The effect of addition of nitrous oxide to a sevoflurane anesthetic on BIS, PSI, and entropy

Abstract

Objective. N₂O is a commonly used anesthetic that has amnestic and analgesic properties. Recently, devices that estimate depth of consciousness have been introduced in an attempt to better titrate anesthesia, however the effect of N₂O on these monitors is unclear. Methods. General anesthesia was induced and titrated to maintain normal blood pressure and pulse in healthy adults. Data were collected in three 10 minute intervals (Sevo, Sevo + N₂O, Sevo). In Phase A, sevoflurane concentration was held constant during the N₂O trial in 60 subjects monitored with either BIS, PSI, or Entropy. In Phase B, sevoflurane concentration was reduced as N₂O was added, maintaining a constant overall “MAC” in 20 subjects monitored concurrently with BIS and Entropy. Sample size for both phases was designed to detect a 10 unit change in measure of processed EEG with alpha = .05 and statistical power = .80. Results. In Phase A, supplementing sevoflurane with > 65% N₂O increased MAC from 1.3 ± 0.05 to 2.2 ± 0.10, but did not significantly alter BIS nor PSI (p-value for differential MAC is < 0.05). Entropy, however, dropped significantly, with a change in state entropy (SE) from 31.1 ± 7.3 to 18.9 ± 3.7 and a corresponding rise when N₂O was discontinued. In Phase B, supplementing sevoflurane with > 65% N₂O with a concomitant reduction in sevoflurane resulted in an increase in both BIS (from 34 ± 5 to 53.9 ± 11.5) and SE (from 32 ± 8.2 to 55.4 ± 21.3). Conclusion. Supplementing sevoflurane with > 65% N₂O did not result in a significant change in either BIS or PSI when sevoflurane concentration was kept constant. Entropy, however, significantly decreased as anesthetic depth increased. When sevoflurane concentration was reduced during N₂O administration, both BIS and Entropy rose despite maintenance of anesthetic depth, indicating a variable concentration effect between volatiles and N₂O.