The effects of plasma fentanyl concentrations on propofol requirement, emergence from anesthesia, and postoperative analgesia in propofol-nitrous oxide anesthesia

To determine the effects of plasma fentanyl concentrations on intraoperativ e propofol requirements, emergence from anesthesia, and relief of postopera tive pain, we studied 60 ASA physical status I and II patients undergoing s pine fusion. The patients were randomly assigned to four study groups accor ding to the expected intraoperative plasma fentanyl concentrations. Group I received an infusion of saline, and Groups II, III, and IV received fentan yl infusions to maintain the blood levels at 1.5, 3.0, and 4.5 ng/mL, respe ctively. An infusion rate of propofol was adjusted to keep the mean arteria l pressure within 15% of the control value. Inspired nitrous oxide concentr ations were maintained at 67%. The following were investigated in each grou p: 1) an average propofol infusion rate, 2) time to spontaneous eye opening and recovery of orientation (name, date, and place), and 3) total dose of fentanyl used for 24 h after admission to the postanesthetic care unit. Ave rage propofol infusion rates were 10.1 +/- 2.5 (mean +/- SD), 7.5 +/- 1.2, 5.7 +/- 1.1, and 4.9 +/- 1.2 mg . kg(-1) . h(-1), in Groups I, II, III, and IV, respectively. Groups receiving fentanyl infusion had significantly sma ller infusion rates of propofol (P < 0.01) than the group receiving saline. Among the three fentanyl infusion groups, Group II (P < 0.01) had more tha n Groups III and IV. The time to spontaneous eye opening and the recovery o f orientation were directly related to plasma fentanyl concentrations. The plasma fentanyl levels between Groups III and IV were the same. The total a mount of IV patient-controlled analgesia fentanyl during postoperative 24 h increased significantly when the order of plasma fentanyl concentrations w as reversed, 913.1 +/- 58.4, 553.4 +/- 129, 222.7 +/- 73.4, and 135.1 +/- 6 9.5 mu g in Groups I, II, III, and IV, respectively. These results suggest that the addition of fentanyl infusions had ceiling effects that reduce the intraoperative propofol requirements according to the plasma fentanyl conc entrations. The ceiling effect was demonstrated in the recovery of consciou sness but not in the fentanyl requirements for postoperative analgesia.