LEARN THE CONCEPT OF CRITICAL & PSEUDOCRITICAL TEMPERATURE WITH THE EXAMPLE OF ENTONOX

▪️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