CHANGES IN UPPER AIRWAY-RESISTANCE DURING PROGRESSIVE NORMOCAPNIC HYPOXIA IN NORMAL MEN

The effects of normocapnic progressive hypoxia on nasal and pharyngeal resistances were evaluated in nine normal men. To calculate resistances, upper airway pressures were measured with two low-bias flow catheters; one was placed at the tip of the epiglottis and the other in the posterior nasopharynx, and we measured flow with a Fleish no. 3 pneumotachograph connected to a tightly fitting mask. Both resistances were obtained during a baseline period and during progressive normocapnic hypoxia achieved by a rebreathing method. We collected the breath-by-breath values of upper airway resistances, minute ventilation, O2 and CO2 fractions, arterial O2 saturation (Sa(O2)), and changes in functional residual capacity (inductance vest). The central respiratory drive was evaluated by the mouth occlusion pressure 0.1 s after the onset of inspiration (P0.1), and breath-by-breath P0.1 values were estimated by intrapolation from the linear relationship between P0.1 and Sa(O2). In each subject both resistances decreased during the hypoxic test. The slope of the decrease in resistance with decreasing Sa(O2) (%baseline/%Sa(O2)) was steeper for pharyngeal resistance than for nasal resistance [2.67 +/- 0.29 and 1.61 +/- 0.25 (SE), respectively; P < 0.05]. The slope of the decrease in resistance with increasing P0.1 (%baseline/cmH2O) was -0.24 +/- 0.05 for nasal resistance and -0.39 +/- 0.07 for pharyngeal resistance (P < 0.05). Functional residual capacity progressively increased during the test, but the decrease in resistance was greater than expected from an isolated increase in lung volume. We conclude that nasal and pharyngeal resistances decrease during progressive normocapnic hypoxia. Several mechanisms (e.g., increase in central respiratory drive, functional residual capacity, nasal mucosal vasoconstriction) may be involved in this decrease.