40 Hz auditory steady-state response and EEG spectral edge frequency during sufentanil anaesthesia

Purpose: The auditory steady-state evoked response (ASSR) is an evoked potential which provides a sensitive measure of the effects of general anaesthetics on the brain. We used pharmacokinetic-pharmacodynamic (PK-PD) modelling to compare the effects of sufentanil on the amplitude of the ASSR with its effect on spectral edge frequency (SE Fl of the electroencephalogram. Methods: Nine patients scheduled for elective cardiac surgery participated. Midazolam (70 mu g.kg(-1) im) was given 60 min before entering the operating room. Anaesthesia was induced with 5 mu g.kg(-1) sufentanil at a rate of 0.83 mu g.kg(-1).min(-1). The ASSR, SEF and plasma sufentanil concentrations were measured for 30 min after induction of anaesthesia before surgery. The half-life between the central and effect site compartments (t(1/2)K(eo)), the 50% inhibitory concentration (IC50) and the slope factor (gamma) were computed, Results: The amplitude of the ASSR increased during the first three minutes of infusion of sufentanil by up to 40%. This was followed by a rapid decrease between the fourth and fifth minutes to 16% of baseline. The SEF decreased progressively during the first five minutes of infusion to 18% of baseline. Both measures subsequently showed modest recovery. The parameters gamma, IC50 and t(1/2)K(eo) for ASSR were (mean +/- SD) 6.0 +/- 3.7, 2.1 +/- 1.2 ng.ml(-1) and 7.3 +/- 2.4 min. For SEF the values were 5.9 +/- 5.2, 1.4 +/- 0.7 ng.ml(-1) (P < 0.05 compared with ASSR) and 6.8 +/- 2.4 min. Conclusion: The sensitivity of ASSR to sufentanil is less than that of the SEF.