Total cross-sectional areas were computed from direct measurements made on two human central airway casts. Acoustic pulse-response measurements were obtained on rigid-walled positive replicas of these casts. From the acoustic response data of each cast, we computed the area-distance function of the acoustically equivalent structure (i.e., the structure with regular branching and negligible viscous losses, but with similar acoustic properties). The acoustic data predicted equivalent areas that compared favorably to the total cross-sectional areas in the casts at all points from the beginning of the trachea to distances about 6 cm beyond the carina corresponding to airways of the third, fourth, or fifth generation. These results indicate that, at least in the central airways, branching asymmetry and internal energy losses introduced negligible errors in estimates of cross-sectional areas derived from acoustic pulse-response measurements. This rapid noninvasive technique thus shows promise as a method of detecting upper and central airway obstruction.
Pulse response of a nonlinear layer
