Nitrous oxide

Nitrous oxide

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Nitrous oxide is a potent short-acting inhaled analgesic gas that has a rapid and predictable onset and offset. In high concentrations (eg 70% with 30% oxygen), it is used in general anaesthesia. In lower concentrations, it is used for its analgesic properties to provide short-term analgesia for minor surgical procedures or incident pain. For analgesia, it is available as a 50:50 mix with oxygen, or can be delivered by variable concentration devices, which provide from 30% to 70% nitrous oxide in oxygen. These devices can:

• be constant gas flow devices (eg nasal masks used by dentists)

• use a demand valve where gas flow is generated by inspiration (eg used in labour wards). This requires a good seal between the mask and the face.

Nitrous oxide has a synergistic effect if given with other analgesics and sedatives. Appropriate fasting is needed. Monitoring, especially pulse oximetry, is essential and a protocol should be in place for its use. Resuscitative equipment should be available.

A gas scavenging system should be used in closed areas to minimise room air contamination and avoid chronic occupational exposure.

 

At the conclusion of administration of nitrous oxide for ten or more minutes, 100% oxygen should be administered for three to five minutes to prevent an abrupt decrease in oxygen saturation of arterial blood on ceasing the nitrous oxide (termed ‘diffusion hypoxaemia’).

The main complications of nitrous oxide use relate to the development of hypoxia. Even in the commonly used concentration of 50%, it is capable of occasionally producing loss of consciousness and impaired cough and gag reflexes. It is contraindicated if there is an impaired level of consciousness. Nitrous oxide expands gas-containing cavities within the body and so is contraindicated in undrained pneumothorax, large lung cysts, and recent middle ear surgery. Other adverse effects can rarely occur, including hypotension, respiratory effects, and nausea and vomiting. Prolonged and repeated inhalation of nitrous oxide can be associated with methionine synthetase inactivation, causing megaloblastic anaemia, and neuropathy due to subacute combined degeneration of the spinal cord. Folate and vitamin B12 supplementation may be required.