Visual predictive check. The blue circles represent the observed data for S(+) ketamine (arterial and venous) and R(–) ketamine (arterial and venous), respectively; the red line represents the 50% of the observed concentrations at each time point; and the black lines represent the ninety-fifth, fiftieth, and fifth percentiles of the simulated concentrations at each time point. The light blue line is at the lower limit of quantitation for the assay (2 ng/ml).