[Attempt at a definition of the individual anesthesia level by repetitive pain stimulation: correlation with EEG findings].

Aim: Previous studies using EEG for assessment of depth of anaesthesia correlate anaesthetic concentration with the anaesthetic stage. This procedure neglects the well known effect of individual different susceptibility to anaesthetics. Thus, patients receiving similar concentrations of anaesthetics may not necessarily be at the same level of "anaesthetic depth". The aim of this study was to define an interindividual comparable level of anaesthesia by recording the autonomic cardiovascular reaction to a standardised painful stimulus (tetanic stimulus, 80 mA, 100 Hz).

Methods: In 61 patients undergoing orthopaedic surgery general anaesthesia was performed with isoflurane in 66% N2O. Starting from 0.4% isoflurane, endtidal isoflurane concentration was increased in a stepwise manner (0.1% isoflurane) until the patient did not show any relevant cardiovascular reaction (increase of heart rate and/or blood pressure < 10%) after tetanic stimulation of the ulnar nerve. If patients demonstrated no haemodynamic changes at 0.4% isoflurane, the concentration was decreased until a relevant cardiovascular reaction was registered. During each steady state period multichannel EEG was recorded and mean values of power density (median: microV2/Hz) were computed.

Results: Comparing EEG-results between both groups exhibiting a cardiovascular reaction (CVR+ , median endtidal Iso: 0.5%) and without reaction (CVR- , median endtidal Iso: 0.6%) an increase in low frequency bands and a significant decrease in high frequencies was found (Wilcoxon-test, p < 0.05). In contrast, comparing EEG-data only in relation to endtidal isoflurane concentration neglecting individual haemodynamic responses, no differences of power density in high frequency bands were detected.

Conclusion: This method to define individual depth of anaesthesia as described, results in more consistent EEG patterns and may be useful in relating EEG to depth of anaesthesia.