Nitrous oxide (1844) is a gas with a slightly sweetish smell. It is neither flammable nor explosive. It produces light anaesthesia without demonstrably depressing the respiratory or vasomotor centre provided that normal oxygen tension is maintained.
Advantages. Nitrous oxide reduces the requirement for other more potent and intrinsically more toxic anaesthetic agents. It has a strong analgesic action; inhalation of 50% nitrous oxide in oxygen (Entonox) may have similar effects to standard doses of morphine. Induction is rapid and not unpleasant although transient excitement may occur, as with all agents. Recovery time rarely exceeds 4 min even after prolonged administration.
Disadvantages. Nitrous oxide is expensive to buy and to transport. It must be used in conjuction with more potent anaesthetics to produce full surgical anaesthesia.
Uses. Nitrous oxide is used to maintain surgical anaesthesia in combination with other anaesthetic agents, e.g., isoflurane or propofol, and, if required, muscle relaxants. Entonox provides analgesia for obstetric practice, for emergency management of injuries, and during postoperative physiotherapy.
Dosage and administration. For the maintenance of anaesthesia, nitrous oxide must always be mixed with at least 30% oxygen. For analgesia, a concentration of 50% nitrous oxide with 50% oxygen usually suffices.
Contraindications. Any closed, distendable air-filled space expands during administration of nitrous oxide, which moves into it from the blood. It is therefore contraindicated in patients with: demonstrable collections of air in the pleural, pericardial or peritoneal spaces; intestinal obstruction; arterial air embolism; decompression sickness; severe chronic obstructive airway disease; emphysema. Nitrous oxide will cause pressure changes in closed, noncompliant spaces such as the middle ear, nasal sinuses, and the eye.
Precautions. Continued administration of oxygen may be necessary during recovery, especially in elderly patients (see diffusion hypoxia, above).
Adverse effects. The incidence of nausea and vomiting increases with the duration of anaesthesia. Nitrous oxide interferes with the synthesis of methionine, deoxythymidine and DNA. Exposure of to nitrous oxide for more than 4 hours can cause megaloblastic changes in the bone marrow. Because prolonged and repeated exposure of staff as well as of patients may be associated with bone-marrow depression and teratogenic risk, scavenging systems are used to minimise ambient concentrations in operating theatres.
Drug interactions. Addition of 50% nitrous oxide/ oxygen mixture to another inhalational anaesthetic reduces the required dosage (minimum alveolar concentration, MAC) of the latter by about 50%.
Storage. Nitrous oxide is supplied under pressure in cylinders, which must be maintained below 25°C. Cylinders containing premixed oxygen 50% and nitrous oxide 50% (Entonox) are available for analgesia. The constituents separate out at -7°C, in which case adequate mixing must be assured before use.
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