In Reply:--Although Mahla and Sulek suggest that the "ideal" monitor to detect cerebral ischemia during carotid endarterectomy has not been found, there are convincing data from human studies that somatosensory evoked potentials (SEPs) after median nerve stimulation are highly specific and sensitive to detect cerebral ischemia during carotid surgery, [1-3]which, in our opinion, may make SEP monitoring superior to electroencephalography (EEG) for three reasons. First, EEG monitoring appears to be "oversensitive" as ischemic changes will occur in up to 40% intraoperatively, [4,5]whereas new postoperative neurologic deficits even using a "no shunt" regimen will not exceed 8-10% in experienced centers. The latter percentage is remarkably close to the findings of our group [1,6,7]that SEP changes indicating ischemia (amplitude reduction > 50%, central conduction time (CCT) prolongation > 20%) will occur in 10-15%. Thus, it is not surprising that Kearse et al. found SEP much less sensitive when compared with the high incidence of ischemic EEG changes (43%) in their study. Second, EEG monitoring methods may not be sensitive enough to detect ischemia (false negative) in patients suffering from preoperative neurologic symptoms, [9,10]whereas false-negative SEP findings in carotid surgery have been reported in only one study. Third, EEG monitoring relies on the detection of multiple patterns that, in the absence of computer-aided analysis, are difficult to quantify and require an experienced neurophysiologist. SEP recordings, in contrast, allows the identification of ischemia using only few quantitative parameters (latency and/or CCT, amplitude).
However, in the absence of carefully controlled clinical studies demonstrating the superiority of one monitoring method, EEG remains undoubtedly an appropriate means to detect cerebral ischemia during carotid surgery. We agree with Mahla and Sulek.
Achim Thiel, M.D., Abteilung fur Anaesthesiologie und Operative Intensivmedizin, Justus-Liebig-Universitat, Klinikstrasse 29, 35385 Giessen, Germany
(Accepted for publication July 27, 1995.)