Women who report being satisfied with nitrous oxide may not show a decrease in pain scores, she added. With nitrous oxide, they say, "It still hurts, but I don’t care."
Inhaling the gas typically provides some degree of pain relief in less than a minute, and the effect dissipates after another breath or two. Since the first study of its use in labor in 1880, nitrous oxide has proved to be safe, Ms. Bishop said. It does not build up in the mother or fetus, and does not seem to affect contractions, labor progression, or the ability to push. It can be used through the second stage of labor, and there’s no evidence that it affects newborns or breastfeeding.
"You can’t kill somebody with 50/50 nitrous oxide and oxygen," she said.
In the United States, the woman initiates and controls the gas flow through a mask, with the negative pressure from inhalation opening a demand valve that stops gas flow when inhalation ceases. Excess nitrous oxide is scavenged out by suction. It’s meant for intermittent, not continuous, use.
Dosimeter badges worn by obstetrics nurses at UCSF consistently show that staff exposure to nitrous oxide is less than 2 parts per million in an 8-hour period, far below the 25-ppm limit set by the National Institute for Occupational Safety and Health.
It’s important to counsel family members who are trying to be "helpful" that only the woman should hold the mask to her face so that she controls the gas flow. Not all women find it helpful, and some may experience dizziness, drowsiness, or nausea, although those effects usually occur with higher doses of nitrous oxide, not the 50/50 blend with oxygen, Ms. Bishop said.
Usually, the nitrous oxide is more effective if the woman breathes it just before a contraction starts instead of waiting for a contraction, but each woman will find what works for them.
Nitrous oxide use at UCSF increased by 50% after the university expanded the privileges of certified nurse-midwives in 2007 to include initiation of the gas mixture, instead of having to call an anesthesia resident. Now the university is moving toward a standing order allowing registered nurses to initiate nitrous oxide use, similar to a standing order for fentanyl initiation. "I think that’s going to be a huge improvement," Ms. Bishop said.
Ms. Bishop reported having no financial disclosures.
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