From the Editor

Nitrous oxide for labor pain

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Should your birthing unit make nitrous oxide available for your laboring patients?


 

References

Neuraxial anesthesia, including epidural and combined spinal-epidural anesthetics, are the “gold standard” interventions for pain relief during labor because they provide a superb combination of reliable pain relief and safety for the mother and child.1 Many US birthing centers also offer additional options for managing labor pain, including continuous labor support,2 hydrotherapy,3 and parenteral opioids.4 In 2012, the US Food and Drug Administration (FDA) approved equipment to deliver a mixture of 50% nitrous oxide and 50% oxygen, which has offered a new option for laboring mothers.

Nitrous oxide is widely used for labor pain in the United Kingdom, Finland, Sweden, Canada, Australia, and New Zealand.5 In the United States, nitrous oxide has been a long-standing and common adjunct to general anesthetics, although it recently has fallen out of favor in place of better, more rapidly acting inhalation and intravenous general anesthetics. With these agents not suitable for labor analgesic use, however, nitrous oxide is undergoing a resurgence in popularity for obstetric analgesia in the United States, and we believe that it will evolve to have a prominent place among our interventions for labor pain.6 In this editorial, we detail the mechanism of action and the equipment’s use, as well as benefits for patients and cautions for clinicians.

How does nitrous oxide work?
Pharmacology.
Nitrous oxide (N2O) was first synthesized by Joseph Priestley in 1772 and was used as an anesthetic for dental surgery in the mid-1800s. In the late 19th Century, nitrous oxide was tested as an agent for labor analgesia.7 It was introduced into clinical practice in the United Kingdom in the 1930s.8

The mechanism of action of nitrous oxide is not fully characterized. It is thought that the gas may produce analgesia by activating the endo­genous opioid and noradrenergic systems, which in turn, modulate spinal cord transmission of pain signals.5

Administration to the laboring mother. For labor analgesia, nitrous oxide is typically administered as a mix of 50% N2O and 50% O2 using a portable unit with a gas mixer that is fed by small tanks of N2O and O2 or with a valve fed by a single tank containing a mixture of both N2O and O2. The portable units approved by the FDA contain an oxygen fail-safe system that ensures delivery of an appropriate oxygen concentration. The portable unit also contains a gas scavenging system that is attached to wall suction. The breathing circuit has a mask or a mouthpiece (according to patient preference) and demand valve. The patient places the mask over her nose and mouth, or uses just her mouth for the mouthpiece. With inhalation, the demand valve opens, releasing the gas mixture. On exhalation, the valve shunts the exhaled gases to the scavenging system.

Proper and safe use requires adherence to the principles of a true “patient-controlled” protocol. Only the patient is permitted to place the mask or mouthpiece over her nose and/or mouth. If the patient becomes drowsy, such that she cannot hold the mask to her face, then the internal demand valve will not deliver nitrous oxide and she will return to breathing room air. No one should hold the mask over the patient’s nose or mouth, and the mask should not be fixed in place with elastic bands because these actions may result in the inhalation of too much nitrous oxide.

Nitrous oxide has a rapid onset of action after inhalation and its action quickly dissipates after discontinuing inhalation. There is likely a dose-response relationship, with greater use of the nitrous oxide producing more drowsiness. With the intermittent inhalation method, the laboring patient using nitrous oxide is advised to initiate inhalation of nitrous oxide about 30 seconds before the onset of a contraction and discontinue inhalation at the peak of the contraction.

There is no time limit to the use of nitrous oxide. It can be used for hours during labor or only briefly for a particularly painful part of labor, such as during rapid cervical ­dilation or during the later portions of the second stage.

Patients report that nitrous oxide does not completely relieve pain but creates a diminished perception of the pain.9 As many as one-third of women are nonresponders and report no significant pain improvement with nitrous oxide use.10

The main side effects of inhalation of the gas are nausea, vomiting, dizziness, and drowsiness. Nausea has been reported in 5% to 40% of women, and vomiting has been reported in up to 15% of women using nitrous oxide.11

Cautions
Contraindications to nitrous oxide include a baseline arterial oxygenation saturation less than 95% on room air, acute asthma, emphysema, or pneumothorax, or any other air-filled compartment within the body, such as bowel obstruction or pneumocephalus. (Nitrous oxide can displace nitrogen from closed body spaces, which may lead to an increase in the volume of the closed space.12)

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