Plourde G, Villemure C
Department of Anaesthesia, McGill University, Montreal, Quebec, Canada.
Anesth Analg. 1996 Jan;82(1):75-83. doi: 10.1097/00000539-199601000-00013.
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 AM-LRs 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-II women undergoing reduction mammoplasty were tested. Anesthesia was induced with fentanyl (3 micrograms/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 microgram/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 microV) 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.
听觉中潜伏期反应（AMLR）是在以10次/秒或更低频率呈现听觉刺激开始后12 - 50毫秒出现的一系列正负波。当刺激呈现频率增加到约40次/秒时，AMLR相互重叠会形成一个持续的、近乎正弦波的波形，称为“40赫兹听觉稳态反应”（40-Hz ASSR）。AMLR和40-Hz ASSR已被用于研究全身麻醉药对大脑的影响。本研究的主要目的是确定全身麻醉药恩氟烷对40-Hz ASSR的影响是否可以从其对AMLR的影响中预测出来。次要目的是研究麻醉苏醒过程中意识水平与40-Hz ASSR之间的关系。对12名接受乳房缩小成形术的美国麻醉医师协会（ASA）I-II级女性患者进行了测试。静脉注射芬太尼（3微克/千克）和硫喷妥钠（3 - 5毫克/千克）诱导麻醉，并用恩氟烷（呼气末浓度为0.5%、0.8%或1.1%；每个浓度4例患者；随机分配）在氧化亚氮（呼气末浓度66%）中维持麻醉，必要时追加芬太尼（1微克/千克）。在诱导前、麻醉和手术期间记录40-Hz ASSR和AMLR。在苏醒期间也记录40-Hz ASSR。在麻醉和手术期间，40-Hz ASSR的振幅显著降低（P < 0.001）。这种衰减不是剂量依赖性的，并且比恩氟烷对AMLR的影响所预测的要明显得多。在两名患者中，即在1.1%恩氟烷组的两名患者中，麻醉期间40-Hz ASSR出人意料地大（0.09和0.11微伏）。恢复听从言语指令的能力与40-Hz ASSR振幅显著增加（P < 0.001）相关。我们得出结论，虽然在恩氟烷 - 氧化亚氮麻醉期间听觉神经元仍能够以较慢的刺激频率做出反应，但其以较快刺激频率被驱动的能力明显受限。40-Hz ASSR可能有助于检测术中无意的知晓，因为意识恢复与振幅明显增加相关。
