Brain energy metabolism and blood flow during sevoflurane and halothane anesthesia: effects of hypocapnia and blood pressure fluctuations

The effects of halothane and sevoflurane on cat brain energy metabolism and regional cerebral blood flow (rCBF) were evaluated during normo- and hypocapnia. Brain energy status was evaluated with phosphorous nuclear magnetic resonance spectroscopy (31P-MRS) and rCBF was measured by the hydrogen clearance method. A high concentration of halothane (3 MAC) impaired brain energy metabolism, while even a higher concentration of sevoflurane (4 MAC) had no untoward effect on brain energy metabolism. At 3 MAC of halothane, there were measurable decreases in brain phosphocreatine (69% of the control) and increases in brain inorganic phosphate (about 250% of control Pi), even though CBF was about 70% of the control value. During hypocapnia, the phosphocreatine levels began to decrease at a Paco2 of 2.7 kPa with 2 MAC of sevoflurane (90% of the control), and at a Paco2 of 4.0 kPa with 2 MAC of halothane (92% of the control). rCBF had decreased to less than 50% of the control value when Paco2 was < or = 2.7 kPa with 2 MAC of sevoflurane and < or = 4.0 kPa with 2 MAC of halothane. Abnormal brain energy metabolism was only observed when rCBF was decreased to less than half of the control (non-anesthetized and normocapnic) value. Following administration of a vasopressor, metaraminol, the abnormal brain energy metabolism induced by 2 MAC of halothane at a Paco2 of 1.33 kPa was normalized in parallel with the improved rCBF values. We conclude that hyperventilation and fluctuating blood pressure contribute to the occurrence of abnormal brain energy metabolism during halothane and sevoflurane anesthesia. This is more pronounced with halothane than with sevoflurane. The hypocapnia-induced abnormality during exposure to 2 MAC of either agent was due to decreased CBF associated with low perfusion pressure, indicating that there was no direct effect of these anesthetics on cerebral energy metabolism.