Three-dimensional response surfaces as predicted by the sevoflurane/opioid-additivity model (XA). A and B, Response surface for, respectively, movement and hemodynamic responses after incision in the control group (fentanyl, sevoflurane, no nitrous oxide). C and D, Response surface for, respectively, movement and hemodynamic responses after incision in the nitrous oxide (N₂O) group (fentanyl, sevoflurane, 66 vol.% nitrous oxide).