March 18, 2004

Entonox (nitrous oxide) use in labour

I have just obtained the information on Entonox supplied by its manufacturer (BOC Medical). It makes very interesting reading. Following details about its product name, dosage and method of administration, contraindications, interaction with other medications and its effects on the ability to drive or operate machinery, it lists the following:

Undesirable effects
The nitrous oxide constituent of Entonox causes inactivation of vitamin B12, which is a co-factor of methionine synthase. Folate metabolism is consequently interfered with and DNA synthesis is impaired following prolonged nitrous oxide administration. These disturbances result in megablastic bone marrow changes. Exceptionally heavy occupational exposure and addiction have resulted in myeloneuropathy and subacute combined degeneration. Theoretically similar adverse results could occur from heavy and prolonged Entonox exposure. All these effects are well documented, are extremely rare and may follow prolonged exposure to levels of nitrous oxide over 5000ppm or to frequent (more than once every 2 days) exposure to analgesic concentrations. It has been suggested that prolonged occupational exposure to high levels of nitrous oxide may affects a woman’s ability to become pregnant.
Use in pregnancy and lactation
Mild skeletal teratogenic changes have been observed in pregnant rat embryos when the dam has been exposed to high concentrations of nitrous oxide during the period of organogenesis. However no increased incidence of fetal malformation has been discovered in 8 epidemiological studies and case reports in human beings. There is no published material which shows that nitrous oxide is toxic to the human fetus. Therefore there is no absolute contra-indication to its use in the first 16 weeks of pregnancy.
Special warnings and precautions for use:
Administration of Entonox more frequently that every 4 days should be accompanied by routine blood cell counts for evidence of megoblastic change in red cells and hypersegmentation of neutrophils. Thorough ventilation or scavenging of waste gases should reduce operating theatre and equivalent treatment room levels of ambient nitrous oxide to a level below 200ppm. Entonox is non flammable but strongly supports combustion. It is highly dangerous when in contact with oils, greases, tarry substances and many plastics.

These comments have many implications for midwives, who regularly, even routinely, offer women Entonox during labour and birth. Apart from the potential impact on the labouring woman and her baby, for midwives there are risks as well, and it is clear that pregnant midwives should be considering their exposure to Entonox carefully. Scavenger units are being installed in some maternity wards, but most have no such protection for their staff. The recommendation for regular blood testing is also noteworthy.

The final comment about the danger of contact with oils, greases and plastics is also of concern. These dangers are not spelled out in these notes. Many women use massage oils during labour and plastics of various kinds can be found in labour rooms. I will try and discover the nature of these “dangers” and report further if I can.

The statement that there is no published material regarding the risks to the unborn baby and the conclusion that therefore there is no absolute contraindication to its use during labour, once again demonstrates the assumption that no evidence means no problems. Until research is undertaken to test this hypothesis, it may be wise to err on the side of caution.

Entonox is being sold rather heavily at the moment, it would seem, from the advertising being undertaken. It is very freely used in UK hospitals and elsewhere, including Australia. Perhaps it is time to reconsider its use in maternity care, where frequent and prolonged exposure of the mother, midwife and the un born baby is a common occurrence.

Posted by andrea at March 18, 2004 10:39 PM