iBRAIN and Nitrous Oxide (Enotonox)

by Asheya on March 5, 2013

By Lisa Baker

In our February 19 blog post, we discussed the elements of informed decision making. Today we will use the iBRAIN framework to analyze inhaled nitrous oxide (Entonox) as a form of pain management in labour.


A popular option for pain management in Canadian hospitals is nitrous oxide (NO). This colorless, sweet smelling gas is inhaled as a mixture of 50% NO and 50% oxygen. If you request this mixture, commonly referred to by its trade name Entonox or laughing gas, you will be given a mask that covers your nose and mouth and will be instructed to breath deeply through the mask when you wish to receive pain relief. As you inhale you will receive the gas mixture and the analgesic (pain reducing) effect will be noticed within 30-60 seconds. Because of the lag time between first breath and pain relief, you should begin to inhale the gas as soon as you sense a contraction begin. In this way, you will find relief from the peak of the contraction. Nitrous oxide is very quickly broken down and expelled from the body. Once the mask is removed, the majority of the gas will be exhaled from the body within 3 or 4 breaths.  Most mothers breathe through the mask for the duration of a contraction and remove the mask in between contractions.

It is believed nitrous oxide works by triggering a release of endorphins (natural pain-killers) within the midbrain. Although this does not completely block pain, it lowers the perception of pain. Some have described the effect of nitrous oxide as ‘taking the edge off the peak of contractions’ or ‘numbing the sensation of the pain’.

Entonox is available in many Canadian hospitals and birthing centers and can be easily administered any time during labour.  It is important that the gas is self-administered. As a mother begins to feel the effects of the NO she will naturally relax her hand and the mask will fall away from her mouth and nose. This will prevent an overdose and loss of consciousness. If a partner or other individual holds the mask, the risk of inhaling too much NO increases.



  • When compared to no nitrous oxide (NO), mothers inhaling a 50% NO mixture during labour contractions reported feeling less intense pain.
  • NO is self-administered and can be stopped or started at any time in labour.
  • NO is quickly expelled from maternal and fetal blood.
  • The use of NO does not increase the rate of assisted birth (vacuum or forceps) or cesarean birth.
  • The use of NO does not affect APGAR scores of the newborn.



  • When compared to no nitrous oxide (NO), mothers inhaling a 50% NO mixture during labour contractions reported more nausea, vomiting, dizziness, and drowsiness. These side-effects increase the longer NO is used.
  • NO does not completely block pain sensations.
  • Some mothers initially dislike the feeling of breathing through a mask. This feeling usually goes away after a couple of breaths.


Note that each of the following alternatives has a set of risks and benefits. Some of these alternatives will be reviewed in future posts.

Non-medicinal forms of pain management:

Medicinal forms of pain management:

  • Epidural analgesia
  • Narcotics (fentanyl, morphine)
  • Local anesthetic blocks of vagina and perineum area (used when an episiotomy, vacuum, or forceps are used during the pushing and birth stage)


Women who choose not to use Entonox will experience less nausea, vomiting, and dizziness and more pain.



(1)  Jones L, Othman M, Dowswell T, Alfirevic Z, Gates S, Newburn M, Jordan S, Lavender T, Neilson JP. Pain management for women in labour: an overview of systematic reviews. Cochrane Database of Systematic Reviews 2012, Issue 3. Art. No.: CD009234. DOI: 10.1002/14651858.CD009234.pub2.

(2)  Klomp T, van Poppel M, Jones L, Lazet J, Di Nisio M, Lagro-Janssen ALM. Inhaled analgesia for pain management in labour. Cochrane Database of Systematic Reviews 2012, Issue 9. Art. No.: CD009351. DOI: 10.1002/14651858.CD009351.pub2.

(3)  Likis FE, Andrews JC, Collins MR, et al. Nitrous Oxide for the Management of Labor Pain [Internet]. Rockville (MD): Agency for Healthcare Research and Quality (US); 2012 Aug. (Comparative Effectiveness Reviews, No. 67.) Available from: http://www.ncbi.nlm.nih.gov/books/NBK100802/

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