INFLUENCE OF SUSTAINED HYPOXIA ON THE SENSATION OF DYSPNEA

We assessed the effect of sustained isocapnic hypoxia (PCO2 = 40 Torr, Sa(O2) = 80%) on the sensation of dyspnea in 16 normal healthy males. Subjects rated the sensation of dyspnea (psi) on 15 cm visual analog scales during 20 min of sustained hypoxia. Following this hypoxic peri od, 8 subjects undertook mild exercise (10-50 W on a bicycle ergometer for 3 min) under the continuation of hypoxia. During sustained hypoxi a, psi increased initially with ventilation from 0.6 +/- 0.2 (n = 16, mean +/- SE) to 2.9 +/- 0.6 at peak ventilation, but it decreased with ventilatory depression to 1.6 +/- 0.4. Dyspnea intensity during hypox ic exercise was significantly smaller than that at peak ventilation in the resting hypoxic period (2.3 +/- 0.7 vs. 3.9 +/- 1.0), although th e ventilation was greater during exercise (24.0 +/- 3.0 vs. 19.7 +/- 1 .4 l/min). These results indicate that sustained hypoxia has a biphasi c, i.e., initial stimulatory and delayed depressant, effect on dyspnea and on ventilation. It is suggested that the dyspnea sensing mechanis m is suppressed during mild exercise under sustained hypoxia.