Hypercapnia and response of cerebral blood flow to hypoxia in newborn lambs

Individual effects of hypoxic hypoxia and hypercapnia on the cerebral circulation are well described, but data on their combined effects are conflicting. We measured the effect of hypoxic hypoxia on cerebral blood flow (CBF) and cerebral O₂ consumption during normocapnia (arterial PCO₂ = 33 ± 2 Torr) and during hypercapnia (60 ± 2 Torr) in seven pentobarbital-anesthetized lambs. Analysis of variance showed that neither the magnitude of the hypoxic CBF response nor cerebral O₂ consumption was significantly related to the level of arterial PCO₂. To determine whether hypoxic cerebral vasodilation during hypercapnia was restricted by reflex sympathetic stimulation we studied an additional six hypercapnic anesthetized lambs before and after bilateral removal of the superior cervical ganglion. Sympathetectomy had no effect on base-line CBF during hypercapnia or on the CBF response to hypoxic hypoxia. We conclude that the effects of hypoxic hypoxia on CBF and cerebral O₂ consumption are not significantly altered by moderate hypercapnia in the anesthetized lamb. Furthermore, we found no evidence that hypercapnia results in a reflex increase in sympathetic tone that interferes with the ability of cerebral vessels to dilate during hypoxic hypoxia.