Entonox is a 50:50 mixture of oxygen and nitrous oxide. It is produced by
bubbling oxygen through liquid nitrous oxide. The two gases dissolve into
each other, creating a gas mixture that does not behave in a way that could be
predicted from their individual properties – the Poynting effect. Entonox is
presented as a gas in French-blue cylinders with white and blue checked
shoulders at a pressure of 137 bar when full. Below temperatures of –7°C
(pseudocritical temperature), Entonox can separate into its constituent parts
because of liquefaction of nitrous oxide. This can potentially result in the
delivery of a hypoxic mixture as the cylinder empties.
1. Inhalational anaesthetic agent (in conjunction with a volatile agent).
2. Inhalational analgesic agent.
a. During labour.
b. For breakthrough pain when regional anaesthesia is used
(e.g. tourniquet pain and caesarean section).
c. During painful short procedures, such as change of burns
dressings, manipulations of fractures/dislocations and
cleaning of wounds in paediatric patients.
