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CLINICAL PEARL

by Brigid McCue MD, PhD

No Laughing Matter: Alternative Uses of Nitrous Oxide in Labor and Delivery

A 23 yo G2P1 presents to the OB ED for cerclage removal. She has a history of a 24 week delivery thought to be due to cervical insufficiency, and had a McDonald cerclage placed at 16 week gestation. An uneventful pregnancy followed, and the patient was scheduled to have the cerclage removed at 36 weeks in her obstetrician’s office. The patient was unable to tolerate removal of her cerclage and she is referred for removal under anesthesia in the OR.

Due to high volume of cesarean sections and the non-emergent nature of her procedure, the patient is faced with a long wait. She consents to cerclage removal with the assistance of nitrous oxide analgesia in the OB ED.

Patient is instructed as to the proper use of patient controlled nitrous oxide in a 50/50 blend. The procedure is carried out quickly with excellent patient compliance. After a period of observation the patient is discharged to home. She presents two weeks later in active labor.

Nitrous oxide (N2O) is an inhaled anesthetic gas commonly used in general anesthesia and dental care. Use in labor is common in several European countries (used by 50%-75% of laboring patients in the UK), and began growing quickly in the US after re-introduction of portable, self-administered delivery devices (e.g. Nitronox or Pro-Nox). N2O has an onset of action approximately 50 seconds after inhalation and dissipates seconds after cessation of use. When used as instructed, the delivery device scavenges the exhaled N2O and prevents contamination of the environment.

Contraindications are few, but include women with a baseline arterial oxygenation saturation less than 95% on room air, acute asthma, emphysema, pneumothorax, bowel obstruction or pneumocephalus (as N2O can cause expansion of closed, air-filled compartments). B12 deficiency is a relative contraindication as N2O inactivates ­vitamin B12, however intermittent use for a limited time is associated with minimal to no hematologic effects.  

Limited studies show no significant adverse neonatal effects measured by Apgar score or umbilical artery and vein blood gases 1. N2O crosses the placenta but negative effects on human fetuses exposed to N2O have not been described.

In addition to the mild analgesic effect of 50% nitrous oxide, patients describe a marked anxiolytic effect which facilitates procedures in the antepartum, intra-partum and postpartum periods. Patients describe feeling a detachment and relief from anxiety, enabling increased cooperation with caregivers; “I knew I was in pain but I didn’t care”. N2O can facilitate placement of IV or spinal for the needle-phobic patient, placement of the cervical ripening balloon, repair of the difficult perineal laceration, or retrieval of a retained placenta.

Nitrous oxide analgesia is an additional tool in the safety armamentarium of the ob/gyn hospitalist.

Patient self administration of nitrous oxide in labor.

References:

1)     Clinical trials of different concentrations of oxygen and nitrous oxide for obstetric analgesia. Report to the Medical Research Council of the Committee on Nitrous Oxide and Oxygen Analgesia in Midwifery. Br Med J. 1970;1(5698):709–713.

2)     Likis FE, Andrews JC, Collins MR, et al. Nitrous oxide for the management of labor pain: a systematic review. Anesth Analg. 2014;118(1):153–167.

Brigid McCue, MD, PhD, FACOG
Lead OB/GYN Hospitalist Ochsner Baptist Hospital
2700 Napoleon Avenue
New Orleans, LA
Immediate Past President, SOGH