PROCESSING OF AUDITORY-STIMULI DURING ANESTHESIA - EFFECTS OF NONSPECIFIC AND RECEPTOR-SPECIFIC ANESTHETICS ON MIDLATENCY AUDITORY EVOKED-POTENTIALS

Awareness during general anaesthesia is a problem of modern anaesthetic practice. It is a well known fact that especially auditory information can be perceived intraoperatively. It is not yet clear which anaesthetic agents suppress auditory stimuli perception most reliably. Mid-latency auditory evoked potentials (MLAEP) are the electrophysiological correlate of the primary cortical processing of auditory stimuli. Therefore we studied the effects of different anaesthetics on MLAEP. Institutional approval and informed consent was obtained in 1 00 patients scheduled for elective gynaecological or urological procedures. Auditory evoked potentials were recorded in the awake state and during general anaesthesia with Isoflurane (1.2 Vol. %) (group I, n = 10), Enflurane (1.6 Vol. % (group II, n = 10), Thiopentone (5 mg/kg) (group III, n = 10), Etomidate ( 0.3 mg/kg) group IV, n = 10), Propofol (2.5 mg/kg) (group V, n = 10), Midazolam (0.2 - 0.3 mg/kg) group VI, n = 10), Diazepam (0.3 - 0.4 mg/kg (group VII, n = 10), Flunitrazepam (0.03 -0.04 mg/kg (group VIII, n = 10), Fentanyl (0.01 - 0.02 mg/kg) group IX, n = 10) und Ketamine (2 mg/kg (group X, n = 10). For each MLAEP 1 000 successive stimulus responses were averaged over a 100 ms poststimulus period and analysed off-line. Latencies of the peak V, Na, Pa were measured. V belongs to the brainstem generated potentials, which demonstrates that auditory stimuli were correctly transduced. Na, Pa are generated in the primary auditory cortex of the temporal lobe and are the electrophysiological correlate of the primary cortical processing of the auditory stimuli. A Fast-Fourier transformation was performed to calculate powerspectra of the AEP. In the awake state AEP peak latencies were in the normal range. Powerspectra indicated high energy in the 30-40 Hz frequency range. During general anaesthesia with Isoflurane, Enflurane, Thiopentone, Etomidate and Propofol a marked increase in latencies and decrease in amplitudes of Na, Pa could be observed. The primary cortical complex Na/Pa was completely suppressed. The energy in the 30-40 Hz range was reduced and the energy maxima of the MLAEP shifted to low frequencies. In contrast, during general anaesthesia with Midazolam, Diazepam, Flunitrazepam, Fentanyl and Ketamine there was no increase in latencies and no decrease in amplitude of the peaks Na, Pa. The primary cortical complex Na/Pa could be identified like in the awake patients. Power spectra frequencies in the range of 30-40 Hz retained high energy. Primary cortical processing of auditory stimuli is suppressed under non-specific anaesthetics and widely preserved under receptor-specific agents. This must be seen in connection with cases of intraoperative awareness and high incidence of dreams during anaesthetic regimens where Benzodiazepines, high-dose Opioids or Ketamine are used to suppress consciousness. Non specific anaesthetics seem to suppress auditory stimuli perception more reliably than receptor-specific agents.