• P Bischoff, E Kochs, D Droese, W H Meyer-Moldenhauer, and J Schulte am Esch.
• Abteilung für Anästhesiologie, Universitäts-Krankenhaus Eppendorf, Hamburg.
• Anaesthesist. 1993 Mar 1;42(3):142-8.

AbstractIncreases in slow-wave (delta) activity in the EEG may reflect increased depth of anaesthesia provided that hypoxia, haemodynamic instability and drug overdose have been excluded. In contrast, similar intraoperative EEG responses have been described as paradoxical arousal reactions. The aim of this study was to assess the effects of surgical stimulation on spatial EEG changes during anaesthesia with 0.6% isoflurane/66% nitrous oxide. METHODS. The present study investigated changes in EEG power and frequencies in 20 patients (mean age 36 +/- 8 years; ASA I or II) scheduled for elective urological surgery during steady-state anaesthesia with 0.6% isoflurane and 66% nitrous oxide. The following variables were measured: heart rate (HR), mean arterial blood pressure (MAP), end-tidal isoflurane (PetISO) and carbon dioxide concentrations (PetCO2), arterial oxygen saturation (SaO2%) and body temperature (degree C). Patients were randomly assigned to one of two groups: group 1 (n = 10; without surgery) and group 2 (n = 10; with surgical procedure). The EEG was recorded over 20 min. The first 5 min were taken as baseline. In group 2 surgical stimulation (skin incision with subsequent surgical preparation) was started 1-2 min after recording of baseline values. Topographical distribution of EEG output was recorded from 17 electrodes (international 10-20 system), digitized and stored on disk (CATEEM) after establishment of steady-state anaesthesia (PetISO: 0.6%; PetCO2: 35-37 mmHg). Data are given as medium (microV2/Hz) and relative changes (%) +/- SD with respect to baseline. Statistical significance was tested for F4 versus C4 for the delta- and alpha-1-frequency bands using Wilcoxon's test (P < 0.05). RESULTS. In group 1 (without surgical stimulation) all parameters did not change over time. EEG slowing with an increase in power (> 100%) was noted in 8 patients of group 2 (n = 10; during surgical stimulation). By visual inspection of the original EEG tracings paradoxical arousal patterns were seen in these patients. In group 2, delta output changed from 69.6 microV2/Hz (baseline) to 147.4 microV2/Hz at frontal leads (F4) 5-6 min after the start of surgery. Only minimal changes were observed for theta activity. At the same time, in most cases fast wave activity (alpha 1, alpha 2, beta 1 and beta 2) was decreased by more than 50% at identical cortical areas. Increases in MAP were noted continuously after start of surgery up to a maximum of 19.8% from baseline, which became significant at the 16-min level. Heart rate did not change over time. DISCUSSION. Our data demonstrate that EEG slowing may be induced by surgical stimulation during steady-state anaesthesia with 0.6% isoflurane/66% nitrous oxide in oxygen. These findings are consistent with previous reports indicating the occurrence of slow wave patterns following sensory stimulation in comatose patients. Since these events occur predominantly at frontal areas they may not be detected with single-channel parietal recordings. Our data suggest that topographical EEG monitoring may useful for assessing painful events during surgery. Using EEG monoparameters like spectral edge frequency or median the occurrence of paradoxical arousal reactions may be falsely interpreted as an increased depth of anaesthesia.

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