Entonox is Nitrous oxide mixed 50:50 with oxygen

It provides analgesia with maintenance of consciousness.

Usually administered via a demand valve for self administration.

Takes 30 seconds to act and continues for approx. 60 sec after inhalation has stopped

For optimum effect inhalation should start when the contraction tightens. This will co-ordinate the maximal effect with the central painful part of the contraction.

20% N20 is equivalent to 15 mg of subcutaneous morphine.

The optimal analgesic concentration was found to be 70% but some mothers lost consciousness at this concentration

50% N20 in oxygen is safer and this has become standard now

Entonox is the BOC trade name for this gas mixture.

Poynting effect

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.

Critical & Pseudocritical temperature

The critical temperature of a gas is the maximum temperature at which compression can cause liquefaction. Or it is the temperature above which a substance cannot be liquefied however much pressure is applied. Mixing gases may change their critical temperature.

The pseudocritical temperature applies to a mixture of gases, such as Entonox, and is the temperature at which gas mixtures separate into their component parts.

The Poynting effect produces a 50:50 mixture which reduces the crtical temperature of N20 so Entonox has a pseudocritical temperature of -6 degree.

Entonox

Highest -5.5°C @117 bar
Cylinder -7°C @137 bar
Pipeline -30°C @4 bar

In cylinders it is supplied at a pressure of 137 bar and must be stored above its pseudocritical temperature of -6°C.

Below this temperature the N2O liquefies in a process called lamination. If this occurs a high concentration of O2 will be delivered first with little analgesic effect, but as the cylinder empties the mixture will become progressively more potent and hypoxic as it approaches 100% N2O.

If a cylinder has been exposed to cold below -6 degree C it should be warmed for 5 minutes in a 37 degree C water bath or for 2 hours in a room at 15 degree C. It should then be inverted three times before use.

When delivered via a pipeline at 4.1 bar the pseudocritical temperature is less than -30°C.

Altitude per se has no effect on Entonox.

Reference: www.frca.uk