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Exposure of Postoperative Nurses to Exhaled Anesthetic Gases

Sessler, Daniel I. MD; Badgwell, J. Michael MD

doi: 10.1213/00000539-199811000-00019
Economics and Health Systems Research
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SDC

The National Institute of Occupational Safety and Health (NIOSH) has established recommended exposure limits of 25 parts per million (ppm) as a time-weighted average for nitrous oxide and a ceiling of 2 ppm for volatile anesthetics. We quantified exposure of postanesthetic nurses to exhaled anesthetic gases. This study was conducted in the postanesthesia care unit (PACU) of a medium-sized hospital. PACU air exchanges averaged 8 vol/h; however, much of this air was recirculated. We evaluated 50 adults anesthetized with either isoflurane (n = 19) or desflurane (n = 31). Roughly half the patients were tracheally extubated in the operating room, whereas the others were extubated just after admission to the PACU. Exhaled anesthetic gases were sampled through a 20-m hose attached to the participating nurses' shoulders (breathing zone). We also evaluated nursing exposure to exhaled anesthetic gases during recovery of 15 patients who had been anesthetized with nitrous oxide. Exposure was quantified with lapel dosimeters. Anesthetic and recovery durations were each approximately 1 h, with most patients being tracheally extubated in the PACU. Breathing-zone anesthetic concentrations in the patients given isoflurane exceeded NIOSH recommendations in 37% of the patients, representing 12% of recovery time. Breathing-zone anesthetic concentrations in the patients given desflurane, however, exceeded NIOSH limits in 87% of the patients, representing 49% of recovery time. Altogether, noncompliant episodes were detected in 68% of these patients, representing 35% of the entire recovery duration. Breathing-zone anesthetic concentrations in the patients given nitrous oxide exceeded NIOSH limits in 53% of the patients. Our data suggest that postoperative nurses' exposure to exhaled anesthetic gases exceeds NIOSH limits under some circumstances. Implications: Some epidemiological evidence suggests that exposure to waste anesthetic gases may be associated with reproductive toxicity. Accordingly, the National Institute of Occupational Safety and Health has established recommended exposure limits for nitrous oxide and volatile anesthetics. Our data suggest that exposure of healthcare personnel may exceed recommended levels in poorly ventilated postanesthesia care units.

(Anesth Analg 1998;87:1083-8)

(Sessler) Department of Anesthesia, University of California-San Francisco, San Francisco, California; Ludwig Boltzmann Institute for Clinical Anesthesia and Intensive Care; Outcomes Research[trade mark sign] and Department of Anesthesia and General Intensive Care, University of Vienna, Vienna, Austria; and (Badgwell) Perioperative Services, Department of Anesthesia, Cook Children's Medical Center, Fort Worth, Texas.

Supported by Apotheus Laboratories, Inc. (Lubbock, TX), the Fonds zur Forderung der Wissenschaftlichen Forschung (Vienna; Austria), and the Joseph Drown (Los Angeles, CA) and J. William Fulbright (Washington, DC) Foundations.

Presented in part at the annual meeting of the American Society of Clinical Investigation, Washington, DC, April 25-27, 1997.

JMB is a paid consultant for Apotheus, Ltd., a company that produces a postanesthetic scavenging system. DIS does not consult for, accept honoraria from, or own stock or stock options in any company related to this research.

Accepted for publication July 31, 1998.

Address correspondence to Daniel I. Sessler, MD, Department of Anesthesia, 374 Parnassus Ave., University of California, San Francisco, CA 94143-0648. Address e-mail to sessler@vaxine.ucsf.edu.

SECTION EDITOR: PETER G. DUNCAN.

Numerous epidemiological evaluations of healthcare personnel exposed to waste anesthetics have been conducted [1-19]. Most large studies [1-9], including those incorporating the best statistical controls [17-19], conclude that there is a statistically significant correlation between reproductive toxicity and exposure to waste anesthetic gases. The major identified toxicities were spontaneous abortion and infertility; the relative risk for spontaneous abortion was roughly 1.3-1.9.

