I passed your question along to Mike Civitello at Porter Instruments (makers of N2O equipment) and he replied
Many other countries use a premixed gas with demand valve. One cylinder has both N2O and O2. This is not FDA approved. The safety concern is that at varying temperatures the two gases will separate - and you have no way to verify what percentage mixture the patient is receiving. That leaves us with connecting each gas to a device that will mix/blend them at a 50/50 ratio - and can be administered through a demand valve. This type of device was not available until January of this year. Similar devices existed years ago but had gone out of production with the development of other systems that are more commonly used for nitrous oxide and oxygen.
Although N2O and O2 have been widely used in the US in dental offices and other hospital departments - they use a different type of system that is not appropriate for labor applications. This was one of the main obstacles of why N2O for labor was not more readily available than it is now. This is not an obstacle anymore - but it will take some time for facilities to make it available.
