Correlation of regional cerebral blood flow (rCBF) with EEG changes during isoflurane anesthesia for carotid endarterectomy: critical rCBF.

A prospective evaluation of regional cerebral blood flow (rCBF) (ipsilateral middle cerebral artery distribution) was determined using a 133Xe clearance technique in 31 ASA P.S. II-III patients anesthetized with isoflurane-50% N2O in O2 for carotid endarterectomy. Each patient was monitored with 16-channel EEG throughout anesthesia and surgery. Critical rCBF was defined as that flow below which EEG signs of ischemia occurred. Critical rCBF (T1/2 method of analysis) was less than 10 ml X 100 g-1 X min-1 (mean +/- SE 5.9 +/- 1.2) in the six patients in whom transient EEG changes occurred at the time of temporary surgical carotid artery occlusion. No EEG changes occurred with occlusion in the other 25 patients; mean (+/- SE) occlusion rCBF in this group was 18.9 +/- 1.3 ml X 100 g-1 X min-1 (P less than 0.001). Preocclusion flows were not significantly different in the two groups. Critical rCBF during isoflurane anesthesia was less than that previously determined during halothane anesthesia (18-20 ml X 100 g-1 X min-1), and is compatible with the effects of isoflurane on CMRO2 and CBF.