Arousal and cardiopulmonary responses to hyperoxic hypercapnia in lambs

Experiments were done to investigate the arousal and cardiopulmonary responses to hyperoxic hypercapnia in 8 lambs. Each lamb was anaesthetized and instrumented for recordings of electrocorticogram, electro-oculogram, nuchal and diaphragm electromyograms and measurements of arterial blood pressure and haemoglobin oxygen saturation. No sooner than 3 days after surgery, measurements were made in quiet sleep and active sleep during control periods when the animal was breathing 21% oxygen and during experimental periods of hyperoxic hypercapnia when the animal was breathing 10% carbon dioxide and 30% oxygen. Hyperoxic hypercapnia was terminated during each epoch by returning the inspired gas mixture to 21% oxygen once the animal aroused from sleep. Arousal occurred from both sleep states during hyperoxic hypercapnia but was delayed in active sleep compared to quiet sleep (active sleep 58 +/- 17 s; quiet sleep 21 +/- 10 s; mean +/- 1SD). There were no significant changes in heart rate or blood pressure during hyperoxic hypercapnia before arousal. However, respiratory rate and diaphragm electrical activity did increase during hyperoxic hypercapnia before arousal. Thus, our data provide evidence that hypercapnia can initiate arousal from sleep in young lambs. The mechanisms responsible for this response are yet to be determined.