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Created: 9/6/2005
Updated: 26/1/2009

Nitrous oxide mixed 50:50 with oxygen is a ubiquitous method of obstetric analgesia with maintenance of consciousness. It may also be used in the setting of minor procedures in the trauma department where transient analgesia is required.  It is most often administered via a demand valve for self administration. A safety feature is that the mother controls administration.

Entonox takes 30 seconds to act and continues for approximately 60 sec after inhalation has ceased.  For optimum effect inhalation should start when the contraction tightens to co-ordinate the maximal effect with the central painful part of the contraction. There are minimal cardiovascular, respiratory or neurological side effects.  It should not be used in patients with a pneumothorax.

N20 is a strong analgesic. 20% N20 is equivalent to 15 mg of subcutaneous morphine. The optimal analgesic concentration was found to be 70% but some mothers became unconscious. 50% N20 in oxygen is safer and has become standard. Entonox is the BOC trade name for this gas mixture.

Poynting effect

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.

[Poynting, John Henry (1852-1914).  English physicist, mathematician, and inventor.]

Pseudocritical temperature

The critical temperature of a gas is the maximum temperature at which compression can cause liquefaction. Mixing gases may change their critical temperature. The Poynting effect produces a 50:50 mixture which reduces the crtical temperature of N20 so Entonox has a pseudocritical temperature of -6oC.


 Highest -5.5°C 117 bar
 Cylinder -7°C 137 bar
 Pipeline -30°C 4 bar


Entonox is stored in white or blue cylinders with blue and white shoulders.  It is supplied in cylinders at a pressure of 137 bar and must be stored above its pseudocritical temperature of -6°C.  Below this temperature the N2O liquefies in a process called lamination.  If this occurs a high concentration of O2 will be delivered first with little analgesic effect, but as the cylinder empties the mixture will become progressively more potent and hypoxic as it approaches 100% N2O. If a cylinder has been exposed to cold below -6oC it should be warmed for 5 minutes in a 37oC water bath or for 2 hours in a room at 15oC. It should then be inverted three times before use.

When delivered via a pipeline at 4.1 bar the pseudocritical temperature is less than -30°C.

Contraindications to Nitrous Oxide

N2O should not be used for patients with bowel obstruction, pneumothorax, middle ear and sinus disease, and following cerebral air-contrast studies. Many anaesthetists feel that use of N2O should be restricted during the first two trimesters of pregnancy because of its effects on DNA production and the experimental and epidemiological evidence that N2O causes undesirable reproductive outcomes. Since N2O affects white blood cell production and function, it has been recommended that N2O not be administered to immunosuppressed patients or to patients requiring multiple general anaesthetics.
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