Excitatory effects of fentanyl upon the rat electroencephalogram and auditory-evoked potential responses during anaesthesia

Summary Background and objective: Previous studies have shown existence of inconsistent data concerning the use of auditory-evoked potential (AEP) and electroencephalogram (EEG) changes to measure the depth of anaesthesia in regimens involving the use of opioids. The present studies characterize the effects of fentanyl on those responses in rats. Methods: The effects of a bolus of fentanyl (6–10 μg kg−1 intravenously) alone or following naloxone (100 μg kg−1 intravenously) were examined using brain responses in rats during light anaesthesia with either propofol (20–30 mg kg−1 h−1) or isoflurane (0.8%). Electrophysiological data were recorded using silver ball electrodes. The rats' tracheas were intubated and a femoral artery cannula was inserted to monitor blood pressure. Body temperature, respiratory and pulse rate, and pedal withdrawal data were also collected. Parameters measured before and following administration of naloxone and fentanyl or of fentanyl alone were compared using repeated-measures ANOVA. Results: Fentanyl significantly increased the latency of the major peak from the AEP during propofol and isoflurane anaesthesia (F = 13.2 and 13.5, respectively; P < 0.05) and the amplitude differential between two waveform complexes, and the second differential index (F = 28.3 and 57.2, respectively; P < 0.01). The spectral edge frequency and median frequency from the EEG tended to increase. These effects were abolished by the prior administration of naloxone. Conclusions: These excitatory effects were inconsistent with the classical concept of brain activity depression indicating a deepening of anaesthesia.