Fig. 4. Arterial blood S -(+)-ketamine (open symbols) and R -(−)-ketamine (closed symbols) concentration histories for the first 1.5 min (illustrating the first- and second-pass peaks) and for 240 min ( inset ) after right atrial injection in dog 1. The symbols represent drug concentrations, whereas the lines represent concentrations predicted by the models. Note that several open symbols are obscured by being coincident with closed symbols.