Arousal from Sleep during Rapidly Developing Hypoxemia in Lambs

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Abstract
Arousal is an important protective response that may prevent severe hypoxemia and death during sleep. However, very little is known about arousal from sleep in response to respiratory stimuli in newborns. Experiments were therefore done to investigate the arousal response from sleep to rapidly developing hypoxemia in eight lambs. Each lamb was anesthetized and instrumented for recordings of electrocorticogram, electrooculogram, nuchal and diaphragm electromyograms, and measurements of arterial hemoglobin oxygen saturation. A tracheotomy was done and a tracheostomy tube placed in the trachea so that the fraction of inspired oxygen could be changed quickly. No sooner than 3 days after surgery, measurements were made in quiet sleep and active sleep (AS) during 30-s control periods when the animals were breathing 21% oxygen and during experimental periods of hypoxemia when the animals were breathing either 10, 5, or 0% oxygen in nitrogen. During quiet sleep, arousal occurred at similar arterial hemoglobin oxygen saturations (81 ± 6% on 10% O2, 80 ± 5% on 5% O2 and 83 ± 5% on 0% O2) suggesting that arousal was independent of the rate of change of arterial oxygen. However, during AS arousal occurred at different arterial hemoglobin oxygen saturations (76 ± 6% on 10% O2, 55 ± 11% on 5% O2, and 44 ± 17% on 0% O2) suggesting that arousal was dependent on the rate of change of arterial oxygen. During some epochs of AS, electrocortical signs of cerebral hypoxia and primary apnea occurred before arousal. These data provide evidence that arousal from quiet sleep in response to hypoxemia occurs once an arousal threshold has been reached. However, during AS, arousal appears to follow an arousal latency once an arousal threshold has been reached. Furthermore, if the rate of change of oxygen is great enough during AS, signs of cerebral hypoxia and primary apnea may precede arousal. These data would allow one to speculate that if the rate of change of arterial oxygen is great enough during apnea in AS, arousal may fail to occur before hypoxic cerebral depression and ultimately death ensue.

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