Does nitrous oxide affect bispectral index and state entropy when added to a propofol versus sevoflurane anesthetic?

In earlier studies, nitrous oxide (N2O) did not affect bispectral index (BIS) or state entropy (SE) when administered as the sole anesthetic agent. We investigated the effect of adding N2O to sevoflurane or propofol anesthesia on BIS and SE.

A total of 28 patients were randomized to receive general anesthesia using either 2% sevoflurane or 120 mcg/kg/min propofol during surgery. After 20 minutes of stable anesthetic and surgical conditions (baseline), 60% N2O was added for 20 minutes. Another 20 minutes was allowed for washout after N2O was discontinued. Sevoflurane concentration was closely monitored using the standard end-tidal gas analysis and vaporizer output was constantly adjusted to offset the second-gas effect caused by the addition and discontinuation of N2O. Mean values for BIS and SE were calculated for the last 5 minutes of the baseline, nitrous oxide, and washout periods.

In the propofol group, BIS and SE values in baseline versus N2O periods were 40±10 versus 39±10 and 43±11 versus 43±12, respectively (mean±SD, P>0.05). In the sevoflurane group, addition of N2O decreased both BIS (36±7 to 32±7, P<0.001) and SE (37±10 to 31±11, P=0.001). The magnitude of change from the baseline in BIS and SE in the sevoflurane group was 3.6±3.8 (95% CI: 0.8-6.5) and 6.1±5.5 (95% CI: 3.7-8.6), respectively. Average baseline and washout values of BIS and SE were not different in both groups (P>0.05).

N2O decreased both BIS and SE when added to sevoflurane, but not propofol. The observed changes in the sevoflurane group were not clinically significant. Decreases in BIS and SE in the sevoflurane group could result from a true additive effect and second-gas effect of N2O that was unaccounted for despite a meticulous titration of sevoflurane using end-tidal gas monitoring.