Primate Brain Tolerance to Temporary Focal Cerebral Ischemia during Isoflurane- or Sodium Nitroprusside-induced Hypotension

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Abstract
Isoflurane has properties that suggest it may provide cerebral protection from ischemia. This study compared, in primates, neurologic outcome after a 45-min period of temporary focal ischemia during hypotension induced with either isoflurane or sodium nitroprusside (SNP). Fourteen macaque monkeys were studied. Seven animals received halothane and seven isoflurane anesthesia during surgical preparation. After transorbital exposure of the middle cerebral artery (MCA), in the halothane group, the inspired halothane was reduced to 0.75% and the mean blood pressure (BP) reduced to 50 mmHg by the infusion of SNP. In the isoflurane group, the isoflurane was titrated to reduce mean BP to 50 mmHg. Stable hypotension was maintained for 90 min, which included a 45-min period of MCA occlusion. Monitoring included intraarterial blood pressure, arterial blood gases, EEG, somatosensory evoked potentials (SEP), and temperature. After the procedure the animals were allowed to awaken and were assessed neurologically every 8 h. On the third postoperative day, they were reanesthetized and underwent magnetic resonance imaging (MRI) and SEP recording. Thereafter, they were killed with iv KCl and the brains fixed for neuropathology. All animals survived the surgical procedure, but two animals receiving halothane and none receiving isoflurane died prematurely (P < 0.02). The SEP disappeared in all animals within 10 min of MCA occlusion and then returned partially or completely. There was no difference between groups in neurologic scores, in the size of the lesion as assessed by MRI (isoflurane 15.7 .+-. 6%, halothane/SNP 10.5 .+-. 4%), or in the extent or severity of the neuropathologic lesions. If focal cerebral ischemia severe enough to cause complete loss of the SEP occurs during induced hypotension, isoflurane offers no cerebral protection when compared with that of halothane anesthesia with SNP-induced hypotension.