Anesthetic effects on cerebral metabolic rate predict histologic outcome from near-complete forebrain ischemia in the rat

Background: Although reduction of cerebral metabolic rate is thought to contribute to anesthetic neuroprotection, histologic evidence to support this concept has not been provided. In this study, histologic outcome was evaluated in rats subjected to different durations of severe forebrain ischemia while anesthetized with volatile anesthetics that have substantially different effects on cerebral metabolic rate. Methods: Normothermic rats that underwent fasting were anesthetized with 0.75 minimum alveolar concentration (MAC) isoflurane-60% nitrous oxide (N₂O) or 0.75 MAC halothane-60% N₂O. Ischemia was induced with use of a combination of bilateral carotid occlusion and controlled hypotension. Rats in the isoflurane group were subjected to 6.5 min or 8.0 min ischemia, whereas the halothane group received 6.5 min ischemia. Histologic damage was assessed 4 days later. Results: With 6.5 min ischemia, mean ± SD, hippocampal CA1 percent of dead (% dead) neurons was reduced with isoflurane-N₂O (45 ± 18) versus halothane-N₂O (60 ± 23, P = 0.023). Eight minutes of ischemia increased % dead neurons in the isoflurane-N₂O group (60 ± 17, P = 0.017). There was no difference between the isoflurane 8.0-min and halothane 6.5-min groups (P = 0.935). A similar pattern was observed in hippocampal CA4 and the neocortex. Striatal damage was not affected by anesthetic or ischemic duration. Conclusions: At 6.5 min ischemia, isoflurane provided improved outcome versus halothane. Previous research has shown that 0.75 MAC isoflurane-N₂O increases the time to onset of ischemic depolarization by 1.5 min and reduces cerebral metabolic rate by 42% versus 0.75 MAC halothane-N₂O. In the current study, when the duration of ischemia was increased by 1.5 min in the isoflurane-N₂O group, histologic outcome became similar to that in halothane-N₂O-anesthetized rats. These results provide evidence that cerebral metabolic rate reduction has an advantageous effect on outcome from severe brain ischemia, but also suggest that such benefit is likely to be small.