a mixture of oxygen and nitrous oxide (= a gas with a sweet smell), that can be breathed in to reduce pain, for example during childbirth or medical examinations

Hospitals have gas and air, which is inhaled and can be used at any time. 医院有麻醉混合气，可以随时吸入使用。

Gas and air can be used from the onset of your labour. 麻醉混合气可以从分娩一开始就使用。