I'm afraid that the answers provided by TeeGee and barbie jacey are not correct. On the other hand, Hawkwalk�s answer is correct. There are a lot of misconceptions flying about regarding nitrous oxide, so I'll try to put the record straight without going into too much detail.
Inhalation anaesthetics may be either gases or volatile liquids. Now, when the medical profession speak of anaesthetic usage, they are always either speaking of induction of anaesthesia or maintenance of anaesthesia.
Induction of anaesthesia is the use of anaesthetic agents to put a patient to sleep, which is what pb100191 is asking about. Maintenance of anaesthesia is the use of anaesthetic agents to make sure that you stay asleep after you've been put to sleep. It's important to recognise the difference before going any further.
There are a few pure gases which could put a patient to sleep (nitrous oxide is not one of them) but these gases can cause problems for the patient and are potentially toxic, so they're hardly ever used.
Nowadays, most hospitals use volatile liquid anaesthetics administered using vapourisers, which use air, oxygen or nitrous oxide-oxygen mixtures as the carrier gas for induction anaesthesia. These volatile liquid anaesthetics include Isoflurane, Enflurane, Desflurane and Sevoflurane.
Sevoflurane is the best of the lot for what's called mask induction anaesthesia but it's quite expensive (around �50 for 100ml). Halothane is not as popular as it once was due to potential severe liver toxicity, cardiorespiratory depression, cardiac effects and some obscure biochemical changes that may occur in the body. There are also issues over accumulative sensitivity and toxicity.
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