New EEG parameters correlated with sevoflurane concentration in dogs: tau and burst.

Background: Electroencephalographic (EEG) indicators for anaesthesia depth are being developed. Here, we propose novel EEG parameters created by using two kinds of waveforms discriminated by voltage thresholds from a new perspective. Methods: Six young-adult beagles and 6 senior beagles were anaesthetised with end-tidal sevoflurane (SEV) of 2.0%-4.0% in 0.5% intervals, and 2-channel EEG (256 Hz) was recorded. Two events were discriminated: a consecutive part ({\tau}) with a peak-to-peak potential difference (Vpp) within 1-6, 8, 12, or 20 mcrV, and another part (burst) with Vpp outside the threshold. Number of {\tau} (N{\tau}), mean {\tau} duration (M{\tau}), total percentage of {\tau} (SR{\tau}), mean burst duration (Mbst), and amplitude of burst (Abst) were evaluated as anaesthesia depth indicators using Pearsons correlation coefficients (r). Results: When Vpp was near the suppression wave threshold, N{\tau} had the highest correlation with SEV in both groups. As SEV was increased until onset of burst suppression, N{\tau} decreased, M{\tau} remained unchanged, and Mbst and Abst increased. In the young-adult group, mean |r| exceeded 0.95 for N{\tau} with Vpp of 4-6 mcrV, for Mbst with Vpp of 8-12 mcrV, and for Abst with Vpp of 4-12 mcrV. In the senior group, r exceeded 0.90 for N{\tau} with Vpp of 4 mcrV and for Abst with Vpp of 5-12 mcrV. Conclusion: We developed new EEG parameters that were almost perfectly correlated with volatile anaesthetic concentrations using traditional bipolar recording. This study suggests that the effect of anaesthetics on EEG can be investigated from two directions: pacemaker ({\tau}) and generator (burst).