The processed electroencephalograms are useful surrogate measures of quantification of volatile anesthetics effect. We hypothesized that there is a certain relationship between the end-tidal concentration of sevoflurane and approximate entropy (ApEn). The aim of this study was to investigate the effect of sevoflurane on ApEn during induction.

Fourteen patients were enrolled. With recording of the electroencephalogram at the F3, F4, P3, and P4 montages, anesthesia was induced by increasing the vapor setting of sevoflurane by 1 vol% up to 8 vol% via a facemask. When the end-tidal concentration of sevoflurane did not show any further change, we increased vapor setting of sevoflurane by 1 vol%. When the end-tidal concentration of sevoflurane did not show any further change with 8 vol% of sevoflurane vapor setting, measurement of electroencephalogram was finished. The relation between end-tidal sevoflurane concentration and ApEn was tested by Spearman correlation.

ApEn showed a significant correlation (R = -0.7551, P < 0.0001) with end-tidal sevoflurane concentration. The final pharmacodynamic parameters estimated by a sigmoid E _max model were described as follows: E ₀ = 0.91, E _max = 0.28, C _e50 = 1.36 vol%, γ = 1.27, k _e0 = 0.67 min^-1.

Sevoflurane induction with gradual increase of end-tidal sevoflurane concentration increases the amount of regularity in approximate entropy.