Elastic characteristics of the airway wall.

To examine the elastic behavior of the upper airway, we obtained pressure-area plots from data gathered from acoustic images of the airway and measurements of mouth pressure during tidal breathing in 10 adult human volunteers (dA/dP). These plots revealed both tidal hysteresis and a change in slope as a function of distance along the airway. The slope of the regression line of the dA/dP plots decreased from the pharyngeal region to the trachea and became 0 at the thoracic inlet, the location of which was independently assessed. In most subjects the slope became negative distal to the thoracic inlet. Correlation coefficients between pressure and area approached 1 in the pharyngeal region and 0 at the thoracic inlet. When subjects breathed against a small resistive load (10 cmH2O.1(-1).s) pharyngeal, extrathoracic, and intrathoracic pressure-area plots were exaggerated but the slope at the thoracic inlet was unchanged. We conclude that this pressure-area characteristic defines regional differences in upper airway elasticity and delineates the transition point between the intra- and extrathoracic airways.