Surgical stimulation increases median nerve somatosensory evoked responses during isoflurane-nitrous oxide anaesthesia

Median nerve somatosensory evoked responses (MnSSER) were recorded in 15 healthy adult patients, ASA I-II, before and during orthopaedic surgery. After induction of anaesthesia with fentanyl 0.1-0.15 mg, etomidate 0.3 mg kg^-1 and vecuronium 0.1 mg kg^-1, anaesthesia was maintained with 0.6% isoflurane (end-tidal) and 66% nitrous oxide in oxygen. MnSSER were recorded after establishment of steady-state anaesthesia at baseline, during preparation (n = 11) and continuously after the start of surgery. For the last measurement period, four patients were excluded from analysis because additional fentanyl was required. MnSSER were recorded at Erb's point, at C6 (neck) and at the respective contralateral primary somatosensory projection area (C3' or C4'). All MnSSER waveform components remained recordable and easily identifiable during anaesthesia. During intense surgical stimulation (e.g. periosteal stimulation) the peak-to-peak amplitude N20P25 increased significantly by more than 45% (P < 0.05), whereas latencies of all components did not change over time. These data indicate that MnSSER may be reliably monitored in the intraoperative period during steady-state isoflurane-nitrous oxide anaesthesia. In addition, concurrent changes in haemodynamic variables during nociceptive stimulation support the hypothesis that reversal of isoflurane-nitrous oxide-induced suppression of MnSSER may indicate increased nociceptive input when depth of anaesthesia is inadequate.