title: Facilitated assessment of unconsciousness from morphologic changes in the bilateral posterior tibial nerve cortical somatosensory evoked potential under total intravenous propofol anesthesia during spine surgery.
abstract: OBJECTIVE
  To describe a unique morphologic feature of the bilateral posterior tibial nerve cortical somatosensory evoked potential, the S-wave, which varies systematically with propofol infusion rate and which appears to be useful in guiding adequate propofol concentration levels during spine surgery.
  
  
  METHODS
  Two illustrative cases were selected from a pool of 15,000 pediatric and adult patients (ages 8-85 years) who were undergoing corrective spine surgery in operating rooms of university and community hospitals. Anesthesia was maintained with a continuous intravenous infusion of propofol (125-225 microg/kg/min) along with intermittent boluses of narcotic and midazolam (1.0-2.0 mg) as needed. Characteristic metamorphosis of the bilateral posterior tibial nerve cortical somatosensory evoked potential, highlighted by the emergence of an additional middle-latency component labeled the S-wave, served as a neurophysiological marker of "light" propofol anesthesia prompting elevation of propofol infusion rate or bolus injection.
  
  
  RESULTS
  The S-wave was routinely abolished with increased propofol infusion rate or bolus injection. In all instances increased propofol concentration levels produced a characteristic morphologic change in the posterior tibial nerve cortical somatosensory evoked potential consistent with a return to adequate anesthetic depth. Selected cases presented herein compare the S-wave technique to BIS and illustrate the usefulness of the S-wave in identifying inadequate depth of propofol anesthesia.
  
  
  CONCLUSIONS
  The bilateral posterior tibial nerve cortical somatosensory evoked potential changes its morphology in predictable fashion with decreased depth of propofol anesthesia, allowing for anticipation of imminent anesthetic "lightening." It serves as a useful cross-check to Bispectral Index (BIS) or other "level of consciousness" EEG-based algorithms for monitoring depth of propofol anesthesia during prolonged corrective spine surgery.
authors.affiliation: Surgical Monitoring Associates, Inc., 25 Bala Avenue, Suite 105, Bala Cynwyd, PA 19004, USA. danielmschwartz@mac.com
authors.forename: Daniel M
authors.initials: DM
authors.lastname: Schwartz
authors.forename: Anthony K
authors.initials: AK
authors.lastname: Sestokas
mesh_terms: D000328:Adult; D000465:Algorithms; D000771:Anesthesia, Intravenous; D018686:Anesthetics, Intravenous; D002648:Child; D003243:Consciousness / Q000187:drug effects; D004569:Electroencephalography; D005073:Evoked Potentials, Somatosensory; D006801:Humans; D007262:Infusions, Intravenous; D016343:Monitoring, Intraoperative / Q000379:methods*; D015742:Propofol / Q000008:administration & dosage; D013131:Spine / Q000601:surgery*; D013979:Tibial Nerve / Q000502:physiology*; D014474:Unconsciousness / Q000175:diagnosis*
chemical_list: D018686:Anesthetics, Intravenous; D015742:Propofol