Influence of progressive and of transient hypoxia on upper airway resistance in normal humans.

In order to evaluate the influence of hypoxia on upper airway patency, we measured the response of upper airway resistance (UAR) to progressive (P) normocapnic hypoxia (Rebuck's method) and transient (T) hypoxia (three to five breaths to 100% N2) in 11 normal men. Breath-by-breath inspiratory UAR was calculated at isoflow during exclusive nasal breathing. The UAR response to hypoxia was characterized by the changes in nasal resistance and pharyngeal resistance (PR) as a function of SaO2, mean inspiratory flow (VT/Tl), and changes in the end-expiratory lung volume (EELV) measured with an inductance vest. The ventilatory response to hypoxia was greater during T (-0.31 +/- 0.03 L/min/%SaO2; mean +/- SEM) than during P (-0.27 +/- 0.03 L/min/%SaO2, p = 0.05). UAR decreased as SaO2 decreased; this decrease was steeper during T than during P hypoxia (delta PR/%SaO2: 3.9 +/- 0.5 during P and 2.5 +/- 0.2 during T, p = 0.05). For the whole group, there was no difference in the slope of the decrease in UAR with increasing VT/Tl between the two hypoxic tests (delta PR/delta VT/Tl: -0.85 +/- 0.1 during P and -0.70 +/- 0.1 during T, p greater than 0.05). However, in four subjects, the slope of the relationship PR/VT/Tl during T remained steeper than during P. EELV increased as SaO2 decreased, with a greater increase during progressive than during transient hypoxia.(ABSTRACT TRUNCATED AT 250 WORDS)