Fig. 4. Effect of adding remifentanil on the concentration–effect relations of sevoflurane for sedation ( A ) and analgesia ( B ). Each curve represents the concentration–effect relation for sevoflurane with a fixed effect site concentration of remifentanil simulated from the corresponding response surface model. The shift in the curves toward the left indicates that much less sevoflurane is needed when remifentanil is added, demonstrating the significant pharmacodynamic synergy between the sedative and the opioid. Note that the magnitude of the leftward shift decreases as the remifentanil concentration increases ( i.e. , there is a ceiling effect). OAA/S = Observer's Assessment of Alertness/Sedation.