Elsevier

Anesthesiology Clinics

Volume 35, Issue 1, March 2017, Pages 125-143
Anesthesiology Clinics

Should Nitrous Oxide Be Used for Laboring Patients?

https://doi.org/10.1016/j.anclin.2016.09.011Get rights and content

Section snippets

Key points

  • Unlike neuraxial labor analgesia, N2O provides highly variable labor analgesia, ranging from very poor to very good.

  • Despite this variability, parturients who choose to use N2O where neuraxial analgesia is an option (including after trying N2O) report satisfaction similar to that reported by women who use neuraxial analgesia. Parturients using N2O report higher satisfaction than subsets of parturients who experience inadequate neuraxial labor analgesia.

  • Regarding safety, parturient and neonatal

Historical considerations

Several excellent reviews of the early history of N2O use for anesthesia during surgery have been published,5, 6 and Table 1 depicts a brief historical chronology. The discovery of N2O is attributed to Joseph Priestly, who demonstrated its use in 1772.5 In 1800, Humphry Davy reported that N2O was useful at relieving toothache and associated it with pleasurable sensations during self-administration.5 Dentist Horace Wells first suggested the use of N2O as an anesthetic and self-administered it

Nitrous oxide pharmacokinetics and pharmacodynamics

N2O is a vapor anesthetic that is tasteless and odorless. It is a weak anesthetic agent with a minimum alveolar concentration more than 100% at 1 atm, which confers safety during administration of the 50% concentration. N2O has very low blood/gas solubility, and Waud and Waud14 showed that peak brain concentrations occurred 60 seconds after onset of its administration in laboring patients. Its mechanism of action, however, is not well understood. Maze and Fuginaga15 hypothesized that N2O

Nitrous oxide: variable analgesic effectiveness

Despite its introduction as a labor analgesic nearly 6 decades ago, its subsequent routine use throughout Europe, and its growing popularity in the United States, evidence regarding the effectiveness of N2O is surprisingly limited. A 2002 systematic review with strict criteria for inclusion (randomization, adequate control group, effectiveness assessments by parturients at the time of or shortly after intervention) identified only 11 trials of adequate quality for review. Data on 340

What matters: effective pain relief versus satisfaction?

Although effectiveness of pain relief is a primary determinant in many parturients’ reported satisfaction with labor analgesic care, especially with neuraxial modalities,24, 31, 32, 33, 34, 35, 36 it is clearly not the only factor.34, 37, 38, 39, 40 Among 28 women with a priori plans to use labor epidural analgesia, effective pain relief (confidence in timely access to it and analgesic effectiveness) was viewed as beneficial in regaining self-control and ability to focus, think, and participate

Nitrous oxide: 5-year experience at a high-risk academic center

N2O has been offered as an option for all laboring women at Vanderbilt Medical Center since June 2011. Soon after admission, anesthesiology personnel assess every woman with a viable pregnancy and counsel them regarding neuraxial and N2O analgesic options provided by the obstetric anesthesiology service, including benefits, side effects, risks, limitations on mobility, and the relative level of analgesia that may be expected. Consistently, one in 5 laboring women selects N2O analgesia during

Nitrous oxide analgesia versus other non-neuraxial techniques

Unfortunately, there is no useful evidence comparing N2O with other non-neuraxial analgesic modalities or nonpharmacologic interventions. Most studies of such alternative interventions reveal modest or no analgesic effectiveness, including maternal childbirth preparation,44 relaxation techniques,45 water immersion/whirlpool therapy,46 acupuncture/acupressure,47 transcutaneous nerve stimulation (TENS),48 and subcutaneous water injection.49 A single observational study examined pain relief and

Adverse maternal and neonatal effects

N2O is generally well tolerated, without major side effects. Some of the reported adverse reactions and contraindications to N2O for labor are presented in Box 2. Although N2O likely has little direct effect on ventilatory drive, its use during labor has been associated with brief self-limited maternal oxygen desaturations between labor contractions.57 Self-administration techniques involve focused inhalation of the gas to obtain analgesic effects. Hyperventilation during uterine contractions,

