Informed Consent and Nitrous Oxide for Labor Analgesia

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For clarification, our human subjects protection board approved our proposal to conduct a retrospective analysis of a postpartum follow-up quality improvement database, not a prospective observational trial. We introduced nitrous oxide as a standard labor analgesic option on our labor unit in mid-2011. On admission, all pregnant women receive an anesthesiology consultation, including discussion regarding analgesic options, benefits, and risks. Dr Hogan’s comments relate to potential effects of nitrous oxide on the methionine synthase pathway, human consequences of which are unknown. As such, we do not find them useful to clinical decision-making discussions with patients. However, his questions draw attention to the larger issue of the safety of intermittent nitrous oxide self-administered by laboring women, and implications for informed consent. This is particularly timely, given the December 14, 2016, US Food and Drug Administration (FDA) Safety Communication regarding use of general anesthetic and sedative drugs in children and pregnant women.
The FDA notice summarizes evidence regarding histologic and neurocognitive effects of lengthy anesthetic drug exposure in animals (19 studies), and neurocognitive and developmental outcomes after administration of general anesthesia in young children undergoing surgery (24 studies, none in pregnant mothers). The notice acknowledges the unknown clinical significance of nonclinical animal studies, and uncertainty regarding potential effects on brain development after repeated or lengthy anesthetic exposure during surgery in children or pregnant women. Of note, the FDA report cited reassuring results of 2 ongoing well-designed prospective studies of neurocognitive effects of general anesthesia for inguinal hernia repair in young children (total 399 exposed subjects, with controls). Widespread criticism of the FDA recommendations are based on the weakness of supporting evidence.1,2 Extrapolation of current evidence to clinical conclusions regarding fetal risks of maternal nitrous oxide use is even more tenuous. A few animal studies have examined effects of nitrous oxide exposure alone. Rat pups, aged 1–14 days, failed to demonstrate apoptotic neurodegeneration when continuously exposed to 50%, 75%, or 150% nitrous oxide for up to 6 hours.3 Because arterial nitrous oxide tension fluctuates at a fraction of inspired concentrations when used intermittently during labor, fetal exposure would be far less than that in these animal studies.
King and Wong4 have suggested that it may be “impossible to conduct human studies addressing whether in utero exposure to N2O during labor has adverse effects on the infant central nervous system,” and recommend that parturients who choose to use nitrous oxide “should be informed that we lack data about this outcome.” We agree. As with other anesthetic modalities, we candidly discuss the state of evidence regarding safety risks with expectant mothers. Since introducing nitrous oxide in mid-2011, our practice has been to discuss the decades-long worldwide experience of safe nitrous oxide use during labor, the existence of animal studies both demonstrating and failing to demonstrate adverse effects on animal babies, and the possibility that someday there may be proven negative effects on human neonates (see Supplemental Digital Content, Figure, As with neuraxial modalities, we continue to offer nitrous oxide analgesia to laboring women at this time.

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