Sustained isocapnic hypoxia suppresses the perception of the magnitude of inspiratory resistive loads

The sensation of increased respiratory resistance or effort is likely to be important for the initiation of alerting or arousal responses, particularl y in sleep. Hypoxia, through its central nervous system-depressant effects, may decrease the perceived magnitude of respiratory loads. To examine this , we measured the effect of isocapnic hypoxia on the ability of 10 normal, awake males (mean age = 24.0 +/- 1.8 yr) to magnitude-scale five externally applied inspiratory resistive loads (mean values from 7.5 to 54.4 cmH(2)O . l(-1) . s). Each subject scaled the loads during 37 min of isocapnic hypo xia (inspired O-2 fraction = 0.09, arterial O-2 saturation of similar to 80 %) and during 37 min of normoxia, using the method of open magnitude numeri cal scaling. Results were normalized by modulus equalization to allow betwe en-subject comparisons. With the use of peak inspiratory pressure (PIP) as the measure of load stimulus magnitude, the perception of load magnitude (P IP) as creased linearly with load and, averaged for all loaded breaths, was significantly lower during hypoxia than during normoxia (20.1 +/- 0.9 and 23.9 +/- 1.3 arbitrary units, respectively; P = 0.048). Psi declined with t ime during hypoxia (P = 0.007) but not during normoxia (P = 0.361). Our res ult is remarkable because PIP was higher at all times during hypoxia than d uring normoxia, and previous studies have shown that an elevation in PIP re sults in increased Psi. We conclude that sustained isocapnic hypoxia causes a progressive suppression of the perception of the magnitude of:inspirator y resistive loads in normal subjects and could, therefore, impair alerting or arousal responses to respiratory loading.