Anesth Analg 2002;94:1244-1248
© 2002 International Anesthesia Research Society
TECHNOLOGY, COMPUTING, AND SIMULATION
Environmental Monitoring of Sevoflurane and Nitrous Oxide Using the Cuffed Oropharyngeal Airway
Burkhard Gustorff, MD DEAA*,
Norbert Lorenzl, MD*,
Laleh Aram, MD
,
Claus G. Krenn, MD,
Brigitte P. Jobst, MD, and
Klaus H. Hoerauf, MD
Departments of *Anesthesia and Intensive Care B and Anesthesia and Intensive Care A, Vienna General Hospital, University of Vienna, Austria
Address correspondence and reprint requests to Klaus H. Hoerauf, MD, Department of Anesthesiology and General Intensive Care, University of Vienna, Waehringer Guertel 18-20, A-1090 Vienna, Austria. Address e-mail to klaus.hoerauf{at}univie.ac.at
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Abstract
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We compared exposure to sevoflurane (SEV) and nitrous oxide (N2O) during ventilation using the cuffed oropharyngeal airway (COPA) with waste gas exposure using a conventional face mask (FM) without any additional airways or face straps and with the laryngeal mask airway (LMA). Trace concentrations of SEV and N2O were assessed by using a direct reading spectrometer during 33 surgical procedures under general anesthesia. Measurements were made at the patients’ mouths and in the anesthesiologists’ breathing zones. Mean ± SD concentrations of SEV and N2O measured at the patients’ mouths were comparable in the COPA (SEV, 8.1 ± 12.2 ppm; N2O, 213.3 ± 289.2 ppm) and LMA (SEV, 18.5 ± 25.8 ppm; N2O, 283.4 ± 361.0 ppm) groups but differed significantly from the FM group (SEV, 46.5 ± 19.6 ppm; N2O, 750.7 ± 308.3 ppm). These values resulted in a comparable contamination of the anesthesiologists’ breathing zones (SEV, 0.5 ± 0.2 ppm; N2O, 5.7 ± 4.8 ppm) for the COPA group, compared with the LMA group (SEV, 1.0 ± 0.9 ppm; N2O, 12.2 ± 14.3 ppm). This differed significantly from the FM group (SEV, 2.2 ± 0.9 ppm; N2O, 37.5 ± 14.3 ppm). We conclude that the use of the COPA during short surgical interventions has an occupational safety comparable to that of the LMA and that both resulted in less contamination through waste anesthetic gases. Therefore, the COPA may be a valuable alternative to the conventional FM.
IMPLICATIONS: In this study, we have shown that the occupational exposure to waste anesthetic gases is comparable when using the cuffed oropharyngeal airway (COPA) and the laryngeal mask airway and is increased when using the face mask. Therefore, the COPA may be a valuable alternative to the conventional face mask during short surgical procedures.
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Introduction
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Some degree of contamination of the operating room (OR) by waste anesthetic gases is unavoidable when they are administered. Although the health consequences of environmental exposure to OR anesthetic vapor remain controversial, there is considerable epidemiological evidence that trace concentrations of anesthetics are associated with spontaneous abortion and infertility (1,2). Furthermore, chronic exposure to even small concentrations of waste anesthetic gases may cause genetic damage in OR personnel (3,4). Although long-term consequences are not definitively known, US and European health authorities recommend limits ranging from 2 to 75 ppm for volatile anesthetic exposure and from 25 to 100 ppm for nitrous oxide (N2O) to minimize personnel health hazards (5).
Concerning one confounding factor of OR pollution, the standard OR ventilation, the American Institute of Architects and the United States Department of Health and Human Services set the OR ventilation to six air exchanges per hour, with only two being fresh (6). In our hospital, the ventilation is usually more frequent and ranges from 15 to 25 exchanges, all being fresh.
Despite ventilation rates, the likelihood of OR contamination also increases when non-gas-tight airway devices, large concentrations of inhaled anesthetics (5), or both are used. Especially during short surgical procedures lasting <30 min, airway management is often performed by the traditional face mask (FM). During the last 10 yr, the laryngeal mask airway (LMA) has been established as an alternative. The cuffed oropharyngeal airway (COPA) is another relatively new device for the management of the airway during general anesthesia, preferably for short surgical procedures; it is an excellent option when intubation is not desired (7–9). Whereas the gas-tightness of the LMA, resulting in low occupational pollution, has been demonstrated (10) and the FM cannot be proven to be gas tight, no data are available with the new COPA compared with these other devices.