The putative link between anesthetic gas exposure and toxicity is controversial and widely disputed [20,21]. The National Institute of Occupational Safety and Health (NIOSH) nonetheless recommends exposure limits for workers. For nitrous oxide, this limit is 25 parts per million (ppm) as a time-weighted average over the time of exposure. For volatile anesthetics (without concomitant nitrous oxide exposure), there is a ceiling of 2 ppm, rather than a time-weighted average [22]. An even stricter limit may apply when volatile anesthetics are coadministered with nitrous oxide [23]. However, these NIOSH recommendations have not been adapted by the Occupational Safety and Health Administration, which is the relevant regulatory agency. NIOSH recommendations are thus suggestions rather than applicable law.

Nitrous oxide and volatile anesthetic concentrations in unscavenged operating rooms (ORs) exceed NIOSH recommended exposure limits [24]. In contrast, gas concentrations in properly scavenged ORs generally comply with NIOSH recommended exposure limits [25,26]. Many previous studies, however, measured gas concentrations near patients' heads-a method likely to overestimate the actual exposure of nurses. Accordingly, we quantified personnel exposure to exhaled anesthetic gases by continuously sampling from the shoulders of postanesthesia care unit (PACU) nurses.

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Methods

This study was conducted June 10-20, 1996 in the PACU of a university-affiliated private hospital in Lubbock, TX. The hospital is a fairly typical acute care hospital with 420 beds and 11,000 operations performed per year. It is approved by the Joint Commission on Hospital Accreditation.

The PACU contains six beds, of which three or four are typically occupied. Nine staff nurses routinely participate in unit care. Both study protocols were approved by the institutional review board, and written, informed consent was obtained from each participating nurse. Neither of the investigators provided anesthesia for the patients in this study.

Air exchange in the unit was measured three times each study day by using an ADM-850 Airdata multimeter (Shortridge Instruments, Inc., Scottsdale, AZ). Hospital engineers initially estimated that the ventilation was 25% fresh air. Subsequent investigation, however, indicated that the recirculated fraction was actually unknown but was directly related to outside temperature. Because prevailing daytime temperatures were high during the study period, it is likely that the recirculated fraction far exceeded 75%.

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Isoflurane and Desflurane

We evaluated nursing exposure to exhaled anesthetic gases during recovery of 50 adult patients who had been anesthetized with either isoflurane (n = 19) or desflurane (n = 31). Per routine, most had also been given nitrous oxide, opioids, and muscle relaxants. No aspect of anesthesia care was proscribed, and patients were not chosen for participation until they arrived in the PACU. Once patients were chosen for study, morphometric and surgical details were obtained from the anesthesia record. Similarly, vaporizer settings (as recorded on the anesthesia records) were integrated over time to estimate volatile anesthetic exposure in minimum alveolar anesthetic concentration (MAC)-hours. Data were accrued sequentially; thus, only one nurse was monitored at any given time. Each set of measurements started when participating patients arrived in the PACU and continued until the patients were discharged from the unit. Gas concentrations were not evaluated between study patients.

Exhaled anesthetic gases were sampled at a rate of 7 L/min through a 20-m, 6-mm polypropylene tube attached to the participating nurse's shoulder, roughly 8 in. from the mouth. We considered this site to represent the nurse's breathing zone. The sampling tube was sufficiently long, lightweight, and flexible to allow nurses complete freedom to move throughout the PACU. An investigator continuously observed the tube to prevent kinking and to assure that it in no way restricted nursing performance. The nurses were asked to follow their normal routine and were encouraged not to restrict their actions or movements. Measurements continued until patients were discharged to the ward.

Halogenated anesthetic concentrations in the gas samples were evaluated by using calibrated Miran 1B2 infrared spectrophotometers (Foxboro, East Bridgewater, MA): one was used for isoflurane and the other for desflurane. The accuracy of these spectrometers is 0.1 ppm, and they have a usable range of 0.2-10 ppm. The 95% response time is <2 min. Measurements were acquired at 7.5 Hz, and the averages were recorded at 1-min intervals.

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Nitrous Oxide

We evaluated nursing exposure to exhaled anesthetic gases during recovery of 32 adult patients, half of whom had been anesthetized with nitrous oxide. Per routine, most had also been given a volatile anesthetic, opioids, and muscle relaxants. No aspect of anesthesia care was proscribed, and patients were not chosen for participation until they arrived in the PACU. Once patients were chosen for study, morphometric and surgical details were obtained from the anesthesia record. Similarly, flowmeter settings were integrated over time to determine nitrous oxide anesthetic exposure in MAC-hours. Data were accrued sequentially (one patient at a time), and breathing zone nitrous oxide concentrations were obtained throughout recovery (until patient discharge).

