Effects of benzodiazepines on mid-latency auditory evoked potentials.

Midlatency auditory evoked potentials (MLAEP) reflect primary cortical processing of auditory stimuli. The effects of benzodiazepines on MLAEP have not yet been studied. We examined the effects of intravenous induction of general anaesthesia using the benzodiazepines midazolam, diazepam and flunitrazepam on MLAEP in 30 patients scheduled for minor gynaecological procedures. Anaesthesia was induced with midazolam (0.2-0.3 mg.kg-1, Group I, n = 10), diazepam (0.3-0.4 mg.kg-1, Group II, n = 10) or flunitrazepam (0.03-0.04 mg.kg-1, Group III, n = 10). Auditory-evoked potentials were recorded before and five to ten minutes after induction of general anaesthesia. Latencies of the peak V, Na, Pa, Nb and Pl (ms) and amplitudes Na/Pa, Pa/Nb and Nb/P1 (microV) were measured. In the awake state, MLAEP had high peak to peak amplitudes and a periodic waveform. After induction of anaesthesia there was no or only a small increase in latencies of the peaks Na, Pa, Nb and P1, which was significant only for P1 in the midazolam group. Amplitudes Na/Pa, Pa/Nb and Nb/P1 decreased only slightly and which reached statistical significance only for Na/Pa in the flunitrazepam group. The MLAEPs do not change markedly in amplitude or latency during induction of general anaesthesia with benzodiazepines. Primary cortical processing of auditory stimuli seems to be preserved under benzodiazepines. This may be seen in connection with cases of intraoperative awareness and especially the perception of auditory stimuli during anaesthetic regimens where benzodiazepines are used to suppress consciousness.