Nitrous Oxide (N20) is otherwise known as laughing gas. It is an anaesthetic gas that has been used since the beginning of the last century. It is commonly used during surgery and as a pain relief method during labour. A lot of people will have come in contact with Nitrous Oxide at their dentist. Nitrous Oxide is mixed with oxygen. The concentration of Nitrous Oxide that a woman in labour receives varies between 30 to 70%. Nitrous Oxide is breathed into the lungs and there it very quickly enters the blood stream. Nitrous Oxide reaches the brain within 15 seconds. The amount of Nitrous Oxide that reaches the brain varies, depending on the strength administered and how long it has been used. The normal function of the brain is depressed to varying degrees. However the way Nitrous Oxide provides pain relief is unknown.

The effectiveness of Nitrous Oxide as pain relief in labour varies from woman to woman. Less than 50 per cent of women find it a satisfactory form of pain relief, 20 per cent obtain some pain relief for some of the time and around 30 per cent find it totally ineffective. There have been reports regarding the declining use of Nitrous Oxide as a pain relief in labour in recent years, due to its inability to provide a reliable form of pain relief.

How is Nitrous Oxide and oxygen administered?

Nitrous Oxide and oxygen can be given from either a fixed or portable apparatus. The fixed apparatus is attached to the wall and varying concentrations of Nitrous Oxide and oxygen can be given. The midwife adjusts the dosage to what is required. However as it cannot be moved, a woman in labour cannot use Nitrous Oxide in the shower, bath or at a homebirth. Nitrous Oxide and oxygen can also be given from a portable cylinder. The dosage is fixed (a 50:50 mixture of Nitrous Oxide and oxygen is in the cylinder – this known as Entonox). With Entonox it is not possible to alter the concentration of Nitrous Oxide that a woman receives.

Nitrous Oxide is breathed in by the woman either using a face mask or mouth piece. To use it correctly the woman begins breathing on the mouth piece or face mask deeply at the beginning of a contraction until a rattling sound is heard. The Nitrous Oxide and oxygen mixture is then breathed in throughout the whole contraction and is stopped as soon as the contraction is over. Nitrous Oxide can be given at any time during labour, but is most likely administered during transition.

How long does the effects of Nitrous Oxide last?

Once Nitrous Oxide enters the blood stream, it is very quickly distributed around the body. It passes just as quickly across the placenta and into the baby’s circulation. Nitrous Oxide (unlike drugs like pethidine) is not broken down by the liver and therefore does not leave by-products. Nitrous Oxide is able to be eliminated quickly from the woman’s body entirely by her lungs.  Also, the Nitrous Oxide is eliminated quickly from the baby’s body. When using Nitrous Oxide it is important to start using it as soon as the contraction starts due to the 15 seconds it takes to become effective. Once the woman stops breathing the Nitrous Oxide and oxygen mixture and begins to breath air again, the effects of  the Nitrous Oxide wears off quickly.

What are the advantages of using Nitrous Oxide during labour?

The advantages of using Nitrous Oxide during labour is that it provides some pain relief for some women. Women using Nitrous Oxide remain awake and in control of their own pain relief. Nitrous Oxide does not interfere with contractions, the time Nitrous Oxide remains effective is short and there are no obvious short term effects on mother and baby that have been noted to date.

What are the disadvantages of using Nitrous Oxide during labour?

Disadvantages of using Nitrous Oxide during labour include:

• Nausea and/or vomiting
• Not a reliable form of pain relief
• Women can become drowsy, confused or disorientated. Some women experience these feelings as quite unpleasant
• The face mask can make some women feel claustrophobic

Whether there are any long term effects of Nitrous Oxide on babies is difficult to answer. I could find only one study on this question. In a well designed case control study conducted in Stockholm by Jacobson et al. (1990) they found that when Nitrous Oxide was given to a woman during labour, that child (in later life) was five and a half times more likely to have an amphetamine addiction than a brother or sister who did not receive Nitrous Oxide via their mother during labour.

Reference List

• Enkin, M., Keirse, M. J. N. C., Renfrew, M., & Neilson, J. (1995). A guide to     effective care in pregnancy and childbirth (2nd ed.). Oxford: Oxford     University Press.
• Jacobson, B. et al. (1990) Opiate addiction in adult offspring through possible imprinting after obstetric treatment. British Medical Journal, 301: 1067-1070.
• Robertson, A. (1999). Preparing for birth: Mothers. Sydney: ACE Graphics.
• www.manbit.com/obstetpain/n201.htm (Website no longer active)

(25th September 2000)