The effects of anesthesia with increasing end-expiratory concentrations of sevoflurane on midlatency auditory evoked potentials.

We studied midlatency auditory evoked potentials (MLAEP) during general anesthesia with increasing end-expiratory concentrations of sevoflurane in 12 patients scheduled for elective gynecologic surgery. After oral premedication with 20 mg clorazepate dipotassium, anesthesia was induced with etomidate (0.2 mg/kg intravenously [IV]). Vecuronium (0.1 mg/kg) was given for neuromuscular block, and controlled ventilation with sevoflurane in 100% O2 was instituted. Auditory evoked potentials were recorded in the awake state and during anesthesia with end-expiratory steady-state concentrations of 0.5, 1.0, 1.5, and 2.0 vol% of sevoflurane on vertex (positive) and mastoids on both sides (negative). Latencies of peaks V, Na, Pa, Nb, and P1 (ms) and amplitudes of Na/Pa, Pa/Nb, and Nb/P1 (microV) were measured. In the awake state, MLAEP had high peak-to-peak amplitudes and a periodic waveform. During general anesthesia with increasing end-expiratory concentrations of sevoflurane, the latency of the brainstem response V increased slightly. In contrast, MLAEP showed marked dose-dependent, statistically significant increases in the latencies of Na, Pa, Nb, and P1 and decreases in the amplitudes of Na/Pa, Pa/Nb, and Nb/P1. Under 2 vol% of sevoflurane, MLAEPs were severely attenuated or abolished. Based on these observations, > or = 1.5 vol% sevoflurane should suppress phenomena such as auditory perceptions, intraoperative wakefulness, and awareness.