Dear Q&A,

Is it feasible to use self-administered nitrous oxide for labor analgesia?

Nitrous oxide is used extensively for labor analgesia in the United Kingdom, Canada, Australia, Finland, and Sweden.¹ Historically, the use of nitrous oxide in the United States for this purpose has been significantly more limited. However, there has been a dramatic growth of this service in the last couple of years as commercially available delivery systems have reemerged onto the market.

Our institution was asked if we would explore expanding options for labor analgesia to include nitrous oxide. After consulting with other institutions offering nitrous oxide for labor analgesia, we agreed to move forward with this initiative. At the time, the only FDA-approved delivery system for nitrous oxide was the Nitronox™ system by Porter Instrument Division, Parker Hannifin Corporation, which had recently acquired the rights and began manufacturing units again. The device delivers nitrous oxide in a 50:50 ratio with oxygen through a demand valve attached to either a mouthpiece or a standard anesthesia face mask. Patients are instructed to exhale into the mask or mouthpiece where waste gas is evacuated through a connection to the central vacuum system.

Prior to launching the service, there were significant concerns expressed among our staff regarding environmental exposure to nitrous oxide. As part of a plan to assuage worries and ensure that we remained below National Institute for Occupational Safety and Health (NIOSH) recommended exposure limits (25 ppm, time-weighted average), we required dosimeters (Assay Technology passive nitrous oxide badges) be worn for anyone anticipated to be in a patient’s room for more than 15 minutes while nitrous oxide was being used. In addition, dosimeters were placed within the patient’s room while nitrous oxide was being used to monitor ambient levels.

The results from our first trial period are illustrated in the Table where samples ranged from <2.8 to 140 ppm (time-weighted averages). We opted to suspend this service while we investigated possible causes. After discussions with the manufacturer and additional patient education efforts that emphasized ensuring a tight mask seal, and advocating that staff remain out of the patient’s immediate space while nitrous was being used, we reinitiated the service. Repeat testing (see table below) again showed that 48% of samples were above recommended thresholds. The decision was made to terminate the use of the Nitronox™ delivery system.

<tr “>Delivery SystemTrialTotal SamplesSamples above NIOSH recommended levelsPercent exceeding NIOSH levels

Nitronox™	1	25	15	68%
	2	31	15	48%
Pro-Nox™	1	5	2	40%
	2	14	10	71%

Given continued interest in offering nitrous oxide to our parturients, we explored an alternative delivery system, the Pro-Nox™ system by CAREstream Medical. Following additional educational sessions for staff, we reinitiated a nitrous oxide option for labor analgesia. Samples were sent off (Liberty Mutual Industrial Hygiene Laboratory, Hopkinton, MA) after the first two patients used the service. Results were consistent with our previous experience and ranged from 1.2 to 180 ppm (Table, above). The service was again temporarily suspended while we investigated ways to mitigate further exposure. A second trial of the Pro-Nox™ delivery system yielded similar results with 71% above the NIOSH recommended limits. The decision to suspend nitrous oxide as an option for labor analgesia indefinitely was unanimous among our workgroup.

Although nitrous oxide for the management of labor pain has enjoyed decades of seemingly innocuous use, there has been a paucity of quality literature regarding its safety and effectiveness. A systematic review of nitrous oxide for the management of labor pain was recently published,² which sought to review the existing evidence for the effectiveness of nitrous oxide for labor analgesia, the influence of nitrous oxide on maternal satisfaction with the birth experience, and adverse effects associated with nitrous oxide. The authors identified 58 publications that met their criteria; however, the majority (46/58) of them were deemed of poor quality. The evidence for occupational harm associated with nitrous oxide is limited, and many of the studies that do exist were performed prior to modern scavenging techniques making evaluation of possible risk difficult. However, it should be noted obstetrical suites may not be designed with scavenging and ventilation systems that are present in modern operating rooms and dental suites.

The available data is insufficient to make evidence-based statements regarding maternal, fetal, or occupational health risks associated with exposures to nitrous oxide during labor. However, the NIOSH recommended exposure limit of 25 ppm for health care workers is the current U.S. standard. We found that despite numerous attempts to decrease nitrous exposure to health care professionals, we were unable to consistently adhere to these guidelines resulting in the elimination of nitrous for labor analgesia at our institution.

