[Monitoring intraoperative processing of acoustic stimuli with auditory evoked potentials]

130 Patients undergoing elective intra-abdominal, gynaecological, urological or cardiac surgery were studied after institutional approval and informed consent. In all patients auditory-evoked potentials (AEP) were recorded in the awake state and during general anaesthesia. Latencies of the peaks V, Na, Pa, Nb and P1 were measured. V belongs to the brainstem-generated potentials, which demonstrates that auditory stimuli were correctly transduced. Na, Pa, Nb, P1 are generated in the primary auditory cortex of the temporal lobe. During anaesthesia with isoflurane, enflurane, thiopentone, etomidate and propofol the peak V remains unchanged, whereas the mid-latency auditory-evoked potentials (MLAEP) show marked increases in latencies and decreases in amplitudes or are even completely suppressed. This indicates a successful stimulus transmission up to the level of the brainstem and midbrain. However, stimulus processing in the primary auditory cortex is blocked. Under increasing end-expiratory concentrations of isoflurane MLAEP show a dose-dependent increase of latencies and decrease of amplitudes. Under surgical anaesthesia with 1.2 vol%, MLAEP are nearly completely suppressed. A different picture can be seen when MLAEP were recorded during anaesthesia with the receptor-specific anaesthetics midazolam, flunitrazepam, diazepam, fentanyl and ketamine. During anaesthesia with receptor-specific anaesthetics, the brainstem peak V as well as the mid-latency components remain nearly unchanged compared with AEP from awake patients. This indicates that auditory stimuli reach the primary auditory cortex and are processed at a primary cortical level. With increasing doses of fentanyl one can observe only a significant decrease of amplitudes for the late component P1.(ABSTRACT TRUNCATED AT 250 WORDS)