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Trace Nitrous Oxide Levels in the Postanesthesia Care Unit

McGregor, Diana G. MBBS, FRCA; Senjem, David H. BA, CSP; Mazze, Richard I. MD

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doi: 10.1213/00000539-199908000-00042
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Abstract

There has been speculation that personnel who work in the postanesthesia care unit (PACU) may be at risk of adverse health effects because these units do not scavenge waste anesthetic gases [1,2]. Most of the reports on the subject of health hazards associated with exposure to waste anesthetic gases examined measurements made in operating rooms (ORs), not PACUs. At the Mayo Clinic, institutional quality assurance data indicate that waste nitrous oxide levels in the PACU are consistently lower than those in the OR. However, under Minnesota law, these data are confidential, and their publication is prohibited. Thus, we performed the present study to document and publish the levels of nitrous oxide to which PACU personnel are exposed on a daily basis in the ambient air.

Methods

After institutional review board approval, all PACU patient care personnel in PACU A and PACU B were included in the study. Nitrous oxide levels were measured on one day using a time-weighted averaging (TWA) monitor (Vapor-Trak; Kem Medical Products Corp, Farmingdale, NY) worn on the lapel by all PACU patient care personnel for the duration of their shifts, except during breaks. Monitors were analyzed by Kem Medical Products at their Deerfield Beach, FL, plant. The Vapor-Trak nitrous oxide monitor measures nitrous oxide by adsorption on the monitor; in the laboratory, the nitrous oxide derivative is desorbed from the solid sorbent on the monitor, and the concentration is determined by infrared analysis [3]. It is specified to have a lower limit of sensitivity of 2 ppm for an 8-h sampling period and accuracy limits of 25% at 25 ppm and 12.5 ppm

Age, weight, type of surgery, anesthetic technique (general or regional anesthesia, monitored anesthetic care) were entered on a data sheet for each patient admitted to the PACU. Additionally, the anesthesia resident or nurse anesthetist reported the last end-tidal mass spectrometer reading obtained for nitrous oxide immediately before leaving the OR. Nitrous oxide was turned off at the end of the surgery, usually 5-15 min before the patient left the OR. This is consistent with Occupational Health and Safety Administration (OSHA) guidelines to reduce waste anesthetic gas levels to which personnel may be exposed.

There were an average of 9 (range 5-12) air changes per hour in PACU A and 20 (range 18-21) air changes per hour in PACU B. These rates are verified annually. A minimum of 25% of each air change is taken from the outside air, the remainder being recirculated air. The American Institute of Architects' (AIA) Guidelines for Hospital and Medical Facilities states the minimal number of total air changes when the PACU is in use should be six per hour, with a minimum of two air changes per hour from outdoor air [4]. PACU A occupies 35,154 cu ft and PACU B occupies 18,468 cu ft.

Two-sided sign rank tests were used for statistical comparisons. P < 0.05 was considered significant.

Results

Eighty-nine patients were admitted to PACU A (Table 1). Eighteen PACU nurses, one anesthesia resident, and one respiratory therapist worked in the unit on the day measurements were made. Their time of exposure (i.e., time working in the PACU) ranged from 0.97 to 9.03 h (mean 7.5 h). The TWA level of nitrous oxide for the personnel in PACU A during their shifts ranged from undetectable (i.e., <2 ppm) to 6.4 ppm (mean 2.0 ppm) (Table 2).

Table 1
Table 1:
Demographic/Procedural Characteristics of Patients Entering the PACU
Table 2
Table 2:
N2 O Exposure Levels for PACU Personnela

Forty-four patients were admitted to PACU B (Table 1). Thirteen nurses worked in PACU B on the day measurements were made. Their time of exposure ranged from 0.5 to 8.83 h (mean 5.40 h). The TWA level of nitrous oxide was undetectable for all 13 nurses in PACU B (Table 2). Median levels of exposure to nitrous oxide were significantly lower than the levels recommended by National Institute of Occupational Safety and Health (NIOSH) and OSHA (25 ppm) (P < 0.001).

Discussion

We found that the levels of nitrous oxide measured in both PACUs were well below the NIOSH and OSHA TWA recommended exposure level (REL) of 25 ppm [5,6]. Values in other studies have spanned a considerable range, and methods of measurement have included "grab" (i.e., single samples) and TWA sampling. In 1972, Bruce and Linde [7] measured grab air samples at a fixed point from the patient's head in the PACU and found serial mean halothane levels to be <0.6 ppm. Halothane levels in the ambient air are approximately 1/50 that of nitrous oxide [8]. Thus, for this study, nitrous oxide levels, if measured, would likely have been approximately 30 ppm. Pfaffli et al. [9] obtained grab nitrous oxide samples in recovery rooms and found levels of 50-500 ppm. In 1978, Berner [10] found that both the number of patients in the PACU and the frequency of air changes influenced the levels of waste anesthetic gas in ambient air; the author reported grab sample nitrous oxide levels of 10-34 ppm. In 1980, Davenport et al. [11] found grab sample nitrous oxide levels of 30 ppm in recovery rooms. In 1990, Kant et al. [12] reported TWA nitrous oxide levels of 9 ppm in their PACU, in which the ventilation system was connected to the OR. In 1997, Kendrick et al. [2] reported TWA levels of isoflurane in two PACUs; the mean level in one unit was 0.5 ppm, and it was 1.66 ppm in the other. The latter unit was located in an old building, and ventilation in the PACU did not meet the AIA's recommended number of air changes [4]. Applying the 50:1 correction factor used above, nitrous oxide levels are estimated at 25 ppm and 83 ppm, respectively. In 1998, Sessler and Badgwell [13] reported nitrous oxide levels up to 30 ppm, but the tracheas of half their patients were extubated in the PACU, and measurements were not made during the entire shift of the PACU staff. Our data reflects our modern ventilation systems and work practices designed to reduce waste anesthetic levels in the ambient air.

