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A computer-assisted continuous infusion (CACI) system which would rapidly attain and maintain a constant plasma level (i.e., constant set point) of the opiod, fentanyl, was designed. This open-loop CACI system was then modified to allow the anesthesiologist (clinician) to adjust the plasma level (set point) intraoperatively (i. e., a variable set point); however, since no sensor is presently available to measure the plasma concentration of fentanyl on line, a mathematical model was implemented to simulate the plasma fentanyl concentration (PFC). This interactive system was tested in 20 patients undergoing coronary artery bypass grafting surgery. The measured PFC/CACI predicted PFC was determined to be 1.04 Â±0.25 (mean Â±S.D.) for the 170 samples analyzed. A correlation coefficient of 0.90 (p < 0.0001) was found between the measured PFC and the CACI system model prediction. These data indicate that the CACI system is an easy way to provide predictable constant plasma fentanyl levels during the precardiopulmonary bypass period of cardiac surgery.