Sevoflurane-induced isoelectric EEG and burst suppression: differential and antagonistic effect of added nitrous oxide

The objective of this study was to investigate whether nitrous oxide influenced the ED50 of sevoflurane for induction of isoelectric electroencephalogram (ED50_isoelectric ) differently from its influence on the ED50 of sevoflurane for electroencephalogram burst suppression (ED50_burst ). In a prospective, randomised, double-blind, parallel group, up-down sequential allocation study, 77 ASA physical status 1 and 2 patients received sevoflurane induction and, after tracheal intubation, were randomly allocated to receive sevoflurane with either 40% oxygen in air (control group) or 60% nitrous oxide in oxygen mixture (nitrous group). The ED50_isoelectric in the two groups was determined using Dixon's up and down method, starting at 2.5% with 0.2% step size of end-tidal sevoflurane. The electroencephalogram was considered as isoelectric when a burst suppression ratio of 100% lasted > 1 min. The subsequent concentrations of sevoflurane administered were determined by the presence or absence of isoelectric electroencephalogram in the previous patient in the same group. The ED50_isoelectric in the nitrous group 4.08 (95%CI, 3.95-4.38)% was significantly higher than that in the control group 3.68 (95%CI, 3.50-3.78)% (p < 0.0001). The values for ED50_burst were 3.05 (95%CI, 2.66-3.90)% and 3.02 (95%CI, 3.00-3.05)% in nitrous group and control group, respectively (p = 0.52). The addition of 60% nitrous oxide increases ED50_isoelectric , but not the ED50_burst of sevoflurane. Neither result indicates an additive effect of anaesthetic agents, as might be expected, and possible reasons for this are discussed.

Minimal alveolar concentration of sevoflurane for induction of isoelectric electroencephalogram in middle-aged adults

BACKGROUND

We determined the minimal alveolar concentration (MAC) of sevoflurane inducing an isoelectric EEG in 50% of adult subjects (MACie).

METHODS

We included 31 middle-aged subjects; 30 subjects finished the study protocol and received sevoflurane at preselected concentrations according to a modified Dixon 'up-and-down' design starting at 1.7 vol% with 0.2 vol% steps size. General anaesthesia was induced and maintained with sevoflurane; tracheal intubation was facilitated with cisatracurium. After a period of 30 min before skin incision, the state of isoelectric EEG was considered as significant when a burst suppression ratio of 100% lasted for >1 min. The haemodynamic responses to skin incision and the vasopressor requirement to maintain stable haemodynamic status were also analysed according to the EEG state.

RESULTS

MACie was 3.5% (95% confidence interval, 3.4-3.7%) in middle-aged subjects. When compared with subjects not in isoelectric EEG state, subjects in isoelectric EEG state received more phenylephrine to maintain stable haemodynamics (10 of 10 compared with 7 of 20 subjects, P=0.001) and experienced less sympathetic responses to skin incision (1 of 10 compared with 11 of 20 subjects, P=0.024).

CONCLUSIONS

MACie for sevoflurane was ∼2.1 times MAC for immobilization in phenobarbital premedicated middle-aged adults. Sevoflurane-induced isoelectric EEG state is associated with significant cardiovascular depression but reduced haemodynamic responses to skin incision.

Effects of music on target-controlled infusion of propofol requirements during combined spinal-epidural anaesthesia

The sedative effects of music were evaluated using the bispectral index (BIS) during target-controlled infusion (TCI) propofol. A total of 110 women undergoing hysterectomy were randomly allocated to receive either music or no music. Propofol was administered using target-controlled infusion and the concentration adjusted gradually to achieve an observer's assessment of alertness/sedation (OAA/S) score of 3 intra-operatively. The haemodynamic and bispectral index values during the sedation phase were recorded. Interleukin-6 was evaluated before, immediately after and 1 h following intervention. The music group had a significant reduction in mean (SD) induction time of sedation: 12 (12) min vs. 18 (12) min, p < 0.01; propofol target concentration: 1.6 (0.3) microg.ml(-1) vs. 2.4 (0.4) microg.ml(-1), p < 0.0001; intra-operative amount of propofol: 171 (98) mg vs. 251 (92) mg, p < 0.0001; and significantly higher levels of satisfaction with their peri-operative care: 9.6 (0.6) compared to the control group: 8.1 (1.0), p < 0.0001. No other differences were found. The results show the influence of music on the induction time of sedation, concentration and level of propofol during surgery, and suggest sedative benefits of music.