Since Vaisman's [14] account from the Soviet Union in 1967 examining the health of OR workers, it has been suspected that trace levels of anesthetic gases may have an adverse effect on the health of OR personnel. As a consequence of this and other reports [15-18], the ASA Ad Hoc Committee on Adverse Reactions to Anesthetic Agents met in June 1972 with members of NIOSH, and the ASA National Health Survey of Operating Room Personnel was instigated [19]. Results of the survey, published in 1974, suggested there may be an increased risk in OR personnel of spontaneous abortion, congenital abnormalities in their children, cancer, and hepatic and renal disease. These data and those from animal research made it reasonable to recommend scavenging waste anesthetic gases in all anesthetizing locations [20]. An important future study, the second national survey, was planned to begin in 1978 to determine whether scavenging waste anesthetic gases would lead to a reduction of the reported problems. If that occurred, it would support the inference that the waste anesthetic gases caused the problem. However, in 1977, without performing the follow-up study, NIOSH made recommendations concerning standards of exposure to waste anesthetic gases [6]. The REL for nitrous oxide measured as a TWA was 25 ppm during anesthetic administration. At the same time, OSHA developed technical instructions concerning waste anesthetic gases [5], but these recommendations were never officially put forth as a standard. No information on RELs for PACU personnel was ever promulgated, but it would be logical to assume that these levels should not exceed those recommended for OR personnel.

In the 1980s, studies questioning the conclusions of the early epidemiologic studies began to appear. In 1985, at the request of the ASA, Buring et al. [21] analyzed the then 17 published studies that examined outcomes after exposure to trace anesthetic gases. They reported that the most consistent adverse finding (i.e., data from more than one or two studies) was an increase in the rate of spontaneous abortion in pregnant OR workers. However, Buring et al. noted that all of the studies were in one way or another flawed and that their positive results could be attributed to these errors (e.g., responder bias, confounding variables). At the same time, Tannenbaum and Goldberg [22] independently reviewed the relevant epidemiology literature and came to a similar conclusion. In another key study, Axelson and Rylander [23] demonstrated the inaccuracy of epidemiological data examining spontaneous abortion when outcome data were not verified. The entire 33% difference between study and control patients disappeared when the medical records of all patients were examined. Spence [24] and Maran et al. [25], using annual questionnaires, prospectively surveyed all British female medical school graduates <or=to40 years working in hospitals during the years 1977-1984. Their analyses showed that these anesthesiologists did not have an increased risk of infertility. Additionally, there was no association between the outcomes of spontaneous abortion and development of congenital abnormalities in live-born children and risk factors that included occupation of the mother, hours exposed to the OR environment, and the use of scavenging equipment. However, in a different venue, the dental operatory, Rowland et al. [26,27] reported reduced fertility and increased risk of spontaneous abortion among dental assistants exposed to nitrous oxide in unscavenged areas, but not in those that were scavenged. Nitrous oxide levels in unscavenged dental operatories may exceed 1000 ppm, a level much higher than those in any report of PACU or OR exposure [6]. Axelsson et al. [28] reported that night work and shift work and staff shortages among Swedish midwives were associated with an increased incidence of late spontaneous abortions, whereas the use of nitrous oxide as an analgesic in >50% of deliveries was not.

In summary, our data indicate that 33 PACU personnel in two separate units were exposed to very low TWA levels of nitrous oxide during the course of their work day. Although measurements were made in only two PACUs and on one day, the results are similar to confidential data obtained at the Mayo Clinic for quality assurance purposes on previous occasions during the last 15 years. The institution follows OSHA recommendations for the maintenance and checking of anesthetic and ventilation system equipment and the use of work practices in the OR designed to reduce contamination of the OR air with waste anesthetic gases. This use of appropriate work practices in the OR and a well maintained ventilation system in the PACU result in low levels of trace anesthetic gases in the PACU. Because there is still no proven hazard to OR workers exposed to slightly higher levels of waste anesthetic gases, it is presumed that lower levels of exposure are safe for PACU personnel.

We acknowledge Ryan Lennon, Department of Biostatistics, Mayo Medical School, for the statistical analysis.

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