Effects of increasing doses of alfentanil, fentanyl and morphine on mid-latency auditory evoked potentials.

We have studied dose-dependent effects of alfentanil, fentanyl and morphine on mid-latency auditory evoked potentials (MLAEP). Anaesthesia was induced with alfentanil 100 micrograms kg-1 every 5 min to a total dose of 500 micrograms kg-1 (group I, n = 10), fentanyl 10 micrograms kg-1 every 7 min to a total dose of 50 micrograms kg-1 (group II, n = 10) or morphine 1 mg kg-1 for 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 every opioid dose on vertex (positive) and mastoids on both sides (negative). Latencies of the peaks V, Na, Pa, Nb, P1 (ms) and amplitudes Na/Pa, Pa/Nb and Nb/P1 (microV) 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 spectra indicated high energy in the 30-40 Hz frequency range. During general anaesthesia with increasing doses of alfentanil, fentanyl and morphine, the brainstem response V was stable. There was a marked increase only in latency and decrease in amplitude of P1. In contrast, for the early cortical potentials Na and Pa, only small increases in latencies and decreases in amplitudes were observed. After the largest doses of alfentanil (500 micrograms kg-1), fentanyl (50 micrograms kg-1) and morphine (3 mg kg-1), Na, Pa and Nb showed a similar pattern as in awake patients.(ABSTRACT TRUNCATED AT 250 WORDS)