Erroneous conclusion from processed electroencephalogram with changing anesthetic depth.

INTRAOPERATIVE monitoring of the electroencephalcgram (EEG) is commonly used for detection of cerebral ischemia during carotid endarterectomy. Although realtime interpretation of analog EEG recordings may be considered the “gold standard” for ischemia detection, various signal-processing techniques have been used to ease the interpretation of intraoperative EEG. Several reviews of various EEGprocessing techniques are now available. ’,’ Appropriate monitoring of intraoperative EEG for cerebral ischemia assumes that anesthetic depth remains relatively constant, because changes in anesthetic depth or surgical stimulation are known to have profound effects on the EEG.3 We report the case of a patient having a carotid endarterectomy in which the processed EEG showed a significant loss of “power,” and a shift of the spectral edge frequency to lower frequencies during anesthetic emergence. If the EEG had been recorded under conditions of a stable anesthetic when the carotid artery was clamped, the record could have been interpreted as cerebral ischemia.