Median nerve somatosensory evoked potentials during isoflurane anaesthesia.
OBJECTIVE
The effect of isoflurane on the subcortical P14 component of the median nerve somatosensory evoked potential (SEP) is poorly known. We studied whether the P14 wave from the upper brainstem, recorded with a nasopharyngeal electrode, was attenuated at the isoflurane-induced EEG burst-suppression level. We also compared the effect of isoflurane on the P14, cervical N13 and cortical N20, N35 and N6, components.


METHODS
Seventeen elective patients were anaesthetized with isoflurane. Somatosensory evoked potentials were recorded prior to anaesthesia, at 0.5 MAC and 1 MAC end-tidal isoflurane as well as at the level when EEG was in burst-suppression (mean 1.9 vol% end-tidal isoflurane).


RESULTS
Isoflurane had varying effects on the subcortical components of median SEP. The amplitude of nasopharyngeal P14 was stable, but the mean latency increased from 14.4 +/- 1.2 msec at 0.5 MAC to 15.2 +/- 1.1 msec at burst-suppression level (P < 0.05). In contrast, the N13 neck response amplitude was attenuated from 3.3 +/- 0.6 microV to 2.6 +/- 0.5 microV (P < 0.005) without latency changes. The latency of the cortical N20 wave was increased from 19.7 +/- 1.1 msec at awake to 24.4 +/- 1.6 msec at burst-suppression level (P < 0.0001) and amplitude was reduced from 3.3 +/- 1.1 microV to 1.3 +/- 0.6 microV (P < 0.0001). The later cortical components were attenuated even during 0.5 MAC isoflurane and were not recordable during EEG burst-suppression.


CONCLUSIONS
We conclude that P14 can reliably be recorded with nasopharyngeal electrodes during isoflurane anaesthesia, even during EEG burst-suppression, when the N20 wave is attenuated. In contrast, the middle-latency SEP components are sensitive to isoflurane anaesthesia.
Porkkala
T
T
International Graduate School in Neurosciences, University of Tampere, Finland.
Kaukinen
S
S
Häkkinen
V
V
Jäntti
V
V
D000293:Adolescent; D000328:Adult; D000704:Analysis of Variance; D000769:Anesthesia, Inhalation; D018685:Anesthetics, Inhalation / Q000008:administration & dosage* / Q000494:pharmacology; D001917:Brachial Plexus / Q000187:drug effects; D001933:Brain Stem / Q000187:drug effects; D002540:Cerebral Cortex / Q000187:drug effects; D004558:Electric Stimulation; D004566:Electrodes; D004569:Electroencephalography / Q000187:drug effects / Q000295:instrumentation; D005073:Evoked Potentials, Somatosensory / Q000187:drug effects*; D005260:Female; D005500:Follow-Up Studies; D006801:Humans; D007530:Isoflurane / Q000008:administration & dosage* / Q000494:pharmacology; D008297:Male; D008475:Median Nerve / Q000187:drug effects* / Q000502:physiology; D008875:Middle Aged; D009305:Nasopharynx; D009434:Neural Pathways / Q000187:drug effects; D011930:Reaction Time / Q000187:drug effects; D015203:Reproducibility of Results; D013990:Tidal Volume
D018685:Anesthetics, Inhalation; D007530:Isoflurane