Intraoperative motor evoked potentials to transcranial electrical stimulation during two anaesthetic regimens

Objectives: To study motor evoked potentials (MEPs) to multi-pulse transcra nial electrical stimulation (MP-TES) during orthopaedic spinal surgery unde r different anaesthetic regimens. Methods: MEPs to MP-TES were recorded from tibialis anterior and abductor h allucis bilaterally in 50 operations. Anaesthesia was maintained with propo fol and nitrous oxide in 29 operations and isoflurane (0.78 +/- 0.17% end-t idal) and nitrous oxide in 23 (two patients received both regimens). Analge sia was provided with fentanyl or remifentanil. Results: Motor stimulation caused neither EEG changes nor seizures. MEPs we re obtained in 97% of patients during propofol anaesthesia. The median ampl itude and coefficient of variation (CV) at baseline (across all muscles) we re 198 muV and 22%, respectively. Amplitudes throughout the operation paral leled the degree of neuromuscular block and were reduced after fentanyl bol us, isoflurane or morphine. Loss of MEPS or persistent amplitude decrements were associated with neurological complications in one patient and severe blood loss in another two patients. MEPs were obtainable in 61% of patients during isoflurane anaesthesia and became inconsistent for end-tidal concen trations > 0.87 +/- 0.08%. Amplitudes were smaller (85 muV) and baseline va riability higher (coefficient of variation 29%) than in the propofol group. The decrease in the number of recordings was greater for isoflurane than p ropofol when the number of pulses/train decreased from 4 to 2. Conclusions: Muscle MEPs to MP-TES are a safe, sensitive and reliable metho d for monitoring motor pathways during propofol/nitrous oxide and fentanyl or remifentanil anaesthesia. MEPs are also obtainable in the majority of pa tients during isoflurane/nitrous oxide anaesthesia, hut quantitative monito ring is not always possible with this regimen. (C) 2001 Elsevier Science Ir eland Ltd. All rights reserved.