title: Motor-evoked potential facilitation during progressive cortical suppression by propofol.
abstract: We characterized the effects of various stimulation patterns on motor-evoked potentials (MEPs) elicited by repetitive transcranial magnetoelectric stimulation at different levels of cortical suppression by propofol. In 20 patients undergoing lumbar disk surgery, propofol target plasma concentrations (PTPCs) were increased incrementally by target plasma-level controlled infusion during the induction of anesthesia. MEPs were recorded from the muscles of the upper extremities after single, double, and quadruple magnetoelectric stimulation at 500, 200, and 100 Hz. The mean PTPC during loss of responsiveness to verbal instructions (CP50) was 3 microg/mL (CP(95), 5 microg/mL). At PTPCs <3 microg/mL, maximal MEP amplitudes were elicited by quadruple stimulation at 100 Hz. At PTPCs > or =3 microg/mL, four pulses at 200 Hz yielded peak MEP amplitudes. Therefore, quadruple magnetoelectric stimulation at 100 Hz yields peak myogenic responses in awake patients. With progressive cortical suppression resulting from PTPCs beyond 3 microg/mL, the most effective stimulation frequency shifts to 200 Hz. This may be explained by a differential dose-dependent action of propofol on GABAergic cortical interneurons, corresponding to the clinically observed state of vigilance. Recording of spinal cord evoked potentials will aid in further elucidation of the modulatory effects of general anesthesia on intracortical facilitation.
  
  
  CONCLUSIONS
  We investigated the effect of different transcranial magnetoelectric stimulation paradigms on motor-evoked potentials during different levels of cortical suppression by propofol. The most effective stimulation frequency seems to change from 100 to 200 Hz during progressive propofol dose escalation, possibly because of specific interaction with cortical interneurons.
authors.affiliation: Department of Neurosurgery, University of Freiburg, Freiburg, Germany. scheufle@nz11.ukl.uni-freiburg.de
authors.forename: Kai-Michael
authors.initials: KM
authors.lastname: Scheufler
authors.forename: Josef
authors.initials: J
authors.lastname: Zentner
mesh_terms: D000328:Adult; D018686:Anesthetics, Intravenous / Q000097:blood / Q000494:pharmacology*; D001132:Arm; D017586:Diskectomy; D004305:Dose-Response Relationship, Drug; D004574:Electromagnetic Fields; D004576:Electromyography; D019054:Evoked Potentials, Motor / Q000187:drug effects*; D005260:Female; D006801:Humans; D008297:Male; D008875:Middle Aged; D009044:Motor Cortex / Q000187:drug effects / Q000502:physiology*; D018482:Muscle, Skeletal / Q000502:physiology; D015742:Propofol / Q000097:blood / Q000494:pharmacology*; D011930:Reaction Time; D014474:Unconsciousness / Q000139:chemically induced
chemical_list: D018686:Anesthetics, Intravenous; D015742:Propofol