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. Sympathectomy 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.
Massik
J
J
Department of Pediatrics, Johns Hopkins Medical Institutions, Baltimore, Maryland 21205.
Jones
M D
MD
Miyabe
M
M
Tang
Y L
YL
Hudak
M L
ML
Koehler
R C
RC
Traystman
R J
RJ
D000818:Animals; D000831:Animals, Newborn; D001794:Blood Pressure / Q000187:drug effects*; D002245:Carbon Dioxide / Q000097:blood / Q000494:pharmacology*; D002560:Cerebrovascular Circulation / Q000187:drug effects*; D006339:Heart Rate / Q000187:drug effects*; D000860:Hypoxia / Q000503:physiopathology*; D010100:Oxygen / Q000097:blood; D010313:Partial Pressure; D012016:Reference Values; D012756:Sheep
D002245:Carbon Dioxide; D010100:Oxygen