(1989; 60 pages) [French]
Group: inhalational anaesthetic gas
Nitrous oxide is a colourless gas with a slightly sweetish odour. It is neither flammable nor explosive. It is a cerebral depressant and produces light anaesthesia without demonstrably depressing the respiratory or vasomotor centre provided that normal oxygen tension is maintained.
Nitrous oxide reduces the requirement for other more potent and intrinsically more toxic anaesthetic agents. It has a strong analgesic action. Induction is rapid and not unpleasant although transient excitement may occur. Recovery time rarely exceeds 1-4 minutes even after prolonged administration.
Nitrous oxide is expensive to buy and to transport. It must be used in conjunction with more potent anaesthetics and muscle relaxants to produce a state of full surgical anaesthesia.
• Maintenance of surgical anaesthesia in combination with other anaesthetic agents (halothane, ether, thiopental or ketamine) and muscle relaxants.
• In subanaesthetic doses, to provide analgesia for obstetric practice, for emergency management of injuries, during postoperative physiotherapy and for refractory pain in terminal illness.
Dosage and administration
For the maintenance of anaesthesia, nitrous oxide must always be mixed with at least 30% oxygen. This is usually accomplished using a compressed-gas anaesthetic machine.
For analgesia, a concentration of 50% nitrous oxide with 50% oxygen usually suffices.
Any closed gas-filled space tends to expand during administration of nitrous oxide. It is therefore contraindicated in patients with demonstrable collections of air in the pleural, pericardial or peritoneal space; intestinal obstruction; occlusion of the middle ear; arterial air embolism; decompression sickness; chronic obstructive airway disease; or emphysema. It is also contraindicated in patients who have recently undergone pneumoencephalography.
Continued administration of oxygen may be necessary during recovery especially in elderly patients.
The incidence of nausea and vomiting increases with the duration of anaesthesia. Because prolonged and repeated exposure may be associated with bone-marrow depression and a teratogenic risk, precautions should be taken to minimize ambient concentrations in operating theatres.
Addition of 50% nitrous oxide/oxygen mixture to an inhalational anaesthetic reduces the required dosage of the latter by about 50%.
Nitrous oxide is supplied under pressure in cylinders, which must be kept below 25°C. It must be obtained from a reliable source since contamination with higher oxides of nitrogen, including nitric oxide and nitrogen peroxide, has caused deaths.
Cylinders containing premixed oxygen 50% and nitrous oxide 50% are available for analgesia in some countries. However, the constituents separate out at -6°C, in which case adequate mixing must be assured before use. When the two components are supplied from separate cylinders a safety device must be installed that interrupts the How of nitrous oxide should the oxygen pressure fall.
Identification of cylinders
An ISO standard1 requires that cylinders containing nitrous oxide should bear the name of the contents in legible and permanent characters and, preferably, also the chemical symbol N2O. The neck, from the valve to the shoulder, should be coloured blue. Cylinders containing nitrous oxide and oxygen mixtures should be similarly labelled, and the neck coloured white and blue.
1International Standard 32. Gas cylinders for medical use - marking for identification of content. Switzerland, International Organization for Standardization, 1977.