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Inhaled analgesia

Last reviewed dd mmm yyyy. Last edited dd mmm yyyy

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Mixtures of nitrous oxide and 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. "Entonox" is a 50:50 mixture of the two gases.

Entonox acts on the central nervous system resulting initially in analgesia and later in anaesthesia.

Entonox takes 30 sec to act and continues for approximately 60 sec after inhalation has ceased.

The advantages of Entonox are:

  • the absence of respiratory depression in the foetus
  • that uterine action is unaffected
  • its rapid clearance from the body, allowing its intermittent use over several hours
  • its suitability for patients with pulmonary or cardiac pathology
  • its simplicity
  • that it is patient controlled
  • its low cost

Disadvantages include:

  • exhaustion - the woman has to take deep breaths to obtain full analgesia
  • its unacceptability to some mothers - the drunken feeling may be disturbing
  • its limited efficacy
  • the need for complicated equipment which must be regularly checked

In summary, Entonox is very useful for short periods of pain relief.

NICE state (1) that ".. Entonox (a 50:50 mixture of oxygen and nitrous oxide) should be available in all birth settings as it may reduce pain in labour, but women should be informed that it may make them feel nauseous and light-headed."


  1. NICE (September 2007).Intrapartum care.

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