure, was significantly (P less than 0.05) smaller for the intrathoracic segment than for the bronchial segment: (2.1 +/- 2.0) X 10(-3) cmH2O-1 vs. (9.1 +/- 2.1) X 10(-3) cmH2O-1. Direct measurements of airway area using acoustic reflections are in good agreement with previous estimates of airway distensibility in vivo, obtained by radiography or endoscopy.