The influence of arterial carbon dioxide tension on the cerebrovascular response to arterial hypoxia and to haemodilution.

Cerebral blood flow (CBF) measurements and blood gas analyses were performed on anaesthetized and artifically ventilated dogs during arterial hypoxia or haemodilution in different ranges of arterial carbon dioxide tension. Arterial hypoxia as well as haemodilution produced a flow increase in all ranges of ventilation. However, this flow increase was elicited at a cerebrovenous oxygen tension which rose with the arterial carbon dioxide tension, but which tended to be maintained by the flow increase during continued decrease of the arterial oxygen content. On the assumption that the cerebrovenous oxygen tension reflects the oxygen tension of the brain tissue, it is suggested that the arterial carbon dioxide tension influences the ability of the brain tissue to maintain the aerobic metabolism during reduced tissue oxygen tension. This means that tissue hypoxia, in the sense of utilisation of anaerobic metabolism, occurs at a tissue oxygen tension which is lower the lower the arterial carbon dioxide tension is.