Figure 1. A representative plasma concentration-time profile from an epidural study day in one participant, and an illustration of the concentration steps chosen for the tailored intravenous alfentanil infusion. Open circles indicate the measured plasma alfentanil concentration after epidural administration of a 400-[micro sign]g epidural loading dose followed by a 400-[micro sign]g/h epidural infusion of alfentanil. The dotted lines represent the series of concentration steps selected as targets for the tailored intravenous infusion day for this participant. The choice of targets and the duration of each step was made by visual inspection of the alfentanil concentration versus time graph.