Anesth Analg 2001;92:389-392
© 2001 International Anesthesia Research Society
TECHNOLOGY, COMPUTING, AND SIMULATION
Online Monitoring of Air Quality at the Postanesthetic Care Unit by Proton-Transfer-Reaction Mass Spectrometry
Josef Rieder, MD*,
Peter Prazeller, PhD
,
Michael Boehler, MD*,
Philipp Lirk*,
Werner Lindinger, PhD, and
Anton Amann, PhD*
*Department of Anesthesiology and Critical Care Medicine, Leopold-Franzens-University of Innsbruck, Austria; and Department of Ion Physics, Innsbruck University, Innsbruck, Austria
Address correspondence and reprint requests to Dr. Anton Amann, The Leopold-Franzens University of Innsbruck, Department of Anesthesiology and Critical Care Medicine, Anichstrasse 35, 6020 Innsbruck, Austria. Address e-mail to anton.amann{at}uibk.ac.at
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Abstract
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The subthreshold exposure to trace anesthetic gases is not associated with considerable risk of adverse health effects. Online control of ambient air exchange at the postoperative workplace may help in supervising air quality and lead to cost reduction. A proton-transfer-reaction mass spectrometer system was used for online monitoring of volatile organic compounds, especially anesthetic gases. The mean exposure to sevoflurane and isoflurane at the urological postanesthesia care unit (PACU) was 15.9 and 9.5 parts per billion, respectively. Sevoflurane and isoflurane concentrations at the urological PACU showed a patient turnover-dependent burden during our investigation period. Because modern PACUs have a high ventilation capacity, the 24-h occupational burden by anesthetic gases at the PACU is relatively low. Monitoring and controlling of ambient air by automatic built-in alarm systems would be useful for quality control of the postoperative workplace. Moreover, energy costs of ventilation systems could be reduced by coupling ventilation capacity to the effective exposure.
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Introduction
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Although no cause-effect relationship regarding unfavorable health responses to volatile anesthetics has been established, the United States and most European public health authorities recommend extremely strict threshold values to minimize possible health risks. It seems very unlikely that adverse health effects can be elicited when these threshold doses are maintained by a functioning room air exchange system (1). However, inadequate ventilation systems may lead to occupational exposure in excess of threshold doses, possibly increasing the risk of occupational illness (2).
In recent decades, trace gas analysis has been performed by means of gas chromatographic methods, which have improved to such an extent that complex mixtures of gas components having volume mixing ratios as small as parts per trillion and even less can be quantitatively analyzed with precision. Nevertheless, gas chromatographic analysis is time consuming, needs supervision, and does not allow online monitoring. Hence, for unsupervised systems without technical assistance, gas chromatographic methods are not suitable. Moreover, the concentrations of anesthetic gases may change rapidly or show interesting circadian rhythms, possibly in correlation with patient turnover and air exchange. Therefore, single isolated measurements of volatile organic compounds (VOCs) as indicators of ambient air quality are not sensible.
Proton-transfer-reaction mass spectrometry (PTR-MS) permits online determination of different molecular species (with molecular weights from, e.g., 20–400 d). Therefore, circadian rhythms can be detected and the average and peak occupational exposure determined. The respective time concentration patterns may give hints concerning the sources of ambient air contamination.
This study focused on the expositional burden of volatile anesthetics (sevoflurane and isoflurane) at our surgical and urological postanesthesia care unit (PACU). Our hypothesis was that high-capacity ventilation systems lead to mean concentrations of anesthetic gases more than 100-fold less than the concentrations recommended by National Institute for Occupational Safety and Health (NIOSH) and Occupational Safety and Health Administration (OSHA) guidelines.
