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British Journal of Anaesthesia, 1992, Vol. 69, No. 2 130-136
© 1992 The Board of Management and Trustees of the British Journal of Anaesthesia

research-article

SOME EFFECTS OF ISOFLURANE ON I WAVES OF THE MOTOR EVOKED POTENTIAL

R. G. HICKS, M.SC.1, I. J. WOODFORTH, M.B., B.S., F.F.A.R.A.C.S.2, M. R. CRAWFORD, M.B., B.S., F.F.A.R.A.C.S.1, J. P. H. STEPHEN, M.B., B.S., F.R.A.C.S., F.R.C.S.3 and D. J. BURKE, M.D., D.SC., F.R.A.C.P.1

1Department of Clinical Neurophysiology, The Prince Henry Hospital Little Bay, N.S.W. 2036, Australia
2Department of Anaesthesia, Prince of Wales Hospital Randwick, N.S.W. 2031, Australia
3Department of Orthopaedic Surgery, Prince of Wales Children's Hospital Randwick, N.S.W. 2031

We have investigated the effects of isoflurane anaesthesia on the motor evoked potential recorded in the extradural space during corrective spinal surgery in 15 patients. Isoflurane was added to a nitrous oxide in oxygen mixture supplemented with fentanyl and a neuromuscular blocking agent. Isoflurane was administered to achieve end-tidal concentrations of 2%, 1 % and 0% in all patients, and also of 1.5% and 0.5% in nine patients. Transcranial electrical stimulation of the motor cortex was used to elicit descending volleys in corticospinal axons (the motor evoked potential). With stimuli of 450-750 V and no isoflurane, multiple I waves were always seen following the D wave. In all patients the number of I waves decreased and individual I waves became smaller in amplitude the greater the isoflurane concentration, but there were only minor changes in the D wave. The greatest depressant effect on I waves occurred at an end-tidal concentration of 0.5%. Given that I waves are an index of synaptic transmission, anaesthetic-induced changes in I waves may provide a useful model for the neuronal events underlying anaesthesia-induced unconsciousness.


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