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, particularly in sleep. We hypothesised that hypoxia, through its central nervous system depressant effects, may decrease the perception of the magnitude of respiratory loads. 

Methods: We measured the effect of isocapnic hypoxia on the ability of ten normal awake males (mean age 24 ±1.75(SEM)) to magnitude scale 5 externally applied inspiratory resistive loads (ranging from 7.5 to 54.4cm HzO l ^-1s). Each subject scaled the inspiratory resistive loads during 37 mins of isocapnic hypoxia (FlO ₂ 0.09, SaOz approx 80%) and 37 mins of normoxia. 

Results: Magnitude perception of load (ψ) calculated for all loaded breaths using peak inspiratory pressure (PPK) as the measure of load stimulus was significantly lower during hypoxia than normoxia (20.1 ± 0.9 (SEM) cf 23.9 ±1.3 (SEM) arbitrary units, respectively, p=0.048), and furthermore, declined with time during hypoxia (p=0.007) but not during normoxia (p=0.361). Our result is remarkable considering that PPK was higher at all times during hypoxia than during normoxia and that previous studies have shown that an elevation in PPK results in increased ψ. 

Conclusion: sustained isocapnic hypoxia causes a progressive suppression of the perception of the magnitude of inspiratory resistive loads in normal subjects and could therefore impair alerting or arousal responses to respiratory loading.