Figure 3. The relation between the peak end-tidal isoflurane concentration (ET-ISO) and the corresponding opioid concentrations after skin incision for patients administered sufentanil (left) and after sternal lift for patients administered fentanyl (right). The curves represent the end-tidal isoflurane concentration predicted by the equations ET-ISO = 7.48 x (serum sufentanil)(^-1).09 (P = 0.0001, r²= 0.452), and ET-ISO = 10.96 x (serum fentanyl)^-1.19 (P = 0.0003, r²= 0.566).