The 40-Hz Auditory Steady State Response for Monitoring Level of Consciousness: Methodological Considerations

Abstract

Delivering auditory stimuli at a rate of approximately 40 per s produces an electrical cerebral response which can be recorded from the scalp. This response, called the 40-Hz auditory steady state response (40-Hz ASSR), consists of a sinusoidal waveform that has the same frequency as that of stimulus delivery [7,11,16,22] (Fig. 1). Recent evidence suggests that the 40-Hz ASSR may offer a way to monitor the level of consciousness during anesthesia. The amplitude of the 40-Hz ASSR was reduced to noise level during anesthesia with isoflurane 1% end-tidal in oxygen [15]. The amplitude remained maximally reduced after decreasing the concentration of the isoflurane to 0.5% end-tidal in oxygen and allowing 10 min for equilibration [15]. The amplitude of the 40-Hz ASSR was also maximally reduced by enflurane 0.5%, 0.8%, or 1.1% and 60% N₂0 (end-tidal concentrations) [25]. The return of the ability to open the eyes on command after termination of the anesthetic seemed reliably associated with a clear, stepwise increase in the amplitude of the 40-Hz ASSR, which until then had remained markedly reduced [15,25]. This suggests that the profound attenuation of the 40-Hz ASSR by enflurane-N₂0 or isoflurane may reflect unconsciousness (unresponsiveness to verbal command). Additional evidence that the attenuation of the 40-Hz ASSR reflects unconsciousness was obtained during induction of anesthesia with thiopental [15], propofol (unpublished observations), or sufentanil [14]. With these agents, profound attenuation of the 40-Hz ASSR and unresponsivenes to verbal command occurred at the same time.