Friedman Y, King B S, Rampil I J
Department of Anesthesia, University of California, San Francisco 94143-0648, USA.
Anesthesiology. 1996 Jul;85(1):135-41. doi: 10.1097/00000542-199607000-00019.
Evoked, recurrent electromyographic activity (F waves) reflect alpha-motor neuron excitability. Based on observations that other inhaled anesthetics do so, we hypothesized that nitrous oxide, alone or in combination with isoflurane, would depress F-wave activity and correlate with depression of movement response to tail clamp or electric stimulation.
In study 1, the authors examined the effect of nitrous oxide in combination with isoflurane in 13 normocapnic Sprague-Dawley rats anesthetized with 1.0% isoflurane (0.7 minimum alveolar concentration) in oxygen. The tibial nerve was stimulated at the popliteal fossa, and evoked electromyographic activity [M (direct neuromuscular junctional response) and F waves] were recorded from ipsilateral foot muscles. The effect of the addition of 30% or 70% nitrous oxide was measured. F-wave amplitude/M-wave amplitude ratio (F/M) was determined from each stimulus-electromyographic response pair. F/M vs. movement response to 60-s tail clamp was assessed after each recording session. F-wave amplitude/M-wave amplitude ratio at adjacent doses that permitted and prevented movement were compared. In study 2, the authors examined the effect of (hyperbaric) nitrous oxide as the sole anesthetic agent on F waves. In 11 rats anesthetized with isoflurane, stimulation and recording electrodes were placed as described above, with additional electrodes for stimulation placed in the tail. Rats were placed in a pressure chamber pressurized with nitrous oxide/oxygen to 3.4 atm. Thirty m were allowed for isoflurane washout. Electromyographic activity was evoked and recorded at 1.0, 1.6, 2.2 and 2.7 atm N2O (random order). Movement in response to 60 s of 15 V, 50-Hz tail stimulation was evaluated after each recording session.
Nitrous oxide with or without isoflurane produced a dose-dependent decrease in F/M. By interpolation of this data, the authors found that 2 atm N2O alone, or 44% N2O added to 1.0% isoflurane at 1.0 atm, produced 1.0 minimum alveolar concentration anesthesia. At the deepest level of isoflurane/ nitrous oxide that permitted movement, mean F/M was 20.6 +/- 17.5%; at the lowest concentration that blocked movement, rats had a mean F/M of 13.7 +/- 13.9% (P = 0.01). At the minimal hyperbaric nitrous oxide blocking movement, rats had a mean F/M of 3.7 +/- 2.9%, whereas the F/M at the highest nitrous oxide dose that permitted movement was 4.4 +/- 2.7% (P < 0.04).
Because nitrous oxide depressed F-wave but not M-wave activity, the data suggest a central (spinal) rather than neuromuscular junctional site of action of this agent. The direct correlation between nitrous oxide dose, F-wave amplitude depression, and surgical immobility suggests the possibility of using F-wave activity to predict the likelihood of anesthetic-induced immobility. However, the mechanism of action of nitrous oxide may differ from that of the potent inhaled agents.
诱发的、反复出现的肌电图活动（F波）反映α运动神经元的兴奋性。基于其他吸入麻醉药有此作用的观察结果，我们推测氧化亚氮单独使用或与异氟烷联合使用时，会抑制F波活动，并与对夹尾或电刺激的运动反应抑制相关。
在研究1中，作者检测了氧化亚氮与异氟烷联合使用对13只正常碳酸血症的Sprague-Dawley大鼠的影响，这些大鼠用1.0%异氟烷（0.7最低肺泡浓度）在氧气中麻醉。在腘窝处刺激胫神经，并从同侧足部肌肉记录诱发的肌电图活动[M（直接神经肌肉接头反应）和F波]。测量添加30%或70%氧化亚氮的效果。从每个刺激-肌电图反应对中确定F波振幅/M波振幅比（F/M）。在每次记录后评估F/M与对60秒夹尾的运动反应。比较允许和阻止运动的相邻剂量下的F波振幅/M波振幅比。在研究2中，作者检测了（高压）氧化亚氮作为单一麻醉剂对F波的影响。在11只用异氟烷麻醉的大鼠中，按照上述方法放置刺激和记录电极，另外在尾部放置刺激电极。将大鼠置于用氧化亚氮/氧气加压至3.4个大气压的压力舱中。留出30分钟用于异氟烷洗脱。在1.0、1.6、2.2和2.7个大气压的氧化亚氮（随机顺序）下诱发并记录肌电图活动。在每次记录后评估对15伏、50赫兹尾部刺激60秒的运动反应。
无论有无异氟烷，氧化亚氮均使F/M呈剂量依赖性降低。通过对这些数据进行内插，作者发现单独使用2个大气压的氧化亚氮，或在1.0个大气压下向1.0%异氟烷中添加44%的氧化亚氮，可产生1.0最低肺泡浓度麻醉。在异氟烷/氧化亚氮允许运动的最深水平时，平均F/M为20.6±17.5%；在阻止运动的最低浓度时，大鼠的平均F/M为13.7±13.9%（P = 0.01）。在最低高压氧化亚氮阻止运动时，大鼠的平均F/M为3.7±2.9%，而在允许运动的最高氧化亚氮剂量时F/M为4.4±2.7%（P < 0.04）。
由于氧化亚氮抑制F波活动而非M波活动，数据表明该药物的作用部位是中枢（脊髓）而非神经肌肉接头。氧化亚氮剂量、F波振幅降低与手术制动之间的直接相关性表明，有可能利用F波活动来预测麻醉诱导制动的可能性。然而，氧化亚氮的作用机制可能与强效吸入麻醉药不同。