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Methods
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PTR-MS permits online monitoring of VOCs with volume mixing ratios as small as a few parts per trillion (3). PTR-MS combines the idea of chemical ionization with the idea of H3O+ as the primary reactant ion, which is most suitable when air samples containing a wide variety of traces of VOCs are to be analyzed (3). Most of the VOCs have proton affinities larger than H2O and can therefore be ionized by chemical ionization. A decisive advantage of using primary H3O+ ions is that most of their proton transfer processes are nondissociative, so that only one product ion species occurs for each neutral reactant, thereby allowing an accurate allocation of potential substances with measured masses.
For our experimental setup, we positioned the PTR-MS apparatus in separate rooms near the urological and surgical PACUs. Several measurements confirmed that anesthetic trace gases with a minimal distance of 2.5 m from patients’ headspace were equally distributed in height. To get information about the occupational exposure, we focused on mean room air concentration and not on headspace concentrations. The air at the PACUs was therefore collected about 10 cm below the ceiling—but far away from fresh-air inlet—through a Teflon® tube with a vacuum pump. To get online information on anesthetic gas workplace concentrations, measurements were taken every 3 min. PTR-MS would allow for five measurements per second.
Patient turnover, anesthesia protocols, and extubation times at the PACUs were documented. All interactions in the PACUs (e.g., room cleaning by housekeeping or disinfection by medical personnel) were protocoled by a member of the research team.
The surface area of the urological PACU was 120.1 m2 and the volume 310 m3. The actual ventilation capacity was 2980 m3/h. In comparison, the Austrian norm (ÖNORM H6020) recommends a ventilation capacity of 32.4 m3/h and per square meter, resulting in 3891 m3/h for the urological PACU. We also collected data in one room at the surgical PACU. The patients were tracheally extubated at the operating theater 10–15 min before arriving at this room. The surface area of this room was 48 m2, with a volume of 149 m3. The actual ventilation capacity was 860 m3/h. In comparison, the Austrian standard (ÖNORM H6020) recommends a ventilation capacity of 32.4 m3/h and per square meter, meaning 1556 m3/h for the surgical PACU.
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Results
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Figure 1 shows the occupational burden with sevoflurane at the local surgical PACU. Small concentrations of anesthetics were detected during the patient-free weekend. At the surgical PACU, we observed a sudden decrease in the anesthetic gas burden each morning at 6 AM.
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Figure 1. Concentration of sevoflurane at the surgical postanesthesia care unit (PACU). Vertical lines indicate 6:00 AM. Because of faulty programming, the room air exchange system was shut off during the night and turned on at 6:00 AM each day.
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Sevoflurane and isoflurane concentrations at the urological PACU show a patient turnover-dependent burden during our investigation period ( Figs. 2 and 3): the peaks of sevoflurane and isoflurane concentration correlate with extubation times.
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Figure 2. Concentration of isoflurane at the urological postanesthesia care unit (PACU) with patient turnover (extubation time–leave from PACU) indicated by bars. Height of bars is proportional to length of anesthesia.
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Figure 3. Concentration of sevoflurane at the urological postanesthesia care unit (PACU) with patient turnover (extubation time–leave from PACU) indicated by bars. Height of bars is proportional to length of anesthesia.
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In correlation with patient turnover (32 patients: 16 patients anesthetized with sevoflurane, 15 with isoflurane, and 1 with propofol), the mean exposure to sevoflurane at the urological PACU on the 3 recorded working days was 15.9 parts per billion-volume (ppbv). For isoflurane, the mean concentration was 9.5 ppbv. Because there were no patients and hence no occupational exposure at the PACU during the weekend, these mean concentrations were calculated only on working days.
Figure 4 shows the time dependence of mass 43 (daltons) concentrations during our investigation period at the urological PACU. This mass is characteristic for cleaning substances and disinfectants, e.g., arising (= 61 - 18) from protonated isopropanol (mass 61) which has lost a water molecule (mass 18).
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Figure 4. Concentration of mass 43 in parts per million (ppmv). This mass is attributed to isopropanol and propanol, respectively, which are contained in cleansers and share the same molecular weight.
