Strategy for Maintaining Partial Neuromuscular Blocking Adequate for Motor Evoked Potential During Neurosurgery

Primary Outcome Measures:

• Amplitude of MEP monitoring [ Time Frame: every 30 min during MEP monitoring ]
  the value of MEP amplitude every 30 min during MEP monitoring
  
  

Secondary Outcome Measures:

• latency of MEP monitoring [ Time Frame: every 30 min during MEP monitoring ]
  latency of MEP monitoring every 30 min during MEP monitoring
  
  
• Incidence of patient's spontaneous movement [ Time Frame: from start to end of the MEP monitoring, an expected average of 4 hours ]
  Incidence of patient's spontaneous movement during MEP monitoring
  
  
• Overall assessment of MEP monitoring quality [ Time Frame: from start to end of the MEP monitoring, an expected average of 4 hours ]
  overall assessment of MEP monitoing quality provided by the electrophysiologist Grade I : no problem all through the monitoring Grade II : difficulty of monitoring for less than 5 min Grade III : difficulty of monitoring for 5 min to 30 min Grade IV : Difficulty of monitoring for more than 30 min
  
  
• Incidence of patient's spontaneous respiration [ Time Frame: from start to end of the MEP monitoring, an expected average of 4 hours ]
  Incidence of patient's spontaneous respiration as determined by end-tidal CO2 curve monitoring
  
  

The maintenance of partial neuromuscular blocking during general anesthesia for neurosurgery is essential for intraoperative motor-evoked potential monitoring. However, the precise strategy of administering neuromuscular blocking agent for obtaining that goal has not been established. Therefore, the investigators tried to find the optimal initial dose of vecuronium infusion and determine the adequate goal of neuromuscular blocking as guided by neuromuscular transmission module (M-NMT Module, Datex-Ohmeda Inc, Helsinki, Finland). Previously, one to two counts of response to TOF stimulation has been considered to be primary goal of partial neuromuscular blocking needed for intraoperative motor evoked potential monitoring. However, the visualization of twitch height of response to TOF stimulation has been possible with the help of NMT module. For adequate motor evoked potential monitoring, twitch height of T1 or T2 is also as important as simple count of TOF stimulation. The investigators tried to establish a vecuronium infusion strategy as guided by not only count of TOF stimulation but also twitch height of T1 or T2.