Effects of hypercarbia on canine cerebral metabolism and blood flow with simultaneous direct and indirect measurement of blood flow.

While it is generally agreed that cerebral blood flow (CBF) increases during hypercarbia, measurements of cerebral metabolic rate (CMRO2) during hypercarbia have yielded divergent results. Accordingly, the authors reexamined the changes in CMRO2 during normocarbia and hypercarbia to 80 and 100 torr employing a canine model designed to minimize potential measurement errors introduced by hypercarbia. Repetitive measurements of arterial-cerebral venous blood oxygen content differences C(a-v)O2 were made, and CBF was measured simultaneously by both a direct (sagittal sinus blood flow collection) and an indirect (133Xe washout) technique. CMRO2, derived from the direct CBF measurements and C(a-v)O2, was unchanged at PaCO2 80 torr and was significantly decreased (by 10 per cent) at PaCO2 100 torr. Comparison of the two techniques for determining CBF showed good agreement between the direct method and fast compartment analysis of the 133Xe washout.