Entonox, referred to colloquially as gas and air, is an analgesic used to ease pain during childbirth and in other situations when other analgesics would not be appropriate. It is a mixture of fifty percent nitrous oxide (N2O or laughing gas) and fifty percent oxygen. It is administered by having the patient inhale it. It is very fast-acting, typically taking effect between twenty seconds to one minute after inhalation.
It should be stored away from sources of heat or flame, and should under no circumstances come into contact with oil or grease due to the 50% oxygen content of Entonox and the potential for combustion. Similarly, no-one should smoke in the vicinity of an Entonox cylinder.
The nitrous oxide constituent of Entonox can cause inactivation of vitamin B12 which is a co-factor of methionine synthase. Folate metabolism is consequently interfered with and DNA synthesis is impaired following prolonged nitrous oxide administration.Prolonged or frequent use may result in megaloblastic bone marrow changes.
Exceptionally heavy occupational exposure and addiction to nitrous oxide have resulted in myeloneuropathy and subacute combined degeneration. Theoretically similar adverse results could occur from heavy and prolonged Entonox exposure.
All these effects are well documented, extremely rare and may follow prolonged exposure to levels of nitrous oxide over 5000ppm or to frequent (more than once every two days) exposure to analgesic concentrations. It has been suggested that prolonged exposure to high levels of nitrous oxide may affect a woman's ability to become pregnant, may result in bowel distension, middle ear damage and rupture of ear drums. Addiction to Entonox has been reported. For a full toxicological review please visit the European Industrial Gases Association website.
Occupational exposure standard (OES)
There is no OES for medical oxygen in the U.K. The OES for medical nitrous oxide is proposed as follows: Long-term exposure limit (8 hour TWA reference period) -100 ppm (ref. HSE EH40).
Entonox may in some circumstances affect the health and safety of midwives at work, please see RCM guidance notes on how it can be used and stored.