Entonox
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Entonox, referred to colloquially as gas and air, is an analgesic used to ease pain during labour, trauma and minor surgical procedures. Entonox is the trade name for a mixture of fifty percent nitrous oxide (N2O or laughing gas) and fifty percent oxygen (O2) so a more appropriate name would be gas and oxygen. Entonox is administered by inhalation from a cylinder at a pressure of 137 bar (in the UK) or from a pipeline supply.
Its analgesic effect is strong (equivalent to 5 to 10 mg of morphine) and characterised by rapid onset and offset (i.e. it is very fast-acting and wears off very quickly).
Nitrous oxide is itself active (does not require any changes in the body to become active), and so has an onset in roughly the lung-brain circulation time. This gives it a peak action 30s after the start of administration; Entonox should thus be used accordingly, e.g. use should begin 30s before a contraction becomes painful in labour. It is removed from the body unchanged via the lungs, and does not accumulate under normal conditions, explaining the rapid offset.
Entonox is self administered i.e. there is no requirement for an anaesthetist to be present to (amongst other things) control the mix of nitrous oxide and oxygen.
Nitrous oxide can act as an anaesthetic in high doses. Self-administration of Entonox remains safe because if enough is inhaled to start to induce anaesthesia, the patient becomes unable to hold the valve, and so will drop it and soon exhale the residual gas. The 50% oxygen in Entonox ensures they will have sufficient oxygen in their system for a short period of apnoea to be safe.
Nitrous oxide and oxygen will separate at low temperatures (<4 °C), which will permit administration of hypoxic mixtures. Thus it should never be given from a cold cylinder, and the cylinder should be inverted a few times to mix the gases before use.
Nitrous oxide is more soluble than oxygen and nitrogen, so will tend to diffuse into any air spaces within the body. This makes it dangerous to use in patients with pneumothorax or who have recently been SCUBA diving, and there are cautions over its use with any bowel obstruction.
Entonox is produced by the BOC Group, which also owns the trademark.
The ability to combine nitrous oxide and oxygen at high pressure while remaining in the gaseous form is due to the Poynting effect (after John Henry Poynting, an English physicist).
The Poynting effect involves the dissolution of gaseous O2 when bubbled through liquid N2O, with vaporisation of the liquid to form a gaseous O2/N2O mixture.
