OBJECTIVETo find a way to discriminate operative reason from anaesthesia reason for the changes of intraoperative transcranial magnetic motor evoked potentials (MEPs).

METHODSIn 26 patients under Etomidate/Fentanyl anesthesia from February 2001 to June 2004, MEPs elicited by transcranial magnetic stimulation were recorded from tibialis anterior muscles, simultaneously bispectral index (BIS) and train-of-four stimulation (TOF) were used to monitor the anesthesia depth and neuromuscular blockade respectively. MEP, BIS and measurements of TOF at different anesthesia depth and muscular relaxation were recorded synchronously, statistical analysis of this data set was done in order to find the inherent relationship between these variables.

RESULTSUnder anesthesia, MEP amplitude was always positively correlated with the corresponding BIS and TOF value. A regression equation could be built, with which the MEP amplitude could be reckoned based on realtime BIS and T(1)/T(c). In case of spinal cord injury, the measured amplitude value would significantly deviate from predicted one, which suggested that the change of MEP was because of the operation, but not the anaesthesia or neuromuscular blockade. Each patient had his or her own regression equation, which was different from each other.

CONCLUSIONSThe establishment of regression equation from MEPs, BIS and TOF is very useful to distinguish reasons of the changes of transcranial magnetic MEPs during surgery, and with this technique, the intraoperative MEP monitoring should be more reliable and practicable.