Entonox Dangers

Warning! ENTONOX NOT to be used in divers suspected of a decompression accident!!

Here is a recent correspondence that I feel should be passed on:


This topic came up during a discussion on paramedic ‘routine’ treatments where it was said by the paramedics that Entonox was a routine treatment for accident victims and that being a diver was ‘not’ a contra-indication. There was general agreement that the 50% O2 (while less good than 100% O2) was not bad there were some very strong statements made by some divers about the undesirability of the N2O. I can understand that in cases of marginal DCS one might not wish to mask symptoms with a general pain killer but there were also claims that the N2O would actively be dangerous as promoting bubble formation.

As this is potentially serious to any diver who requires post dive paramedic treatment, do you have any advice or (preferably simple) references please? I cannot currently find an authorative answer, some of the divers concerned were adamant that they were right and the paramedics freely admitted that being non-divers it was not something they knew about.


The divers were absolutely right!! Entonox is the trade name for the mixture of 50% nitrous oxide and 50% oxygen, recognised by some patients as “gas and air.” Its low fat solubility causes rapid onset of analgesia. Rapid elimination upon cessation of inhalation makes it ideal for procedural pain. Nitrous oxide is eliminated unchanged from the body, mostly by the lungs.

However — Entonox must never be used in any condition where air is trapped in the body and expansion would be dangerous. For example, it will exacerbate pneumothorax and increase pressure from any intracranial air. Air in any other cavities such as the sinuses, middle ear and gut may also expand. Problem areas are:
Head injuries with impaired consciousness.
Artificial, traumatic or spontaneous pneumothorax.
Air embolism.
decompression sickness.
Abdominal distension.
Maxillofacial injuries.

In addition, inappropriate, unwitting or deliberate inhalation of Entonox will ultimately result in unconconciousness, passing through stages of increasing light headedness and intoxication, a very dangerous thing with a diver with a decompression accident.


1.) BOC Gases. (1995). Entonox. Suggested Protocol Document BOC: Guildford.

2.) Report of the Working Party of the Commission on the Provision of Surgical Services. Pain after Surgery. London: Royal College of Surgeons of England and College of Anaesthetists, 1990.

3.) Gudmarsson, A. N. (1994) “Nitrous oxide as analgesic for painful
procedures outside the operating theatre.” British Journal of Anaesthesia, 72: Supp 1:A241:125.

4.) United Kingdom Central Council for Nursing, Midwifery and Health Visiting. (1992). The Scope of Professional Practice. UKCC: London.

5.) Acott CJ, et al. Decompression illness and nitrous oxide anaesthesia in a sports diver.
Anaesth Intensive Care. 1992 May;20(2):249-50.

6.) McIver RG, et al. Experimental decompression sickness from hyperbaric nitrous oxide anesthesia. SAM-TR-65-47. Tech Rep SAM-TR. 1965 Aug;:1-12. PMID: 5294885; UI: 66052804.

7.) Eger EI II, Saidman LJ: Hazards of nitrous oxide anesthesia in bowel obstruction and pneumothorax. Anesthesiology 26:61, 1965


Ernest S. Campbell, M.D., FACS

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