Effects of sevoflurane on the middle latency auditory evoked response and the electroencephalographic power spectrum.

We investigated the effects of sevoflurane on the middle latency auditory evoked response (MLR) and the power spectrum of the electroencephalogram (EEG) in 10 elective surgical patients. The MLR and the EEG power spectrum were recorded with a surface electrode placed at the central (Cz) scalp location. End-tidal sevoflurane concentrations of 0%, 0.25%, 0.5%, 0.75%, 1.0%, and 1.5% in 50% nitrous oxide and oxygen were studied. The Na, Pa, and Nb components of the MLR increased in latency and decreased in amplitude in a dose-dependent manner at increasing concentrations of sevoflurane. The latencies for Na, Pa, and Nb increased in a linear fashion (correlation coefficients: r = 0.81, r = 0.81, and r = 0.89, respectively). The EEG delta power was dominant with increasing sevoflurane concentration, and was significantly increased at sevoflurane concentrations of 1.0%-1.5%. The beta power, median power frequency (MPF), and 95% spectral edge frequency (SEF) decreased significantly according to the increases by 0.5% sevoflurane. Regarding the changes evoked by 0.25% sevoflurane, the Nb latency of the MLR responded significantly under 0.75% of sevoflurane. At these low concentrations of sevoflurane, the MLR seemed to be more sensitive to changes in anesthetic concentration than the various EEG variables.