The interaction between fentanyl and propofol during emergence from anesthesia: monitoring with the EEG-Bispectral index™

Abstract

Study objective To investigate the effect of different plasma levels of fentanyl on the concentration of propofol and the Bispectral Index (BIS) required for patients to regain consciousness and orientation following surgery. Design Prospective, open-label study. Setting Operating room of a university hospital. Patients 28 patients, aging 20 to 50 years, scheduled for elective, 1- to 4-hour surgeries under general anesthesia. Interventions BIS was continuously monitored from bifrontal montage (At1-Fpz and At2-Fpz) using an Aspect A-1,050 EEG system (Aspect, Natick, MA). Anesthesia was induced with bolus injections of fentanyl 2 μg/kg and propofol 2 mg/kg, and maintained with intermittent injections of fentanyl and constant infusion of propofol. Propofol infusion was stopped at the end of surgery. Measurements Consciousness and orientation were assessed as clinical endpoints once every 2 minutes following the end of the surgery. Blood samples were extracted for plasma propofol and fentanyl concentrations (PCp and FCp, respectively), and BIS values were recorded when patients regained consciousness and orientation. Patients were allocated to one of three groups depending on FCp on awakening: Group 1, FCp > 1 μg/L (n = 8); Group 2, FCp < 1 μg/L and >0.45 μg/L (n = 9); and Group 3, FCp < 0.45 μg/L (n = 11). PCp, BIS, recovery time, and other data were compared between the three groups. Main results Demographic values, duration of surgery, and consumption of propofol and fentanyl were not different between the three groups. Group 3 patients regained consciousness with significantly higher propofol concentration (mean PCp = 3.2 mg/L) compared with those in Groups 1 and 2 (p < 0.05). However, the BIS values at both recovery endpoints were not different among the three groups. Conclusions The plasma levels of fentanyl affect the concentrations of propofol required for patients to regain consciousness. The BIS values for wakefulness are unaltered at the different combinations of propofol and fentanyl concentrations. Thus, the BIS appears to be a useful and consistent indicator for level of consciousness during emergence from propofol/fentanyl intravenous anesthesia.