Tibial nerve somatosensory evoked potentials during EEG suppression in sevoflurane anaesthesia

Objectives and Methods: Cortical tibial nerve somatosensory evoked potentia ls (TSEPs) were recorded from 10 subjects in sevoflurane anaesthesia in ord er to study TSEP during EEG suppression. Results: With a stimulation frequency less than one per second the major co mponent was a positive wave which had maximal amplitude parietally ipsilate rally to stimulus and mean latency of 46.1 ms. It probably corresponds to t he P40 wave. It was preceded by a widespread smaller positive wave, which c orresponds to the subcortical P30 wave. In two patients a high amplitude ne gative wave, a couple of milliseconds before the positive wave, and maximal parietally contralateral to stimulus, was seen. All later waves were absen t. Conclusion: The results are in agreement with our previous results from med ian nerve SEPs showing that the first cortical response from primary somato sensory cortex is enhanced, and later waves are suppressed. Hence, recordin g TSEPs during EEG suppression provides a way to record the activity of the primary somatosensory cortex accurately and rapidly due to the very good s ignal to noise ratio, so that even single responses to stimuli can be seen without averaging. Our results suggest that new cortical generators, which are not recordable awake, may be discovered in some patients. (C) 1999 Else vier Science Ireland Ltd. All rights reserved.