• The primary outcome of this study is a composite measure of the motor evoked responses over the duration of the surgical procedure utilizing the "area under the curve" (AUC, volt-seconds) of the response time series [ Time Frame: End of study ] [ Designated as safety issue: No ]
  

• Secondary outcomes include i) Stimulation threshold ii) MEP fade index and iii) Facilitation necessity and stability. [ Time Frame: end of study ] [ Designated as safety issue: No ]
  

Motor Evoked Potential (MEP) monitoring during spine surgery is now used routinely. However it is affected by a variety of anesthetics including fluctuating and/or excessively deep or light levels of anesthesia. A common problem is maintaining an overall constancy and adequacy of anesthesia.

Individual patients' responses to anesthesia can differ greatly from population means and can complicate the interpretation of MEPs. False alarms can have significant consequences. One strategy to minimize the anesthetic induced variability of MEPs, and thereby improve the monitoring overall, is to keep the patient's anesthetic state (or "depth") in a narrow range during surgery. This study tests whether using a commercially available depth of anesthesia monitor in addition to usual clinical practice improves the quality of MEPs.

The hypothesis of this study is that the SedLine Monitor, which uses an EEG derived assessment of anesthetic depth, may facilitate MEP monitoring if it is kept within a narrow range during spine surgery. Because SedLine recommends maintaining their Index between 30 and 40 during surgical anesthesia, this is the range used in the primary hypothesis.