Nitrous oxide inhalation anaesthetic

Nitrous oxide inhalation anaesthetic is a gaseous inhalation agent first described in 1799 by Hamphrey Davy and introduced by Horace Wells (1846), a dentist, into clinical use in the US and it is still in use.

Physical properties: Nitrous oxide inhalation anaesthetic is a colourless, sweet smelling and non-irritant gas, bp-88°C, slightly heaver than air. Non-flammable but supports combustion even in the absence of oxygen. Nitrous oxide inhalation anaesthetic is stable in sodalime.

Nitrous oxide is supplied and stored in blue coloured cylinders under pressure of 5,000 kPa at 20°C. At room temperature the cylinder contains liquid, hence it should be kept upright when in use. The contents of the cylinder can be ascertained by weighing it.

When there is only gas, then the pressure guage will give an indication of the contents of the cylinder. The critical temperature is 36.5°C. Above this temperature nitrous oxide inhalation anaesthetic can only exist as a gas. It is also available as a 50:50 mixture of Oxygen and nitrous oxide (entonox).

The cylinder is coloured blue with a white yoke, it is pressured to 15,000 kPa at 20°C, both are in gaseous form unless the temperature drops to 7°C, below this temperature in cold countries nitrous oxide inhalation anaesthetic will liquefy, the oxygen rises to the top if the cylinder is upright; the oxygen is delivered first and later hypoxic mixture is released. At a temperature of 7°C thorough mixing of gases is necessary before using the cylinder.

The MAC of nitrous oxide inhalation anaesthetic is 105%, blood/gas solubility coefficient is 0.42, which is less than any other inhalation agent except xenon. Nitrous oxide inhalation anaesthetic does not undergo any metabolism and is eliminated unchanged through the lungs and a small amount through the skin.

Uptake and Distribution: Because of its low blood/gas solubility there is a rapid equilibration of FI/FA ratio. It is more soluble in blood than nitrogen, so during induction nitrous oxide inhalation anaesthetic is more rapidly taken up by blood, so the alveolar volume is reduced, thereby maintaining the concentration of nitrous oxide in the alveoli further increasing the fractional concentration; this is the concentration effect.

So the relative concentration of other gases in the alveoli also increases like CO2 or an inhalation agent, if it is also in the mixture; this is the second gas effect.

During recovery breathing air, the nitrous oxide inhalation anaesthetic diffuses out into the alveoli faster than the nitrogen uptake by blood. The alveolar volume reduces the concentration of all gases including oxygen, resulting in “diffusion hypoxia” or the Fink effect.

In case there is a gas-filled cavity in the body, nitrous oxide inhalation anaesthetic diffuses into the cavity more quickly than the original gas can diffuse out, this may be harmful like in a pneumothorax, gas filled bowel, bullae in the lung, gas bubbles in the eye following retinal surgery or in the middle ear in case of eustachian tube blockage. A gas embolus may also considerably increased in size.




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