Figure 1. (Top) The two parts of the empiric model used in protocol 1. Part 1 describes the time lag between the end-tidal (ET) sevoflurane and effect-site partial pressures (EC). The time lag is defined by a rate constant, k (or t^1/2, which equals ln2/k). Part 2 translates the effect-site concentrations via an inhibitory sigmoid Emax model (the Hill equation) into ventilation (Vⁱ). (Left, bottom) A typical example of hysteresis of the ventilation-end-tidal sevoflurane partial pressure relation. Each circle represents one breath. The arrows indicate the time course. To guide the eye, a broken line connects the data points. The data fit is plotted in the adjoining diagram. (Right, bottom) In the upper panel, the input function (end-tidal sevoflurane partial pressure, continuous line) is plotted with the estimated sevoflurane partial pressure at the effect site (broken line). Note the time lag between the two. In the lower panel, each circle represents one breath. The line through the breaths is the model fit. Estimated parameter values +/− SD: t^1/2, 2.9 +/− 0.5 min; V^BL, 0.617 +/− 0.003; PEC⁵⁰, 18.1 +/− 0.22 mmHg; and [Greek small letter gamma], 1.34 +/− 0.04. PEC⁵⁰versus [Greek small letter gamma]: correlation coefficient =-0.45;[Greek small letter gamma] versus t^1/2: correlation coefficient = 0.36. Data are from cat 006.