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Background: It has been suggested that spectral entropy of the electroencephalogram as applied in the M-Entropy S/5TM Module (GE Healthcare) does not detect the effects of nitrous oxide (N2O). The aim of this study was to investigate the effect on entropy by graded increases in N2O concentrations in the presence of a constant concentration of sevoflurane, in the absence of surgical stimulation.Method: This single-blind, randomised study was conducted at an altitude of approximately 1 400 m. Patients received sevoflurane 2% (1.7% at sea level) and N2O, at end-tidal concentrations of 0%, 10%, 20%, 30%, 40%, 50%, 60% or 70% (equivalent to 8.5%, 17%, 25.5%, 34%, 42.6%, 51.1% and 59.6% at sea level). Entropy was measured before, during and after N2O administration. The absolute changes and ratios o f entropy relative to the baseline were calculated. Between- and within-group comparisons were made using analysis of variance and covariance.Results: None of the entropy variables differed significantly within and between groups before and after N2O administration. Within-group analysis revealed that entropy during N2O administration was significantly lower than before or after N2O administration (P < 0.007). While a minor clinical but statistically significant linear relationship was observed between increasing N2O concentration and decreasing entropy from N2O 0% to 60%, a steeper and clinically important decrease (relative change > 20%) was noted at N2O > 60% (> 51% at sea level).Conclusions: The M-Entropy Module S/5TM responds to increasing concentrations of N2O in the presence of 2% (1.7% at sea level) sevoflurane, in the absence of surgical stimulation. There is a linear relationship between increasing N2O concentrations and decreasing entropy with a steep and clinically important decrease at N2O > 60% (> 51% at sea level). The influence of ambient pressure on the partial pressures, which determine the effects of anaesthetic agents, must be taken into account. Keywords: monitoring; depth of anaesthesia; monitoring; electroencephalography; monitoring; entropy; anaesthetics volatile; sevoflurane; anaesthetic gases; nitrous oxide