Hypocapnia prevents the decrease in regional cerebral metabolism during isoflurane-induced hypotension

In neurologic surgery, induced hypotension is often used while the patient is hypocapnic. We investigated, by tissue biopsy methods and scintillation counting, the regional cerebral glucose utilization (rCMRglc) and blood flow (rCBF) in rats subjected to hypocapnia alone and in combination with hypotension. Anesthesia was maintained with 1.0% isoflurane in nitrous oxide/oxygen. Seven rats were maintained at PaCO2 of 40 mm Hg, six rats were ventilated to PaCO2 of 20 mm Hg, and six animals to PaCO2 of 20 mm Hg in combination with arterial hypotension of 50 mm Hg induced by isoflurane 2.5-3.5%. During hypocapnia, rCMRglc tended to increase in all regions, but the increase was statistically insignificant; rCBF was reduced uniformly by 40%. During combined hypocapnia/hypotension, rCMRglc was unaltered when compared to hypocapnia; compared to normocapnia, increases were seen in hippocampus and cerebellum. During hypocapnia/hypotension, rCBF was unaltered in cortical areas, while increases were seen in all subcortical areas compared to hypocapnia. Regional values of the ratio of rCBF/rCMRglc indicated that during hypocapnia and hypotension induced by isoflurane in nitrous oxide/oxygen, the individual brain areas were perfused according to their metabolic needs. It is suggested that hypocapnia may prevent the decrease in rCMRglc, which is usually observed during deep isoflurane anesthesia.