Contraindications

Contraindications to N2O use in laboring patients are few (see Box 2). The potential for N2O to expand closed gas spaces is well known; caution is advised in women who have had a recent pneumothorax, pneumocephalus, venous air embolism, bowel obstruction, retinal surgery, middle ear surgery, or sinus infections.68 N2O should also be avoided in patients with known pulmonary hypertension and certain congenital heart diseases because of its known effects on pulmonary vascular resistance. Risks of N

Health care worker exposure, environmental, equipment, and administrative concerns

Although inhaled N2O offers an alternative to neuraxial analgesia and has advantages compared with other non-neuraxial alternatives, several practical issues must be considered before introducing a N2O program (Box 3).

Health care worker exposure

The National Institute for Occupational Safety and Health (NIOSH) sets an exposure limit of 25 ppm as a time weighted average (TWA) during periods of anesthetic administration.70 This recommendation, set in 1977 and not reviewed since 1994, was not intended to prevent long-term health consequences for workers but rather to prevent possible effects of exposure on health providers’ sight and audio acuity and mental performance. NIOSH based their 25 ppm recommendation on decreased audiovisual

Nitrous oxide exposure and health safety risk

Survey studies linking N2O exposure to health care worker reproductive hazards appeared in the 1960s and 1970s. Study subjects were either operating room personnel or dental assistants, and all were exposed to levels of N2O and other anesthetic gases that far exceeded that found in any current health care environment. Despite such high exposure levels, these studies reported marginal statistical significance for increased risk (relative risk for spontaneous abortion, 1.3; congenital anomalies,

Environmental concerns

N2O is a potent greenhouse gas, approximately 300 times more potent at trapping atmospheric heat than carbon dioxide; most N2O administered for anesthetic purposes ultimately ends up in the atmosphere.82 However, medical use has been estimated to contribute to less than 0.05% of total atmospheric warming and is responsible for less than 1.0% of all N2O in the atmosphere.82 Swedish manufacturers have developed equipment that reclaims and destroys N2O. Ek and Tjus83 recently reported a near

Administrative procedures and clinical use

N2O administration should comply with the anesthesia and sedation policies unique to each institution. Within the United States, these policies should follow the Conditions of Participation (42 CFR 482.52) recently updated by the Centers for Medicare and Medicaid Services, which direct that the department of anesthesia at each facility develop and implement all policies for sedation and analgesia. Current American Society of Anesthesiologists’ practice guidelines for sedation and analgesia

Equipment

The N2O labor analgesia apparatus may include a single gas cylinder containing a one-to-one mix of N2O and oxygen supplied at 2000 psi (both in gas phase, owing to the Poynting effect), most often used in Europe (eg, Entonox). In the United States, the apparatus most commonly draws the gases from separate sources (N2O from an attached E cylinder and oxygen from a wall source or E cylinder) to provide a one-to-one ratio output. On a practical note, N2O remains in a liquid phase inside a

Cost

Several investigators89 have noted that N2O administration is inexpensive, although specific costs of providing care have not been published. Disposable supply costs are similar to those for neuraxial blocks (approximately $20 hospital cost at Vanderbilt), and capital costs are not prohibitive (approximately $5000 per apparatus). The equipment is robust and has a long life expectancy.89 Personnel costs are probably lower than those involving neuraxial blockades if N2O is administered by nurse

Summary

Each parturient has personal preferences and needs, which are shaped by the uniqueness of her life experience. Furthermore, these preferences and needs are dynamic and often change during the course of her labor. Although N2O is less effective in treating labor pain than neuraxial analgesic modalities, it has consistently served the needs and preferences of a small but significant subset of parturients. Including N2O in the repertoire of modalities that obstetric anesthesiologists offer to

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