Airway area by acoustic reflectometry in man

The geometry of the upper airway of the respiratory system in normal adults has been inferred noninvasively by an acoustic reflectometry technique. Short duration pressure transients launched into the respiratory system at the mouth give rise to echoes reflecting from the inhomogeneous airway structure. This data is transformed into an inference of the cross‐sectional area of the airway as a function of distance from the lips. The results indicate a local area maximum in the vicinity of the uvula (velum closed), a local minimum in the oropharynx near the top of the epiglottis, and another local maximum in the vicinity of the hypopharynx. The area contracts passing through the thyroid and cricoid cartilages, and expands to a plateau in the extra‐thoracic and intra‐thoracic trachea. At fixed lung volume the areas of the larynx and extra‐thoracic trachea are sensitive to transrespiratory system pressure while the area in the vicinity of the thyroid and cricoid cartilages and intrathoracic trachea are not. Radiographic determinations indicate that this method tends to systematically overestimate the anatomic area in the subglottic region as a result of nonrigidity of the airway walls. [Supported by NHLBI contract N01‐HR‐6‐2901.]