Nitrous oxide and oxygen

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"Gas and air" redirects here. For the 1923 film, see Gas and Air (film).
Entonox CD cylinder and giving set

A mix of nitrous oxide 50% and oxygen 50% is a medical analgesic gas, commonly known as Entonox (a registered trademark of BOC) or Nitronox,[1] or colloquially as "gas and air",[2] and is frequently used in pre-hospital care, childbirth and emergency medicine situations by medical professionals such as doctors, nurses, midwives and paramedics.[3]

Medical uses[edit]

Nitrous oxide (N2O) is itself active[clarification needed] (does not require any changes in the body to become active), and so has an onset in roughly the lungbrain circulation time. This gives it a peak action 30 seconds after the start of administration;[3] Entonox should thus be used accordingly, i.e. inhalation should start 30 seconds before a contraction becomes painful in labour. It is removed from the body unchanged via the lungs, and does not accumulate under normal conditions, explaining the rapid offset of around 60 seconds.[3]

Nitrous oxide is more soluble than oxygen and nitrogen, so will tend to diffuse into any air spaces within the body. This makes it dangerous to use in patients with pneumothorax or those who have recently been scuba diving, and there are cautions over its use with any bowel obstruction.

Its analgesic effect is strong (equivalent to 15 mg of subcutaneous route morphine[3]) and characterised by rapid onset and offset, i.e. it is very fast-acting and wears off very quickly.


N2O should not be used in patients with bowel obstruction, pneumothorax, middle ear or sinus disease,[3] and should also not be used on any patient who has been scuba diving within the preceding 24 hours[4] or in violently disturbed psychiatric patients.[1] There are also clinical cautions in place for the first two trimesters of pregnancy and in patients with decreased levels of consciousness.[3]


Distinct blue and white cap of an Entonox cylinder
Typical Schrader valve attachment, making the gas usable only with demand based giving sets

The gas is a mixture of half nitrous oxide (N2O or laughing gas) and half oxygen (O2).[1][3] The ability to combine N2O (nitrous oxide is the common name; dinitrogen monoxide, systematic name) and oxygen at high pressure while remaining in the gaseous form is caused by the Poynting effect (after John Henry Poynting, an English physicist).[3]

The Poynting effect involves the dissolution of gaseous O2 when bubbled through liquid N2O, with vaporisation of the liquid to form a gaseous O2/N2O mixture.[3]

Inhalation of pure N2O over a continued period would deprive the patient of oxygen,[5] but the 50% oxygen content prevents this from occurring. The two gases will separate at low temperatures (<4 °C), which would permit administration of hypoxic mixtures. Therefore, it is not given from a cold cylinder without being shaken (usually by cylinder inversion) to remix the gases.


The gas is self-administered through a demand valve, using a mouthpiece, bite block or face mask.[1] Self-administration of Entonox is safe because if enough is inhaled to start to induce anaesthesia, the patient becomes unable to hold the valve, and so will drop it and soon exhale the residual gas. This means that unlike other anaesthetic gases, it does not require the presence of an anaesthetist for administration. The 50% oxygen in Entonox ensures the patient will have sufficient oxygen in their system for a short period of apnea (or apnoea) to be safe.


Pure N2O was first used as a medical analgesic in 1844 by American dentist Horace Wells, and grew in popularity in the late 1800s.[6][7] Negative side effects were found from the hypoxia caused by a lack of oxygen, and the requirement to have at least 21% oxygen content in the gas was discovered (the same percentage as in air).[7]

In 1911, the anaesthetist Arthur Ernest Guedel first described the use of self-administration of a nitrous oxide and oxygen mix.

It was not until 1961 that the first paper was published by Michael Tunstall and others, describing the administration of a pre-mixed 50:50 nitrous oxide and oxygen mix, which led to the commercialisation of the product.[7]


  1. ^ a b c d Fisher, J; Brown, S; Cooke, M (2006). UK Ambulance Service Clinical Practice Guidelines (PDF). Joint Royal Colleges Ambulance Liaison Committee. ISBN 1-84690-060-3. 
  2. ^ "Entonox (gas and air)". Baby Centre. 
  3. ^ a b c d e f g h i "Entonox". Anaesthesia UK. Retrieved 2009-04-10. 
  4. ^ Komesaroff, D (1998). "Oxygen administration in diving accidents.". South Pacific Underwater Medicine Society Journal 28 (3 Supplement). Retrieved 2011-03-29. 
  5. ^ "Breathing Nitrous Oxide". Nitrous Oxide Supplies. 
  6. ^ Erving HW (1933). "The Discoverer of Anæsthesia: Dr. Horace Wells of Hartford". Yale Journal of Biology and Medicine, May 1933; v.5, n.5, p.421–430. 
  7. ^ a b c "History of Entonox". BOC Gases. 

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