Figure 1. Probability of arousal from sleep and corresponding ventilatory responses during repetitive hypoxia.

Probability of arousal from sleep during repetitive hypoxia (▪) during QS (A) and AS (C) plotted against the number of exposures to hypoxia (values for 11 studies in 9 lambs), compared with the probability of spontaneous arousal during normoxia (□). Associated ventilatory responses to repeated hypoxia (measured as a percentage change from normoxia levels) are shown for QS (B) and AS (D). Note that during QS the probability of arousal in response to hypoxia remained elevated above the spontaneous levels, whereas during AS the increment in arousal was abolished after just 10 exposures. * P < 0.05, probability of arousal during hypoxia vs. probability of spontaneous arousal; † P < 0.05, probability of arousal during hypoxia vs. probability of arousal during hypoxia for the first 10 hypoxic exposures. Similarly, there was no depression of the ventilatory response to hypoxia during QS, but in AS there was a depression of the ventilatory response after 10 exposures that paralleled the arousal depression. * P < 0.05, ventilatory response to hypoxia (percentage change from normoxia) vs. ventilation in normoxia; † P < 0.05, ventilatory response to hypoxia vs. ventilatory response to hypoxia for the first 10 exposures to hypoxia.