There is a lack of data concerning the environmental safety of the COPA device. Therefore, the aim of this randomized, controlled study was to measure exposure to sevoflurane (SEV) and N2O while using the COPA during anesthesia with spontaneous or manually assisted breathing in an OR with air conditioning and scavenging systems conforming to National Institute of Occupational Safety and Health (NIOSH) recommendations. These concentrations were compared with the exposure while using an LMA or an FM under identical conditions.
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Methods
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After approval by the institutional ethics committee and obtaining patients’ written informed consent, 33 otherwise healthy patients (ASA physical status I and II) scheduled for elective minor breast surgery procedures were recruited. Exclusion criteria were 1) ASA physical status >III; 2) high probability of a difficult airway; 3) acute or chronic lung disease; 4) obesity (body mass index >25% of the normal range); 5) sore throat; 6) gastroesophageal reflux due to hiatus hernia, or other esophageal or gastric abnormalities; 7) food or drink within 6 h before anesthesia; 8) surgical procedures with a scheduled duration of <20 min or >60 min; and 9) age <19 yr.
Computer-assisted randomized treatment assignments (FM, LMA, or COPA) were contained in sequentially ordered, sealed envelopes, which were opened just before anesthesia induction. Eleven patients per treatment group were managed by using an FM (Group FM), an LMA (Group LMA), or a COPA (Group COPA). All patients were premedicated with oral midazolam (0.1 mg/kg) approximately 1 h before the induction of anesthesia. Anesthesia was standardized by using propofol 1.5–3 mg/kg for induction until loss of the eyelash reflex and SEV and 65% N2O in oxygen for maintenance. The administration of SEV and N2O was started after placement of the airway device.
The COPA and LMA were inserted after loss of the eyelash reflex. Standard positioning of the head was performed in the supine position, with typical padding under the head. In Group FM, conventional FMs No. 2 or 3 (Draeger, Vienna, Austria), without any additional airways or face straps, were used. In Group LMA, LMAs (Logomed, Windhagen, Germany) No. 3 (body weight <60 kg) and No. 4 (body weight >60 kg) were used, and the cuff was inflated with 20–40 mL of air. In group COPA, the airway was established with COPA No. 10 (Mallinckrodt, Vienna, Austria), and the cuff was inflated with 30–40 mL of air.
Fresh gas flow was set to 6 L/min and was provided by a recently serviced ModulusTM (Ohmeda, Vienna, Austria) anesthesia machine. Manually assisted ventilation was provided if necessary before the transition to spontaneous breathing. Vaporizer setting and end-tidal SEV concentrations, as well as ventilatory variables, were recorded.
Care was taken throughout the induction and anesthesia to maintain the best possible mask seal, to avoid excessive peak inspiratory pressure, and to minimize occupational exposure to OR anesthetic vapor concentrations (e.g., immediately turning off fresh gas flow after putting the airway away from the patient and not using inhaled anesthetics during the induction of anesthesia). The study was conducted in the same OR with 17 air exchanges per hour, with all ventilation being fresh air. The anesthesia machine was connected to a scavenging system, which in turn was connected to the hospital vacuum system at an aspiration rate of 45 L/min.
Ambient gas was continuously sampled (30 mL/min) from two different locations by using a Teflon® tube (4 mm internal diameter; Merck, Vienna, Austria). To detect the most important leakage source, one sampling probe was fixed 2–3 cm above the patient’s mouth. Because measurements in the breathing zone best reflect the personal exposure (3,5), a second one was fixed at the shoulder of the anesthesiologist. As in previous studies (3,5), OR anesthetic vapor concentrations of SEV, as well as N2O, samples were assessed at 1-min intervals by using a Brüel & Kjaer (Naerum, Denmark) photoacoustic infrared spectrometer connected to a multipoint sampler. The lower detection limit with this system is 0.09 ppm for SEV and 0.1 ppm for N2O. Before starting the study, the system was calibrated for each gas (with 20.8 ppm SEV in pure nitrogen to provide an accuracy of ±2% over the entire relevant range). Appropriate compensations were included to neutralize the potential confounding effects of humidity, air pressure, and temperature. We similarly compensated for potential interactions among SEV, N2O, isopropanol, water, and carbon dioxide. The anesthesiologist was blinded to the waste gas measurements.
