The bispectral index has limitations in describing the exact depth of anesthesia during nitrous oxide inhalation. This study examined the effect of nitrous oxide on the cerebral entropy measured using an entropy module (M-ENTROPY Module S/5®, Datex-Ohmeda division, Instrumentarium Corporation, Helsinki, Finland) during the stable anesthetic period with isoflurane inhalation.

Sixty ASA 1 or 2 adult patients were randomly allocated to three groups. During the stable maintenance period after the skin incision, the baseline entropy values (response entropy, RE; state entropy, SE) were recorded at 2.5 minutes intervals over a 20 minute period on a single frontal channel at 0.9% end-tidal isoflurane. After this, medical air was used continuously (group C) or replaced with nitrous oxide at 40% (group L) or 60% (group H) with continuous hemodynamic and entropy values monitoring. Each of the variables was recorded and analyzed at 2.5 minutes intervals over a 20 minute period.

Average values (mean ± SD) of the RE and SE during experimental period were lower in group H (29.2 ± 12.3 and 28.5 ± 11.7, respectively) than group L (33.9 ± 7.3 and 33.0 ± 7.3, respectively) and the averaged values were lower in group L than in group C (46.6 ± 14.8 and 45.5 ± 14.2, respectively). The percent reduction was larger in group H (42.1 ± 14.2 and 38.7 ± 16.5, respectively) than in group L (25.3 ± 15.1 and 24.4 ± 14.9, respectively) and the percent reduction was larger in group L than in group C (P < 0.01).

Added nitrous oxide during the anesthetic maintenance period with isoflurane decreases the level of cerebral entropy.