Figure 9. Computer simulation of the decay in blood propofol and plasma alfentanil concentrations, showing the first 40 min after the termination of a computer-controlled infusion that had been given for 180 min, with target propofol and alfentanil concentrations equal to the alfentanil concentration associated with 50% probability of no response determined for intraabdominal surgery (see Figure 6). The decay curves for propofol and alfentanil were determined with a computer simulation program that was provided with the pharmacokinetic data sets of propofol and alfentanil used in this study. [12,13]Bold line = blood propofol and plasma alfentanil concentrations at which 50% of patients regain consciousness (see Figure 7). The interval between the termination of the infusion of propofol and alfentanil and the time that the propofol and alfentanil concentrations had decreased to concentrations equal to those at which 50% of patients regain consciousness was found to be shortest (10 min) after a 180-min infusion of propofol and alfentanil at concentrations of 3.5 micro gram/ml and 85 ng/ml, respectively.