Repetitive hypoxia rapidly depresses cardio-respiratory responses during active sleep but not quiet sleep in the newborn lamb

1. Arousal from sleep is an important protective response to hypoxia that b ecomes rapidly depressed in active sleep (AS) when hypoxia is repeated. Thi s study questioned whether there might also be selective depression of card io-respiratory responses to hypoxia during BS. 2. Nine newborn lambs (7-22 days of age) were studied over three successive nights. The first and third nights were baseline studies (inspired oxygen fraction, F-i,F-O2 = 0.21). During the second night, during every epoch of sleep, lambs were exposed to 60 s episodes of isocapnic hypoxia (F-i,F-O2 = 0.10). 3. During quiet sleep (QS), the probability of arousal in hypoxia exceeded the probability of spontaneous arousal (P < 0.001) throughout repeated expo sures to hypoxia. Similarly, there were persisting increases in ventilation (135 +/- 25 %), blood pressure (3 +/- 1 %) and heart rate (3 +/- 1%). 4. By contrast, rapid depression of all responses occurred during repetitiv e hypoxia in AX. Thus, the probability of arousal in hypoxia exceeded the p robability of spontaneous arousal during the first 10 hypoxia exposures (P < 0.001) but not thereafter. Similarly, during the first 10 exposures to hy poxia, the changes in ventilation (88 +/- 15%) and blood pressure (5 +/- 1 %) were greater than subsequent responses (P < 0.05). 5. We conclude that, when repeated, hypoxia rapidly becomes ineffective in stimulating protective arousal, ventilatory and blood pressure responses in AS, but not in QS. Selective depression of responses during BS may render the newborn particularly vulnerable to hypoxia in this state.