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Some effects of isoflurane on I waves of the motor evoked potential
R G Hicks1, I J Woodforth, M R Crawford
1Department of Clinical Neurophysiology, Prince Henry Hospital, Little Bay, N.S.W., Australia.
Abstract:
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.
Insights
Isoflurane anesthesia reduced motor evoked potentials (MEPs) during spinal surgery. Higher concentrations of isoflurane decreased I-waves, suggesting impaired synaptic transmission, which may model anesthesia-induced unconsciousness.
Area of Science:
- Neuroscience
- Anesthesiology
- Spinal Surgery
Background:
- Motor evoked potentials (MEPs) assess corticospinal tract integrity.
- Isoflurane is a common inhalation anesthetic used in surgery.
- Understanding anesthetic effects on neural pathways is crucial for patient safety.
Purpose of the Study:
- To investigate the impact of varying isoflurane concentrations on motor evoked potentials (MEPs) during corrective spinal surgery.
- To determine the relationship between isoflurane dosage and changes in MEP components, specifically D-waves and I-waves.
Main Methods:
- 15 patients undergoing corrective spinal surgery were studied.
- Transcranial electrical stimulation elicited motor evoked potentials (MEPs) in the extradural space.
- Isoflurane end-tidal concentrations were varied (0%, 0.5%, 1%, 1.5%, 2%) while monitoring MEPs.
Main Results:
- Increasing isoflurane concentration progressively reduced the number and amplitude of I-waves, which reflect synaptic transmission.
- The D-wave, representing direct axonal conduction, showed minimal changes.
- The most significant depressant effect on I-waves was observed at a 0.5% end-tidal isoflurane concentration.
Conclusions:
- Isoflurane significantly depresses I-wave components of MEPs in a dose-dependent manner.
- These findings suggest that anesthetic-induced alterations in I-waves may serve as a model for studying neuronal mechanisms of anesthesia-induced unconsciousness.
- MEPs can be a valuable tool for monitoring anesthetic effects on neuronal function during surgery.