Comparison of the effects of enflurane/N2O on the 40-Hz auditory steady-state response versus the auditory middle-latency response

Author(s): Villemure, C.

Journal/Book: Anesth Analg. 1996; 82: 351 West Camden St, Baltimore, MD 21201-2436. Williams & Wilkins. 75-83.

Abstract: The auditory middle-latency response (AMLR) is a sequence of negative-positive waves occurring 12-50 ms after the onset of an auditory stimulus presented at rates of 10/s or less. When the rate of stimulus presentation is increased to approximately 40/s, overlapping of the AMLRs results in a sustained, nearly sinusoidal wave, called the ''40-Hz auditory steady-state response'' (40-Hz ASSR). The AMLR and 40-Hz ASSR have been used to study the effects of general anesthetics on the brain. The primary aim of this investigation was to determine whether the effects of a general anesthetic, namely enflurane, on the 40-Hz ASSR can be predicted from its effects on the AMLR. A secondary aim was to examine the relationship between the level of consciousness and the 40-Hz ASSR during emergence from anesthesia. Twelve ASA class I-IT women undergoing reduction mammoplasty were tested. Anesthesia was induced with fentanyl (3 mu g/kg) and thiopental (3-5 mg/kg) intravenously and was maintained with enflurane (0.5%, 0.8%, or 1.1% end-tidal; four patients per concentration; random assignment) in N2O (66% end-tidal), along with fentanyl (1 mu g/kg as needed). The 40-Hz ASSR and AMLR were recorded before induction and during anesthesia and surgery. The 40-Hz ASSR was also recorded during emergence. The amplitude of the 40-Hz ASSR was reduced profoundly during anesthesia and surgery (P < 0.001). The attenuation was not dose-dependent, and was much more pronounced than predicted by the effects of enflurane on the AMLR. The 40-Hz ASSR during anesthesia was surprisingly large (0.09 and 0.11 mu V) in two patients, both of the 1.1% enflurane group. The regaining of the ability to follow verbal commands was associated with a significant (P < 0.001) increase in the amplitude of the 40-Hz ASSR. We conclude that, although auditory neurons remain capable of responding at a slow stimulus rate during enflurane-N2O anesthesia, their ability to be driven at a faster stimulus rate is markedly curtailed. The 40-Hz ASSR may be useful for detecting unintentional intraoperative awareness because the return of consciousness is associated with a clear increase in amplitude.

Note: Article G Plourde, Royal Victoria Hosp, Dept Anaesthesia, 687 Pine Ave W, Suite S5-05, Montreal, Pq H3A 1A1, Canada

Keyword(s): EVOKED-POTENTIALS; SPECTRAL EDGE; ANESTHESIA; ISOFLURANE; FREQUENCY; HUMANS; DEPTH; POWER; RATIO

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