We hope our experience will prompt other institutions that utilize nitrous for labor analgesia to routinely monitor levels for both the safety of the patients and health care personnel and prompt further research in this area. Until prospective, multicenter trials studying the short- and long-term effects of ni

trous oxide for use in the obstetrical suites are conducted, we recommend frequent monitoring of nitrous oxide levels and adherence to NIOSH guidelines.

Benjamin Morley, MD
Lebanon, New Hampshire

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References:

1. Rosen MA. Nitrous oxide for relief of labor pain: a systematic review. Am J Obstet Gynecol 2002;186:S110–26.
2. Likis FE, Andrews JC, Collins MR, Lewis RM, Seroogy JJ, Starr SA, et al. Nitrous oxide for the management of labor pain: a systematic review. Anesth Analg 2014;118:153–67.

Dear Dr. Morley,

The first point concerns a proper mask fit and recognition that a good mask fit requires skill and is critical to maintaining the lowest possible level of N₂O in an operating room.^1,2,3 In the surgical suite the excess inspired and exhaled gases are collected from the circle breathing system or one of the Mapleson circuits. In this case the patient receives 50/50 nitrous oxide from a demand valve, which is closed until a negative pressure from the patient opens the valve and provides the 50/50 nitrous oxide mixture for the patient to inhale, very similar to functioning of the self-contained underwater breathing apparatus regulator (SCUBA). If the flow rate on the demand valve exceeds the patient’s inspiratory flow rate, a poor mask fit will enable nitrous to escape into the room. A possible solution to the problem of a mask leak is to use a double mask as described by Reiz et al.⁴ However, mask leak is probably not the most significant source of trace nitrous oxide.

In the case of self-administered nitrous analgesia, one of the sources of environmental nitrous oxide is from the patient’s exhaled breath. The patient is now exhaling significant amounts of nitrous into the room to be taken up by anyone surrounding the patient. After the patient inhales the nitrous oxide and puts the mask aside, the exhaled gas will contain large quantities of nitrous oxide. Byhahn et al. demonstrated that mean concentrations of exhaled anesthetic gases in the recovery room and intensive care unit following surgery, exceed governmental standards for personnel exposure.⁵ If the patient emerges from anesthesia in the operating room, the concentration of anesthetic agents and nitrous oxide are often low enough to be unmeasurable by clinical monitoring systems as the patient is moved to the PACU. In this case, however, the exhaled breath starts at high concentrations of nitrous oxide, much greater than those in the PACU or ICU after surgery, and is exhaled into the patient’s environment.

The solution to this problem may be found in the labor room air turnover rate and the use of non-recirculated air that is properly heated and humidified for each parturient’s room. Each labor room, like each operating room, should have a very high fresh gas turnover rate using non-recirculated, conditioned (temperature and humidity) air. Design and construction of non-recirculated and conditioned air for each labor room may solve the problem of trace nitrous oxide concentrations, but may be prohibitively expensive.

A. William Paulsen, PhD, AA-C Chair, APSF Committee on Technology.

References:

1. Barker J, Abdelatti M. Anaesthetic pollution. Potential sources, their identification and control. Anaesthesia 1997;52:1077–1083.
2. Kanmura Y, Sakai J, Yoshinaka H, et al. Causes of nitrous oxide contamination in operating rooms. Anesthesiology 1999;90:693–696.
3. Hoerauf K, Funk W, Harth M, et al. Occupational Exposure to Sevoflurane, Halothane, and Nitrous Oxide During Paediatric Anaesthesia. Anaesthesia 1997;52:215–219.
4. Reiz S, Gustavisson AS, Haggmark S, et al. The double mask-a new local scavenging system for anaesthetic gases and volatile agents. Acta Anaesthesiol Scand 1986; 30:260–265.
5. Byhahn C, Westphal K, Strouhal U. Maternal protection law and exposure of personnel in the recovery room and surgical intensive care unit by inhalation anesthetics. Gesundheitswesen 1998;60:586–91.

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