On the patient's arrival in the PACU, a lapel dosimeter (Personal Monitoring System for Nitrous Oxide X575AT; Assay Technologies, Palo Alto, CA) was placed on the primary nurse's lapel and uncapped. On the patient's discharge from the recovery room, the nitrous oxide dosimeter was resealed. At this point, the amount of time the dosimeter had been exposed to ambient air, the room temperature, and the relative humidity of the room were recorded. The dosimeters were then stored in sealed containers until they were returned to Assay Technologies for analysis. To assure against defects, 20 badges from the same batch were opened, immediately closed, and also sent for analysis.

The nitrous oxide dosimeters are composed of an inert sampling grid of polyacetal covered by a sorbent zeolite molecular sieve with pore size of 5 Angstrom. Nitrous oxide has a high affinity for the zeolite sieve, and because the pore dimension is similar to the molecular dimension of nitrous oxide, equilibrium is shifted toward absorption. Desorption is accomplished by immersing the dosimeter in water, which displaces nitrous oxide from the sieve. A sample of the displaced gas is then analyzed by gas chromatography and quantified by electron capture detection. Minimal detection levels are 8 ppm nitrous oxide for a 15-min sample and 2 ppm for a 60-min sample. Validation studies performed by the manufacturer indicate that the 95% confidence level over the range of 12.5 to 50 ppm is 25%.

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Data Analysis

Admission to the PACU was designated elapsed time zero for the patients extubated in the OR; extubation was designed time zero for patients extubated in the PACU. Halogenated anesthetic concentrations during the first five elapsed minutes were discarded. Each 1-min average volatile anesthetic concentration was compared with the NIOSH ceiling level (2 ppm) [22]. From these values, the proportion of noncompliant minutes (i.e., minutes in which the average exceeded 2 ppm) was assessed for each case. The noncompliant fraction was then determined for each type of anesthetic based on the duration of recovery (the entire PACU stay). Average time-weighted exposure levels were also calculated. The time-weighted nitrous oxide concentrations (from the lapel dosimeters) were directly compared with the NIOSH limit (25 ppm) [22].

Ambient air turnover and morphometric characteristics of the participants in each group were compared using one-way analysis of variance and Scheffe's F-tests. Data are presented as means +/- SD, or medians, intraquartile ranges, and maximums, as appropriate. P < 0.05 was considered statistically significant.

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Results

The morphometric characteristics of the patients participating in each section of the study were similar and were comparable in the exposed and unexposed nitrous oxide patients (Table 1). PACU air exchanges averaged 8 vol/h with the door closed (the typical situation) and 9 vol/h with the door open. No air exchange values were <6 vol/h. Roughly half of the patients (n = 32) were intubated on arrival to the PACU, whereas 30 were tracheally extubated in the OR (this is a routine ratio in the study hospital).

Table 1

Table 1

Patients in the volatile anesthetic portion of the study were exposed to roughly 1.5 MAC-hours of volatile anesthetic in approximately 1 h. The duration of recovery (time from admission to discharge) was also approximately 1 h. Breathing-zone anesthetic concentrations in the patients given isoflurane exceeded NIOSH limits in 37% of the patients, representing 12% of the total recovery duration. However, breathing-zone anesthetic concentrations in the patients given desflurane exceeded NIOSH limits in 87% of the patients, representing 49% of recovery (Figure 1). Altogether, noncompliant episodes were detected in 68% of the patients, representing 35% of the entire recovery duration (Table 2). The time-weighted medians, percentiles, and upper range for isoflurane are shown in Figure 2, and those for desflurane are shown in Figure 3. Volatile anesthetic concentrations exceeded 10 ppm, the upper limit of our spectrometers in 2% of the 1-min measurement epochs. Reported mean and median concentrations thus somewhat underestimate actual exposure levels.

Figure 1

Figure 1

Table 2

Table 2

Figure 2

Figure 2

Figure 3

Figure 3

The nitrous oxide concentration in the unexposed badges was 2.1 +/- 0.3 ppm, not substantively different from zero. Patients in the nitrous oxide portion of the study were anesthetized for >1 h. The exposed patients received an average of 0.6 MAC-hours of nitrous oxide. The duration of recovery was approximately 1 h. The relative humidity was 40%-60% during the study, which (according to the dosimeter manufacturer) does not have a statistically significant impact on nitrous oxide sorption equilibria. Breathing-zone anesthetic concentration in the patients not given nitrous oxide exceeded NIOSH limits in 12%. Breathing-zone anesthetic concentrations in the patients given nitrous oxide exceeded NIOSH limits in 53%. Altogether, noncompliant episodes were detected in 34% of the patients (Table 3).