Exposure values were calculated as follows: 1) the mean trace concentrations were calculated for each anesthesia; 2) time-concentration products were calculated for each anesthesia; 3) the sum of all time-concentration products was divided by the sum of all exposure times during anesthesia; and 4) the resulting value gives a time-weighted average relating to the period of anesthetic administration. The intention was to detect a 30% difference between the groups with a common SD 1.3-fold of the difference. With 11 probands in each group, we could reach a minimum power of 85% at P < 0.05. The post hoc power of the study was >95% (G-Power for Macintosh, Buchner, Trier, Germany). Data are separately presented for each measurement point as the average exposure of each anesthetic, as medians with 10th, 25th, 75th, and 90th percentiles, and as time-weighted averages for all anesthetic procedures. Values (mean ± SD) were compared by using parametric tests (analysis of variance), and P < 0.05 was considered statistically significant.
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Results
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Patient characteristics are shown in Table 1. Besides the difference of the peak pressure values, no significant differences of any variable were observed between the groups. LMA and COPA could be applied at the first attempt in all patients, ventilatory support was sufficient, and no differences in ventilatory or respiratory variables were observed between the groups. There were no respiratory or cardiopulmonary complications in either group. The vaporizer settings were approximately 3 vol%, resulting in an end-tidal SEV concentration of approximately 2 vol%, and were comparable in all groups.
The distributions of the average waste anesthetic gas concentrations observed during each procedure are presented as box plots (median and 10th, 25th, 75th, and 90th percentile) in Figure 1 (Groups FM, COPA, and LMA). The trace gas concentrations (mean ± SD) at the patients’ mouths observed during anesthesia were 46.5 ± 19.6 ppm SEV and 750.7 ± 308.3 ppm N2O with the FM, 8.1 ± 12.2 ppm SEV and 213.3 ± 289.2 ppm N2O with the COPA, and 18.5 ± 25.8 ppm SEV and 283.4 ± 361.0 ppm N2O with the LMA. The waste gas concentrations during FM ventilation were significantly different from those during COPA and LMA, whereas the values between COPA and LMA did not differ significantly.
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Figure 1. Distribution of average sevoflurane and nitrous oxide concentrations for each patient at each measurement point (median and 10th, 25th, 75th, and 90th percentiles). AnSev = the location of the anesthesiologist’s breathing zones and measured substance sevoflurane; AnN2O = nitrous oxide at the location of the anesthesiologists’ breathing zones; PatSev and PatN2O = sevoflurane and nitrous oxide, respectively, at the location of the patients’ mouths; FM = face mask; LMA = laryngeal mask airway; COPA = cuffed oropharyngeal airway.
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This resulted in a contamination of the anesthesiologists’ breathing zones of 0.5 ± 0.2 ppm SEV and 5.7 ± 4.8 ppm N2O for the COPA group, which was comparable with 1.0 ± 0.9 ppm SEV and 12.2 ± 14.3 ppm N2O for the LMA group. However, both differed significantly from the FM group (2.2 ± 0.9 ppm SEV and 37.5 ± 14.3 ppm N2O).
Concerning potential violations of the threshold values (SEV: 0.5, 2, 20, or >20 ppm; N2O: 25, 50, 100, or >100 ppm) according to the different international occupational standards (3), data are presented as a percentage of time (Table 2). Measured concentrations at the anesthesiologists’ breathing zones were mostly within the strict occupational standards of NIOSH, and all were within the European occupational standards for N2O in the COPA and LMA groups, whereas in the FM group, NIOSH exposure limits for N2O (25 ppm) were exceeded during 60% of the time of anesthesia. For SEV, an exposure limit of 2 ppm was exceeded during 41% of the anesthesia time during FM, whereas this occurred only during 12% of the time with LMA and during 1% of the time with COPA.
