Fig. 6. Sensitivity analysis for the model parameters. (A –D , F ) Estimated parameters and (E , G ) fixed parameters. Δ-2LL is the difference in −2 log likelihood between the simulated data and the best fit with one of the parameters held constant. The axis of each plot shows the parameter value as a fraction of the value observed or fixed in the analysis of the experimental data set. The short dashed line  indicates the P = 0.01 level, and the long dashed line  indicates the P = 0.05 level. •-• input function: step increase in plasma concentration of 1 ng/ml, duration of step is 1 min (rate of rise = 1 ng/ml per min), number of steps is 5. ○-○ input function: step increase in remifentanil concentration is 1 ng/ml, duration of step is 5 min (rate of rise = 0.2 ng/ml per min), number of steps is 5. x-x  input function: step increase in plasma concentration of 1 ng/ml, duration of step is 0.1 min (rate of rise = 10 ng/ml per min), number of steps is 5. C⁵⁰= concentration remifentanil causing 50% respiratory depression; G = gain of the ventilatory control system or slope of the hypercapnic ventilatory response; k^e0= blood effect site rate constant; τ= time constant of the ventilatory control system; Q = cardiac output; V^ALV= alveolar volume; V^TS= tissue volume.