Fig. 4. Sensitivity analysis for each parameter of the propofol-induced ventilatory depression model, showing the change in −2 log likelihood (LL) with parameter deviation from the estimate value. Positive values indicate a worse fit; negative values indicate an improved fit. The improvement in the equilibration constant between end-tidal pressure of carbon dioxide and partial pressure of carbon dioxide at the effect site (ke0, CO2) and the gain in the carbon dioxide response (F) is expected because these parameters were from the model of carbon dioxide response and were not optimized to estimate propofol-induced ventilatory depression. C50= propofol concentration causing 50% depression of minute ventilation under constant Paco2and 50% of the maximum decrease in carbon dioxide production; γ= slope factor of the fractional sigmoid Emax models for respiratory depression and depression of carbon dioxide production; kel, CO2= effect compartment equilibration constant of propofol; kel= elimination constant.