nitrous oxide oxygen mixture

Nitrous Oxide Oxygen Mixture

A mixture of fifty percent Nitrous Oxide and fifty percent Oxygen. The ability to combine nitrous oxide and oxygen at high pressure while remaining in the gaseous form is due to the Poynting effect. The Poynting effect involves the dissolution of gaseous O2 when bubbled through liquid N2O, with vaporization of the liquid to form a gaseous O2/N2O mixture.

Inhalation of pure nitrous oxide over a continued period would render a human hypoxic, and the 50% oxygen content prevents this from occurring. The two gases will separate at low temperatures (<4oC), which would permit administration of hypoxic mixtures. Therefore, it is not given from a cold cylinder without being shaken (usually by cylinder inversion) to remix the gases.

The gas is self administered though a demand valve, using a mouthpiece, bite block or face mask. Self-administration of unox remains safe because if enough is inhaled to start to induce anesthesia, the patient becomes unable to hold the valve, and so will drop it and soon exhales the residual gas. This means, that unlike with other anesthetic gases, it does not require the presence of an anesthetist for administration. The 50% oxygen in unox ensures the patient will have sufficient oxygen in their system for a short period of apnoea to be safe.

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 30 seconds after the start of administration; unox should thus be used accordingly, e.g. use should begin 30 seconds 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 of around 60 seconds.

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.