MONITORING OF INTRAOPERATIVE AUDITORY STI MULUS-PROCESSING USING AUDITORY-EVOKED POTENTIALS

130 Patients undergoing elective intra-abdominal, gynaecological, urol ogical or cardiac surgery were studied after institutional approval an d informed consent. In all patients auditory-evoked potentials (AEP) w ere recorded in the awake state and during general anaesthesia. Latenc ies of the peaks V, Na, Pa, Nb and P1 were measured. V belongs to the brainstem-generated potentials, which demonstrates that auditory stimu li were correctly transduced. Na, Pa, Nb, P1 are generated in the prim ary auditory cortex of the temporal lobe. During anaesthesia with isof lurane, enflurane, thiopentone, etomidate and propofol the peak V rema ins unchanged, whereas the midlatency auditory-evoked potentials (MLAE P) show marked increases in latencies and decreases in amplitudes or a re even completely suppressed. This indicates a successful stimulus tr ansmission up to the level of the brainstem and midbrain. However, sti mulus processing in the primary auditory cortex is blocked. Under incr easing endexpiratory concentrations of isoflurane MLAEP show a dose-de pendent increase of latencies and decrease of amplitudes. Under surgic al anaesthesia with 1.2 vol%, MLAEP are nearly completely suppressed. A different picture can be seen when MLAEP were recorded during anaest hesia with the receptor-specific anaesthetics midazolam, flunitrazepam , diazepam, fentanyl and ketamine. During anaesthesia with receptor-sp ecific anaesthetics, the brainstem peak V as well as the midlatency co mponents remain nearly unchanged compared with AEP from awake patients . This indicates that auditory stimuli reach the primary auditory cort ex and are processed at a primary cortical level. With increasing dose s of fentanyl one can observe only a significant decrease of amplitude s for the late component P1. This effect does not depend on the given dosages and can be seen after the first fentanyl bolus injection. In c ontrast, brainstem AEP and the early cortical response Na, Pa and Nh d o not change significantly compared with the awake state. During gener al anaesthesia with isoflurane, enflurance, thiopentone, etomidate and propofol MLAEP are suppressed in a dose-dependent way. This indicates that the primary sensory information processing in the brain is block ed at the level of the primary sensory cortex. In contrast, receptor-s pecific anaesthetics like midazolam, diazepam, flunitrazepam, fentanyl and ketamine do not suppress MLAEP This fact supports the assumption that under the influence of these agents auditory information processi ng remains intact to some extent. This corresponds well with the clini cal observation that the incidence of intraoperative awareness and the perception of auditory stimuli during general anaesthesia is higher w ith receptor-specific anaesthetics than with general anaesthetics when used to suppress consciousness. General anaesthetics seem to provide a more effective suppression of auditory stimulus processing and uncon scious perceptions during general anaesthesia than receptor-specific a gents.