Nitrous oxide

Inhaled nitrous oxide is used for acute pain (eg labour pain), procedural sedation and analgesia and anxiolysis (minimal sedation) for dental procedures. The onset of analgesic effect is rapid (occurring after 5 to 8 inhalations) and effects quickly wear off once administration is stopped. Co-administration of opioids or other drugs used for procedural sedation enhances the efficacy of nitrous oxide but may also increase the depth of sedation.

Nitrous oxide concentrations range from 50 to 75% in oxygen and can be administered as a variable (ie adjusted according to response) or fixed dose (eg Entonox). Nitrous oxide delivery can be patient-triggered or not.

• Patient-triggered nitrous oxide (ie delivered on demand when a patient breaths in) requires a good mouth seal and adequate inspiratory effort. Deep sedation is less common with patient-triggered administration because it requires patient coordination to maintain effect.
• If patient-triggered delivery is not suitable (eg young children, sedated patients), nonpatient-triggered delivery (ie continuous flow) is used. Closer observation is required because of the increased risk of complications such as deep sedation, especially in children, and vomiting.

Nitrous oxide use should be supervised by a trained healthcare practitioner, and limited to settings where resuscitation facilities are available. Furthermore, it should be used in a well-ventilated environment and the delivery system (except fixed-concentration patient-triggered delivery systems) should be attached to a scavenger to minimise nonpatient exposure¹.

Other than dizziness, nausea and brief disinhibition, adverse effects from nitrous oxide are rare. Vomiting is more frequent with higher nitrous oxide concentrations and when opioids are co-administered.

After administering nitrous oxide, it is common practice to administer high-flow supplemental oxygen to prevent an abrupt decrease in oxygen saturation of arterial blood (diffusion hypoxaemia).

Nitrous oxide must not be used in patients with a condition in which air is trapped in a body cavity (eg pneumothorax, bowel obstruction) because of its potential to increase volume and pressure within the space.

Avoid nitrous oxide use in patients who are vitamin B₁₂ deficient or have disorders of folate or homocysteine metabolism, because nitrous oxide can reduce vitamin B₁₂ activity (causing bone marrow suppression and irreversible neuropathy). In otherwise healthy patients, seek specialist advice if prolonged or repeated administration (eg 3 times per week for 2 weeks) is required.