Correlation of regional cerebral blood flow with ischemic electroencephalographic changes during sevoflurane-nitrous oxide anesthesia for carotid endarterectomy

Carotid endarterectomy necessitates temporary unilateral carotid artery occlusion. Critical regional cerebral blood flow (rCBF) has been defined as the rCBF below which electroencephalographic (EEG) changes of ischemia occur. This study determined the rCBF50, the rCBF value at which 50% of patients will not demonstrate EEG evidence of cerebral ischemia with carotid cross-clamping. Fifty-two patients undergoing elective carotid endarterectomy were administered 0.6-1.2% (0.3-0.6 minimum alveolar concentration) sevoflurane in 50% nitrous oxide (N2O). A 16-channel EEG was used for monitoring. The washout curves from intracarotid 133Xenon injections were used to calculate rCBF before and at the time of carotid occlusion by the half-time (t(1/2)) technique. The quality of the EEG with respect to ischemia detection was assessed by an experienced electroencephalographer. Ischemic EEG changes developed in 5 of 52 patients within 3 min of carotid occlusion at rCBFs of 7, 8, 11, 11, and 13 ml x 100 g(-1) x min(-1). Logistic regression analysis was used to calculate an rCBF50 of 11.5 +/- 1.4 ml x 100 g(-1) x min(-1) for sevoflurane. The EEG signal demonstrated the necessary amplitude, frequency, and stability for the accurate detection of cerebral ischemia in all patients within the range of 0.6-1.2% sevoflurane in 50% N2O. The rCBF50 of 0.6-1.2% sevoflurane in 50% N2O, as determined using logistic regression analysis, is 11.5 +/- 1.4 ml 100 g(-1) x min(-1). Further, in patients anesthetized in this manner, ischemic EEG changes due to carotid occlusion were accurately and rapidly detected.