Effects of surgical stimulation on midlatency auditory evoked potentials during general anaesthesia with propofol/fentanyl, isoflurane/fentanyl and flunitrazepam/fentanyl.

During general anaesthesia, midlatency auditory evoked potentials are suppressed in a dose dependent manner by a number of general anaesthetics. The activating effects of surgical stimuli on midlatency auditory evoked potentials have been demonstrated during light inhalational anaesthesia, and indicate that midlatency auditory evoked potentials reflect the activity of the central nervous system and not only anaesthetic concentrations. We investigated the effect of surgical stimulation (skin incision, sternotomy) on midlatency auditory evoked potentials under high dose opioid analgesia in 30 patients undergoing elective cardiac surgery. High dose opioid analgesia was maintained using fentanyl (1.2 mg.h-1) and combined with either propofol (4-8 mg.kg-1.h-1) (group I, n = 10), isoflurane (0.6-1.2 vol%) (group II, n = 10) or flunitrazepam (1.2 mg.h-1) (group III, n = 10). Midlatency auditory evoked potentials were recorded in the awake state, during general anaesthesia before skin incision, after skin incision and after sternotomy. During general anaesthesia there were marked statistically significant increases in latencies and decreases in amplitudes of midlatency auditory evoked potentials in the propofol/fentanyl and isoflurane/fentanyl groups. In contrast, in the flunitrazepam/fentanyl group there were only small changes of midlatency auditory evoked potentials. The latencies of the early cortical potentials were similar to those in the awake state. After skin incision as well as after sternotomy no significant changes of midlatency auditory evoked potentials could be observed in any of the experimental groups. These results indicate that activation of the auditory pathway by surgical stimuli may be blocked when analgesia is provided by high dose fentanyl.(ABSTRACT TRUNCATED AT 250 WORDS)