EFFECTS OF INCREASING DOSES OF ALFENTANIL, FENTANYL AND MORPHINE ON MIDLATENCY AUDITORY-EVOKED POTENTIALS

We have studied dose-dependent effects of alfentanil, fentanyl and mor phine on mid-latency auditory evoked potentials (MLAEP). Anaesthesia w as induced with alfentanil 100 mug kg-1 every 5 min to a total dose of 500 mug kg-1 (group I, n = 10), fentanyl 10 mug kg-1 every 7 min to a total dose of 50 mug kg-1 (group II, n = 10) or morphine 1 mg kg-1 fo r induction and 0.5 mg kg-1 every 15 min to a total dose of 3 mg kg-1 (group III, n = 10). MLAEP were recorded before and 3-15 min after e v ery opioid dose on vertex (positive) and mastoids on both sides (negat ive). Latencies of the peaks V, Na, Pa, Nb, P1 (ms) and amplitudes Na/ Pa, Pa/b and Nb/1 (muV) were measured. Fast-Fourier transformation was used to calculate power spectra of the AEP. In the awake state, MLAEP had high peak-to-peak amplitudes and a periodic waveform. Power spect ra indicated high energy in the 30-40 Hz frequency range. During gener al anaesthesia with increasing doses of alfentanil fentanyl and morphi ne, the brainstem response V was stable. There was a marked increase o nly in latency and decrease in amplitude of P1 In contrast, for the ea rly cortical potentials Na and Pa, only small increases in latencies a nd decreases in amplitudes were observed. After the largest doses of a lfentanil (500 mug kg-1), fentanyl (50 mug kg-1) and morphine (3 mg kg -1), Na, Pa and Nb showed a similar pattern as in awake patients. In t he power spectra, high energy persisted in the 30-Hz frequency range. There were no dose-dependent effects of opioids on MLAEP and no differ ences between alfentanil, fentanyl, and morphine could be found.