Effect of extradural analgesia on the paradoxical arousal response of the electroencephalogram

Despite widespread recognition of the potential role of the processed electroencephalogram (pEEG) as a monitor of depth of anaesthesia, few studies have examined the effects of surgical stimuli on the intraoperative pEEG. Two groups of gynaecological patients (n = 10 in each group), with or without pre-incisional extradural analgesia, undergoing gynaecological laparotomy under nitrous oxide and isoflurane anaesthesia were monitored with routine haemodynamic observations and pEEG. Patients who received pre-incisional extradural analgesia showed no significant changes in pEEG variables during surgery (mean spectral edge frequency 95 percentile (SEF95) 13.3 (SD 1.4) Hz), whereas in all patients without pre-incisional extradural analgesia, a significant decrease in SEF95 was noted (6.5 (1.1) Hz after incision compared with a pre-incisional value of 12.5 (1.4) Hz) together with an increase in arterial pressure (paradoxical response). In the latter group, SEF95 and arterial pressure returned to pre-incisional values after extradural analgesia was established during operation. During emergence from anaesthesia, both groups showed a significant increase in SEF95 (25.7 (1.4) Hz). This study suggests that intraoperative pEEG arousal response was different from changes detected when anaesthesia was terminated at the end of surgery. Surgical stimuli in the absence of adequate analgesia induced a paradoxical arousal response.