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Although N2O is delivered to the patient as a gas, it can exist as a liquid at room temperature if pressurised. Thus, strictly speaking, N2O is a vapour not a gas; N2O cylinders contain liquid N2O in equilibrium with its vapour. It is used as a 'carrier gas' and reduces the MAC required for a volatile agent as well as having a useful analgesic action.
N2O has a very low B:G partition coefficient (0.47) so alveolar concentration approaches inspired concentration more rapidly than for sevoflurane and isoflurane. It even diffuses into the blood faster than nitrogen can diffuse out, since nitrogen (N2) has a higher B:G partition coefficient. As a result, the alveolar concentration of N2O increases even more rapidly than predicted from partition coefficient alone; this is known as the concentration effect. Fig 1 shows that the higher the concentration of N2O, the more rapidly equilibrium is reached. Select play to watch the animation.
This difference between partition coefficients for N2O and N2 has two important implications for anaesthetists:
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The second gas effect When a high inspired concentration of N2O is used for induction along with a volatile, not only does the alveolar concentration of N2O rise more rapidly than predicted, but so does that of the volatile. As a result induction is more rapid than might be expected. |
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Diffusion hypoxia At the end of an anaesthetic N2O will diffuse back into the alveoli more rapidly than N2 can diffuse into blood. If air is given without oxygen supplementation this will have the effect of diluting the amount of oxygen, and so reducing its concentration in the alveoli. This carries a risk of hypoxia, so all patients should be given supplementary oxygen at the end of an anaesthetic when N2O has been used. |