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Discussion
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Online monitoring of the ambient air at two of our PACUs recorded concentrations of the commonly used anesthetics sevoflurane and isoflurane. The concentrations of sevoflurane were about twofold larger than those of isoflurane. Because sevoflurane has a faster elimination rate than isoflurane, larger concentrations of sevoflurane are exhaled and therefore cause a higher occupational exposure. Because modern PACUs have a high ventilation capacity, the 24-hour occupational burden by anesthetic gases at the PACU is altogether relatively small. NIOSH recommends that occupational exposure to volatile anesthetics, when they are used as the sole anesthetic, should not exceed 2 parts per million-volume (ppmv) (4). In European countries, recommended threshold values for these drugs range from 2 to 20 ppmv for the volatile anesthetics over an eight-hour working day (5). Mean sevoflurane and isoflurane concentrations of 15.9 and 9.5 ppbv, respectively, are more than 100-fold less than the recommended threshold values, and this confirms our hypothesis.
The aim of our study was to evaluate the mean concentration of anesthetic gases in ambient air within our PACUs. We therefore did not focus on local peak concentrations near patients’ headspace, but on mean workplace concentrations inhaled by the personnel. Several measurements confirmed that anesthetic trace gases with a minimal distance of 2.5 m from patients’ headspace were equally distributed in height. Hence, our measurements 10 cm below the ceiling—far away from fresh-air inlet—were representative for the mean room concentration.
At the surgical PACU, we detected a sudden and unexpected decrease in the concentrations of both anesthetic gases at 6 AM. As the most plausible interpretation of this result, we suggested that the room ventilation had been turned off during the night. Although the Austrian law requires PACUs to be ventilated 24 hours a day (corresponding to NIOSH and OSHA guidelines), our suspicion was confirmed by ventilation technicians previously unaware of this faulty room air exchange programming.
Besides volatile anesthetics, PTR-MS covers the entire spectrum of masses up to 500 daltons. The range of VOC relevant to indoor air quality (6) is therefore accessible to online control. For example, the VOCs vaporized from cleaning substances and surface disinfectants show a characteristic time course. This time course can be used to control cleaning. Moreover, we demonstrate that cleaning also causes contamination of the ambient air over several hours.
Although monitoring is very important in anesthetic practice, online monitoring of the various anesthetic workplaces does not exist. Quality control of ambient air by detection of VOCs as potential health hazards may also help prevent claims for indemnification for occupational illness caused by an inefficient, defective, or stopped ventilation system. Effective ventilation is of paramount importance to ensure small levels of trace anesthetic gases (1). Recommended levels may be exceeded in poorly ventilated workplaces (2). Interrupted ventilation during the night can go undetected for months or even years, because annual control measurements are usually performed during the day. Guidelines for the prevention of occupational exposure do not dictate any controlling or monitoring system for workplace safety. In particular, automatic built-in alarm systems do not exist.
We therefore propose an online control of the ambient air at anesthetic workplaces. Only online monitoring permits ventilation capacity to be regulated to meet actual contamination: VOCs caused by high patient turnover, cleaning, or disinfection should be eliminated immediately by increasing air exchange. In contrast, when exposure is low, the ventilation capacity could be reduced without diminishing the quality of ambient air. Up to a level of 20 ppbv, 100-fold below the recommended level, room air exchange capacity could be easily downregulated without danger of adverse health effects. For the investigational period on working days, this was the case for 40% of the time, for example, when patients having predominantly undergone regional or total IV anesthesia were located at the PACU. Including weekends, this share was even larger (70%), resulting from patient turnover. This could lead to a substantial reduction of energy costs for ventilation while ensuring adequate quality of ambient air.
In conclusion, anesthetic gas concentrations at our PACU are more than 100-fold smaller than those recommended by international guidelines. Defects of the ventilation systems can be detected by PTR-MS online monitoring.
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Acknowledgments
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This study was supported by the Forschungsförderungspreis der Universität Innsbruck (Award for the promotion of scientific research, Innsbruck University).
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Accepted for publication October 3, 2000.
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Abstract
Introduction