Table 3

Table 3

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Discussion

Our major finding is that, under the circumstances of this study, postoperative nurses were frequently exposed to exhaled anesthetic gas concentrations exceeding NIOSH recommended exposure limits. Exposure levels were excessive 12% of the time when nurses cared for patients given isoflurane. However, this increased to 49% when desflurane was the principal anesthetic and to 53% in patients given nitrous oxide.

Interestingly, volatile anesthetic exposure curves did not demonstrate the expected exponential decrease over time. Similarly, "background" concentrations of nitrous oxide seemed substantial, as indicated by the fact that one fourth of the nurses caring for patients not given nitrous oxide demonstrated time-weighted averages that exceeded the 25-ppm NIOSH mandate. These data are most consistent with insufficient ventilation with fresh air.

The Guidelines for Design and Construction of Hospital and Health Care Facilities, a national standard set by the American Institute of Architects and the United States Department of Health and Human Services, specifies six air exchanges per hour, two of which must be fresh [27]. The air exchange in our PACU easily exceeds this standard. However, the major limitation of our study is that the fraction of recirculated air is unknown but likely far exceeds the designated 75%. Presumably, recirculation contributed to the observed relatively high noncompliance levels and to the absence of the typical exponential concentration decay curves. A corollary of this conclusion is that exposure levels would be considerably less were the fraction of fresh air ventilation greater.

The second limitation of our study is that it was conducted in a single PACU; exposure levels in other units surely differ. Exposure levels further depend on the patient population, amount and type of anesthesia administered, and duration of recovery. However, the PACU we evaluated seems typical in terms of size, number of patients, and staffing levels. Furthermore, the type of patient, surgery, and intraoperative anesthetic exposure were fairly typical. Half of the patients in this study were extubated in the PACU. Although typical for the hospital we studied, extubation in the OR is routine in most hospitals. Exposure of nurses to exhaled anesthetic gases may be considerably lower in such hospitals. Various durations have been used to evaluate ceiling concentrations (short-term exposure limits); longer durations (e.g., 15 min) would reduce the proportions of measurements exceeding the designated level. There are thus numerous aspects of our protocol and methodology that increase apparent exposure of postanesthesia care workers; it would thus be reasonable to consider this a worst-case analysis.

It is reasonable to question NIOSH-recommended exposure limits; similarly, it is reasonable to question the data on which they are based. Both topics are contentious and have been reviewed extensively [20,21,28-30]. One might especially question application of the 2-ppm recommended exposure limit for volatile anesthetics to both isoflurane and desflurane, although their anesthetic potencies differ by a factor of 6. Furthermore, recommended exposure limits are much higher in Europe: 50-100 ppm nitrous oxide as a time-weighted average over an 8-h work day. The European community has yet to establish exposure limits for sevoflurane; however, the recommended levels for other volatile anesthetics are much higher than those in the United States [31].

In summary, numerous extremely controversial studies correlate exposure to exhaled anesthetic gases with reproductive toxicity. Based on these data, NIOSH has established recommended exposure limits for volatile anesthetics and nitrous oxide. We evaluated exposure of postanesthesia care nurses to each class of exhaled anesthetic gas. Half the patients we evaluated were tracheally extubated in the PACU. Furthermore, the proportion of PACU air that was recirculated likely far exceeds recommended levels. Exposure levels were thus obtained under suboptimal conditions.

Isoflurane, desflurane, and nitrous oxide concentrations were measured at the shoulders of nurses during the course of their routine work. Isoflurane and desflurane concentrations exceeded the NIOSH ceiling of 2 ppm in 68% of the patients, representing 35% of the entire recovery duration. Time-weighted concentrations exceeded the NIOSH recommendation of 25 ppm in 53% of the patients given nitrous oxide. Our data thus suggest that postoperative nurse exposure to exhaled anesthetic gases exceeds NIOSH limits under some circumstances.

We thank the postanesthesia nurses for their generous participation in this study. We also greatly appreciate the assistance of Brett Moore and Scott Wehmeyer with data acquisition and analysis.

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