Medicines Q&As
Q&A 304.2
What effect does low temperature have on cylinders containing a
mixture of 50% nitrous oxide and 50% oxygen?
Prepared by UK Medicines Information (UKMi) pharmacists for NHS healthcare professionals
Before using this Q&A, read the disclaimer at www.ukmi.nhs.uk/activities/medicinesQAs/default.asp
Date prepared: 23rd February 2012
Summary
50% nitrous oxide (N2O) / 50% oxygen (O2) is a medical gas mixture which is used as a fast acting
analgesic. It is commonly used for pain relief during normal labour, but is also used, for example,
when changing painful dressings or for short-term relief during dental procedures (1). It is available to
the NHS as Entonox® and Equanox®.
This particular gas mixture is available as a compressed gas, in cylinders. These cylinders should be
stored under cover, preferably inside, in a clean and dry environment and must not be subjected to
extremes of temperature (1,2).
If the cylinder temperature is allowed to fall below minus 6ºC, there is a risk that the two gases in the
mixture will start to separate (2). To prevent this from happening, the cylinders should be protected
from the cold (3) and stored above 10ºC for at least 24 hours before use (1,2).
If cylinders are allowed to get too cold, they should not be used for administration to the patient until
the gases have been allowed to remix properly by bringing the cylinder back to a useable temperature
as per the guidance below (4). This may be done by bringing the cylinder temperature above 10 ºC for
at least 24 hours (1,2,4). Alternatively, the gases can be remixed by agitating the cylinder once its
temperature has been raised to above 10ºC to obtain a homogenous mixture (2,4). This is only
recommended for cylinders that are small enough to handle safely (4).
Before use and to ensure the gases are properly mixed, three options are available:
1. Cylinders should be stored for at least 24 hours at a temperature above 10ºC (1,2,3) but not
exceeding 38ºC (2).
2. For smaller sized cylinders (AD, D, EA and ED size), if this is not possible, before use the
cylinders must be maintained at a temperature above 10ºC for at least 2 hours and then
completely inverted 3 times (2,4). BOC recommends that cylinders should not be immersed in
water as this can lead to water entering the cylinder (4). For larger sized cylinders it is not
recommended that they are used if stored below minus 6ºC for any significant length of time
(4).
3. Cylinders required for use on pipeline manifold systems (G size), should be stored in the
manifold room for at least 24 hours before use. The manifold room should be kept at a
temperature of 20ºC (4).
Limitations
This Q&A refers only to the Entonox® and Equanox® brands of 50% nitrous oxide / 50% oxygen
cylinders.
From the NHS Evidence website www.evidence.nhs.uk
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Medicines Q&As
References
1. Summary of Product Characteristics. Entonox. BOC Ltd. Accessed via
http://www.boconline.co.uk/pdf_downloads/safety_data_sheets/entonox.pdf on 27th February
2012. Date of revision of the text 29th September 2010.
2. Summary of Product Characteristics. Equanox. Air Liquide Ltd. Date of revision of the text 3rd
January 2011. Available from Air Liquide Ltd on request.
3. Nitrous oxide monograph. Sweetman SC, editor. Martindale: The Complete Drug Reference.
Electronic edition. London: Pharmaceutical Press. Accessed via
http://www.medicinescomplete.com/ on 23rd February 2012.
4. Personal communication. Head of Quality and Regulatory Affairs. BOC Healthcare. 14 th
March 2012.
Quality Assurance
Prepared by
Kate Pickett, Medicines Q&A Pharmacist, Wessex Drug and Medicines Information Centre, University
Hospital Southampton NHS Foundation Trust.
Date Prepared
23rd February 2012
Checked by
Sue Gough (based on the Q&A originally checked by Sandra Hicks), Critical Evaluation Pharmacist,
Wessex Drug and Medicines Information Centre, University Hospital Southampton NHS Foundation
Trust.
Date of check
28th March 2012
Search strategy
Electronic Medicines Compendium (accessed via http://emc.medicines.org.uk/).
British National Formulary No. 62 September 2011 (accessed via http://www.bnf.org.).
Martindale: The Complete Drug Reference. Electronic edition. London: Pharmaceutical Press.
Accessed via http://www.medicinescomplete.com/
BOC website accessed via www.boconline.co.uk
Air Liquide website accessed via www.airliquide.com
Personal communication with Sales Manager and Head of Quality and Regulatory Affairs, BOC.
From the NHS Evidence website www.evidence.nhs.uk
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