MIDLATENCY AUDITORY-EVOKED POTENTIALS DURING INDUCTION OF GENERAL-ANESTHESIA WITH S-(-KETAMINE VERSUS KETAMINE-RACEMAT())

Mid-latency auditory evoked potentials (MLAEP) reflect the primary cor tical processing of auditory stimuli. They are widely suppressed durin g general anaesthesia with volatile anaesthetics. Under ketamine, in c ontrast, they seem to be preserved, which has been interpreted as indi cating insufficient suppression of consciousness during ketamine anaes thesia. Ketamine exists in two optical isomeres, S-(+)-ketamine und R- (-)-ketamine, which differ in their pharmacodynamic properties: S-(+)- ketamine has higher anaesthetic-hypnotic and analgesic potency than R- (-)-ketamine. It thus appears obvious to question whether S-(+)-ketami ne has a different effect on the primary cortical processing of sensor y, i.e., auditory stimuli. We therefore studied the effects of S-(+)-k etamine versus ketamine-race-mate on MLAEP. Patients and methods. Inst itutional approval and informed consent were obtained for 40 patients scheduled for minor gynaecological procedures. The patients were assig ned randomly to one of the two experimental groups. All experimental e valuations were conducted under double-blind conditions. Anaesthesia w as induced with S-(+)-ketamine 1 mg/kg (group 1, n = 20) or ketamine-r acemat 2 mg/kg (group 11, n=20). MLAEP were recorded before, during, a nd after induction of general anaesthesia from the vertex (positive) a nd mastoids on both sides (negative). Auditory clicks were presented b inaurally at 70 dBnHL at a rate of 9.3 Hz. Using the electrodiagnostic system Pathfinder I (Nicolet), 1000 successive stimulus responses wer e averaged over a 100-ms poststimulus interval and analysed off-line. Latencies of the peak V, Na, Pa, Nb, P1, NI, and amplitudes Na/Pa, Pa/ Nb, and Nb/P1 were measured. V belongs to the brainstem-generated pote ntials, which demonstrates that auditory stimuli were correctly transd uced. Na, Pa, Nb, P1, and N1 are generated in the primary auditory cor tex of the temporal lobe and are the electrophysiological correlate of the primary cortical processing of the auditory stimuli. A Fast-Fouri er transformation calculated powerspectra of the AEP. Results. In the awake state, AEP peak latencies were in the normal range. MLAEP had hi gh amplitudes and a periodic wave form. Power-spectra indicated high e nergy in the 30-40-Hz frequency range. After induction of general anae sthesia with (S+)-ketamine or ketamine-racemat, there was no increase in the latencies of the peaks V, Na, Pa, Nb, P1, and N1. No decrease i n amplitudes Na/Pa, Pa/Nb, or Nb/Pl could be observed. In the power sp ectra, frequencies in the range of 30-40 Hz retained high energy. Conc lusions. MLAEP do not change in amplitude or latency during induction of general anaesthesia with S-(+)-ketamine or ketamine-race-mat. Prima ry cortical processing of auditory stimuli seems to preserved under S- (+)-ketamine and ketamine-racemat. This must be viewed in connection w ith dreams and hallucinations and could be interpreted as inadequate s uppression of auditory information processing during general anaesthes ia with S-(+)-ketamine and ketamine-race-mat.