Fig. 3. ( A ) Relation between spectral edge frequency (SEF) and end-tidal isoflurane concentration (EtIso) during a decrease in inhalation concentration from 2% to 0.5% (the lower limb of the loop) and then an increase back to 2% (the upper limb) under control conditions for the same pig shown in figure 2 . ( B ) Relation between SEF and effect site isoflurane concentration (Ce) after hysteresis minimization.