As already mentioned, nitrous oxide is a relatively weak anaesthetic. A concentration of at least 50 per cent is required to produce worthwhile analgesia. For this reason, nitrous oxide is always mixed with oxygen instead of air. Two types of apparatus are used.

The first mixes the two gases (supplied separately) before delivery to the patient. This apparatus is usually fixed permanently to the wall, and the concentration of nitrous oxide can be adjusted within the range of 0-50 per cent or 0-70 per cent (depending on national regulations). These upper limits ensure that patients will not lose consciousness and can never receive less oxygen than exists in room air (21 per cent). This type of apparatus is popular in Australasia. Being a wall fixture, it is not portable and so cannot be used for a home delivery.

The other kind ot apparatus which is commonly used to supply nitrous oxide is called 'Entonox'. In this case, a 50:50 mixture of nitrous oxide and oxygen is contained in a single cylinder . A special valve at the top of the cylinder reduces the pressure to safe levels (so there is absolutely no danger of being 'blown up'!). It is not possible to alter the concentration of nitrous oxide when using Entonox, but in practice, this does not seem to be important. One advantage of Entonox is that it is portable, and can therefore be used for home deliveries (in Britain, but not in Australia). Entonox is also used in many countries by paramedical personnel to provide pain relief at accident sites and in ambulances.