Effect of nitrous oxide on bispectral index values at equi-minimum alveolar concentrations of sevoflurane and...

Bispectral index (BIS) values may be anaesthetic agent-specific, depending on their ability to suppress the electroencephalogram (EEG) signals. We carried out a prospective, randomised clinical trial to study the effect of nitrous oxide (N₂O) on the BIS values at an equi-minimum alveolar concentration (MAC) of sevoflurane and desflurane.

Sixty adult patients undergoing spine surgery were randomised into two groups; Group S (sevoflurane; n = 30) and Group D (desflurane; n = 30) for the maintenance of anaesthesia in oxygen and air or oxygen and N₂O mixture (FiO₂-0.4) (Stage 1). BIS and fraction of inspired and end-tidal concentration of agents were noted at 1.0 MAC. In Stage 2, air or N₂O was discontinued and the other carrier gas was introduced. At steady state of this carrier gas, values were again noted as in Stage 1. Statistical analysis was performed using two-way analysis of variance followed by Bonferroni correction, and Student's t-test for paired data. P<0.05 was considered statistically significant.

With air-oxygen as the carrier gas, sevoflurane and desflurane resulted in comparable BIS values (P = 0.44). With addition of 60% N₂O, there was a significant increase in BIS values at 1.0 MAC for both the agents. Furthermore, higher BIS values were observed with sevoflurane compared to desflurane (P = 0.01).

Sevoflurane and desflurane at equi-MAC concentration exert similar effect on BIS values when used with air-oxygen. N₂O results in higher BIS values; this effect is more pronounced in combination with sevoflurane.