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Discussion
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The main finding of our study was that the COPA provided a sufficient airway sealing during short surgery procedures and resulted in significantly decreased contamination through waste anesthetic gases compared with the FM. Because the COPA provides only supraglottic ventilation and does not seal the trachea directly, we expected leakage and significant environmental waste anesthetic gas contamination, especially at the patients’ mouths. However, there are few data available in the literature about the tightness of the COPA compared with the conventional mask. The observed small waste gas concentrations of the COPA may be explained by its favorable characteristics that avoid relevant leakage. The COPA provided more effective ventilation in positive-pressure ventilation than the FM (8). Nakata et al. (11) compared the COPA with a tracheal tube and found comparable end-tidal PCO2 measurements in spontaneously breathing adult patients.
Another interesting finding was that using the COPA resulted in waste gas concentrations comparable to those during the usage of the LMA, both at the patients’ mouths and for the anesthesiologist. This had not been expected, because in a comparison of the LMA and COPA in anesthetized adult patients, the LMA showed significantly superior airway stability together with a significantly lower oropharyngeal leak pressure of the COPA compared with the LMA (12). However, this difference of the leak pressure (16 vs 21 cm H2O) may not be relevant in spontaneously breathing patients and may therefore explain the comparable waste gas contamination of COPA and LMA in our study.
Casati et al. (13) determined a smaller target plasma concentration of propofol for the placement of the COPA versus the LMA. Consequently, our finding could be due to a lower dosage of anesthetics at induction for the COPA and an earlier transition to spontaneous breathing after placement of the COPA compared with the LMA group; this could lead to a lower ventilatory peak pressure. However, in this study, the induction of anesthesia was standardized by using propofol until loss of the eyelash reflex before placement of either airway, and the dosage was equal in all groups. Furthermore, we observed lower ventilatory peak pressures in the COPA group compared with the other groups. However, the absolute leak pressures of both COPA and LMA were far below the oropharyngeal leak pressure of both airways and gave proof of spontaneous breathing in all groups. A difference of 3.3 mm Hg (LMA versus COPA) and 4.8 mm Hg (FM versus COPA) was therefore clinically not relevant.
OR anesthetic waste gas concentrations during the usage of both the COPA and the LMA were considerably smaller than during the use of the FM. Our COPA data are comparable to those obtained during tracheal intubation (14) and induction with the FM reported during adult and pediatric anesthesia (5,15). In the COPA and LMA groups, measured concentrations at the anesthesiologists’ breathing zones were mostly within the strict occupational standards of NIOSH, and all were within the European occupational standards, indicating the occupational safety of both the COPA and the LMA. Our study showed that the FM is a potential source of environmental waste gas exposure. NIOSH exposure limits for N2O (25 ppm) were exceeded 60% of the time of anesthesia at the anesthesiologists’ breathing zones with use of the FM.
Whether this resulting chronic exposure to trace concentrations of inhaled anesthetics may lead to unfavorable health consequences is still under discussion. Nevertheless, most international health authorities set occupational standards ranging from low to relatively high levels, demonstrating their uncertainty in this field. Whether these occupational standards are safe or not is still unclear. Recently published data clearly demonstrate that chromosomal damage can be detected not only with high exposure (16), but even with exposure that is well within the recommended standards (3,4). However, no long-term outcome of these changes could be established. Although these studies investigated the effects of combined exposure to isoflurane and N2O, it seems necessary to measure and control the amount of waste anesthetic gases during different OR settings, different standard anesthetic procedures, and different airway devices.
We conclude that the use of the COPA for airway management in patients undergoing short surgical interventions is not necessarily associated with increased waste gas exposure, especially when air conditioning and scavenging devices are available. During COPA use, the NIOSH maximal SEV exposure levels were exceeded in only approximately 1% of total anesthesia time, whereas European occupational standards were never violated. Similar waste gas concentrations were demonstrated during anesthesia with the LMA. The COPA represents a tight-sealing airway without an increased risk of environmental air pollution. The COPA is environmentally as safe as the LMA and may be the better alternative to the traditional FM.
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Footnotes
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Presented in part at the annual meeting of the European Society of Anaesthesiologists, Gothenborg, Sweden, April, 2001.
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Accepted for publication January 16, 2002.
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Abstract
Introduction
