Willis B A, Rosen M
Anaesthesia. 1977 Jun;32(6):573-6. doi: 10.1111/j.1365-2044.1977.tb10008.x.
Mothers in labour using a self-administered 50% N2O and 50% O2 (Entonox) demand system, experience a time lag between making a demand at the onset of a contraction and pain relief. A practical method is described of reducing this time lag by supplementing the intermittent supply from the demand valve system with a continuous nasal flow of N2O and O2. Consequently a basal arterial N2O tension is maintained and when a demand is made, a higher level of N2O is attained in any time interval compared with that using the demand system alone. In six trained volunteers under laboratory conditions, the N2O levels measured at the mouth on a breath-by-breath basis using a mass spectrometer, showed a substantial and statistically significant reduction in the time taken to attain end-tidal N2O concentrations in the range 10-25%. This method seems worthy of a clinical trial.
正在分娩的产妇使用自行给药的50%氧化亚氮和50%氧气（恩托诺克斯）按需供气系统时，在宫缩开始时提出需求与疼痛缓解之间存在时间延迟。本文描述了一种实用方法，即通过持续经鼻给予氧化亚氮和氧气来补充按需阀系统的间歇性供气，从而减少这种时间延迟。因此可维持基础动脉氧化亚氮张力，且在提出需求时，与仅使用按需系统相比，在任何时间间隔内都能达到更高水平的氧化亚氮。在实验室条件下，对六名经过训练的志愿者进行测试，使用质谱仪逐次呼吸测量口腔处的氧化亚氮水平，结果显示，达到呼气末氧化亚氮浓度在10%-25%范围内所需的时间大幅缩短，且具有统计学意义。该方法似乎值得进行临床试验。
