TRACHEOBRONCHIAL DILATION DURING ISOCAPNIC HYPOXIA IN CONSCIOUS HUMANS

To assess the effects of isocapnic hypoxia on the pharynx, glottis, ex trathoracic trachea (ET), intrathoracic trachea (IT), and main bronchi (MB), we measured the cross-sectional areas of these airways by acous tic reflection technique in 15 healthy volunteers. Measurements were m ade during tidal volume breathing while subjects were normoxic [arteri al O2 saturation (Sa(O2)) >95%] or were made hypoxic by a rebreathing procedure. Under hypoxemic conditions, airway cross-sectional areas in creased significantly at ET, IT, and MB levels (P < 0.001). The magnit ude of this dilation was similar for both levels of hypoxemia studied (Sa(O2) 80-85% and 70-75%); at the milder of the two hypoxemic conditi ons, ET cross-sectional area increased by 12.4 +/- 4.2% (SE), IT by 10 .2 +/- 5.9%, and MB by 19.1 +/- 3.2%. No significant changes were foun d in the pharyngeal or glottic areas. Dilation was not produced by nor moxic isocapnic hyperventilation, and the use of hypoxic airway gas mi xtures did not artifactually alter acoustic reflection measurements in a mechanical model. Vagal airway tone, as reflected by airway constri ction during pauses in tidal breathing, was unaffected by isocapnic hy poxia. We conclude that isocapnic hypoxia produces dilation of the tra chea and major bronchi, an effect unaccounted for by an alteration in the ventilatory pattern.