- Analgesia:
- Pre-hospital
- Labour ward
- Emergency department
- Other areas e.g. burns unit for dressing changes
Entonox
Entonox
- A compressed gas mixture of 50% oxygen and 50% nitrous oxide by volume
- Nitrous oxide and oxygen dissolve into each other don't behave in the way that would be predicted from their individual properties
- Oxygen is bubbled through liquid nitrous oxide, with vaporisation of the liquid nitrous oxide to form Entonox
- The mixture has a critical temperature and pressure different to that of either individual gas
- Compressed into blue cylinders with white/blue quartered shoulders at 13,700kPa
- In theory shouldn't be able to make nitrous oxide gaseous at this pressure because it is below its critical temperature (36.5'C) and above its critical pressure (72bar)
- However the Poynting effect confers a pseudocritical temperature of -7ºC
- Therefore the nitrous oxide is above its critical temperature and can be gaseous at this pressure
Safe storage
- Separation of Entonox into its liquid components occurs if the temperature is decreased to below 5.5ºC
- This is most likely at a pressure of 117bar; separation is less likely at pressures above or below 117bar
- At 4bar the pseudocritical temperature is -30ºC
- Separation can be dangerous as it causes the cylinder to contain:
- A gas with high oxygen content, which is initially breathed off first
- A liquid with maximum 20% oxygen content e.g. a hypoxic mixture that is breathed off once the initial gas has been removed
- Therefore entonox storage should involve:
- Horizontal cylinder placement to increase area for diffusion
- An environment above 5ºC - 10ºC for 24hrs before use
- A dip tube whose tip is at the base of the cylinder, causing the liquid nitrous oxide to be used first and prevent administration of hypoxic gas mixtures
- A two-stage pressure demand regulator is attached to the cylinder
- First stage is a pressure-reducing valve
- The second stage is a demand valve, which allows gas to flow when the pressure is below atmospheric pressure
- As the patient breaths in, gas flow is allowed to occur
- Gas flow stops at the end of inspiration