BIS And Aepex During Nitrous Oxide Administration

H. Kang*, N. Scott#, N. Sutcliffe# and G.N.C. Kenny*

*Glasgow University Department of Anaesthesia, Royal Infirmary, 8-16, Alexandra Parade, Glasgow G31 2ER, UK

#HCI International Medical Centre, Beardmore Street, Clyde bank, Glasgow G81 4HX, UK

Background and Goal of Study:

 The bispectral index (BIS) and the Auditory Evoked Potential (AEP),   electroencephalographic-derived variables, have been shown to be altered by volatile anaesthetics, suggesting that both variables correlate with the sedative and hypnotic components of anaesthesia produced by those anaesthetics. Nitrous oxide is the most commonly used volatile anaesthetic as an adjuvant to other intravenous or volatile anaesthetics. As a weak hypnotic and good analgesic, it did not appear to alter AEP markedly in some comparative studies, compared with other volatile anaesthetics.¹ Similarly, this unique effect of nitrous oxide on AEP may also be identical for the BIS. In reality there were some reports in which nitrous oxide was shown to have no effect on the BIS in sub-anaesthetic concentration^{2 3} . This may suggest that the BIS and AEP do not accurately monitor the depth of anaesthesia with nitrous oxide. The effects of nitrous oxide, when used as a sole sedative agent or as an adjuvant to intravenous anaesthesia using propofol, on the BIS and Auditory Evoked Potential Index (AEPex), were compared during normothermic, normocapnic and normotensive conditions.

Materials and Methods:

This study was conducted in two groups: 15 healthy volunteers (group I) and nine patients (group II). In group I, volunteers inhaled slowly increasing nitrous oxide by 10% increments up to a maximum end-tidal concentration of 70% through a face mask. In group II, 9 unpremedicated ASA physical status I patients undergoing lower extremity surgery under spinal anaesthesia were provided with a random sequence of seven concentrations (10, 20, 30, 40, 50, 60 and 70%) of nitrous oxide after they lost consciousness with target-controlled infusion (TCI) of propofol. Simultaneous recording of BIS, AEPex and other measurements was made at predetermined end-tidal concentrations of nitrous oxide when stabilised for 10 minutes for both groups. Level of sedation was also assessed in group I using a validated sedation scoring system, the Observer Assessment of Alertness/Sedation (OAA/S) scale.

Results: In group I, most volunteers lost verbal contact at concentrations between 50%-70% of nitrous oxide. The increasing concentration of inhaled nitrous oxide resulted in significant reduction in mean values of AEPex at concentrations of 60% and 70% (P<0.05) but with large inter-individual variability, while there was no change in mean values of BIS. The increment of inhaled nitrous oxide concentration resulted in significant reduction in mean values of OAA/S scale at concentrations over 50% (P<0.01), and there was a moderate correlation between AEPex and OAA/S scale (r=0.457, P<0.01). In group II, the addition of nitrous oxide in concentrations between 10%-70% when administered in combination with propofol or withdrawal did not have any significant dose dependent effects on mean values of both BIS and AEPex.

Conclusion:

It was concluded that AEPex could reflect the degree of sedation produced by nitrous oxide as a sole agent while BIS was not changed by nitrous oxide. However, large interindividual variability in AEPex values (at the same concentrations) limited the reliability of AEPex as a real time monitoring measure of sedation level by nitrous oxide, suggesting that AEPex values should be interpreted cautiously when using nitrous oxide alone for sedation. AEPex correlated to a moderate degree with clinically assessed levels of sedation by nitrous oxide without other anaesthetic supplementation, as measured by OAA/S scale. We also concluded that both BIS and AEPex were not affected by the use of nitrous oxide when administered with background infusion of propofol.

References

1. Goto T, Nakata Y, Saito H, Ishiguro Y, Niimi Y, Morita S. The midlatency auditory evoked potentials predict responsiveness to verbal commands in patients emerging from anaesthesia with xenon, isoflurane and sevoflurane but not with nitrous oxide. Anesthesiology 2001; 94:782-9

2. Rampil I, Kim J-S, Lenhardt R, Negishi C, Sessler D. Bispectral EEG index during nitrous oxide administration. Anesthesiology 1998; 89: 671-7

3. Barr G, Jakobsson JG, Owall A, Anderson RE. Nitrous oxide does not alter bispectral index: study with nitrousoxide as sole agent and as an adjuvant to i.v. anaesthesia. Br J Anaesth 1999; 82:827-30