DYNAMIC IMAGING OF THE UPPER AIRWAY DURING RESPIRATION IN NORMAL SUBJECTS

The present study was conducted to determine the effects of quiet resp iration on upper airway caliber in 15 normal subjects by using cine co mputed tomography. The cine computed tomography (Imatron) scanner was programmed to obtain 8-mm-thick axial slices every 0.4 s during inspir ation and expiration at four anatomic levels, from the nasopharynx to the retroglossal region. Airflow (pneumotachograph) was measured, and tidal volume was obtained by integration. Upper airway area, determine d by an objective edge detection algorithm, was plotted as a function of tidal volume to generate a loop describing upper airway area change s at each level during a respiratory cycle. The results demonstrate a 17% change in airway size across all anatomic levels during respiratio n. The maximum upper airway cross-sectional area at all four anatomic levels was significantly greater during expiration than during inspira tion. Other major findings include 1) upper airway cross-sectional are a decreases slightly during early inspiration, enlarges toward end ins piration, and is larger at end inspiration than at the beginning of in spiration; 2) upper airway cross-sectional area enlarges from end insp iration to the first point in expiration; the airway enlarges further, reaching its maximum early in expiration, and then narrows toward end expiration; and 3) the changes in upper airway dimensions during rest ing tidal breathing are greater in the lateral than in the anteroposte rior direction. The data suggest that during inspiration, the action o f negative intraluminal pressure may be largely balanced by the action of the upper airway dilator muscles, whereas during expiration, posit ive intraluminal pressure produces expansion of the upper airway.