Isoflurane Waste Anesthetic Gas Concentrations Associated with the Open-Drop Method

Abstract

The open-drop technique is used frequently for anesthetic delivery to small rodents. Operator exposure to waste anesthetic gas (WAG) is a potential occupational hazard if this method is used without WAG scavenging. This study was conducted to determine whether administration of isoflurane by the open-drop technique without exposure controls generates significant WAG concentrations. We placed 0.1, 0.2, or 0.3 ml of liquid isoflurane into screw-top 500 or 1000 ml glass jars. WAG concentration was measured at the opening of the container and 20 and 40 cm from the opening, a distance at which users likely would operate, at 1, 2, or 3 min WAG was measured by using a portable infrared gas analyzer. Mean WAG concentrations at the vessel opening were as high as 662 ± 168 ppm with a 500 ml jar and 122 ± 87 ppm with a 1000 ml jar. At operator levels, WAG concentrations were always at or near 0 ppm. For measurements made at the vessel opening, time was the only factor that significantly affected WAG concentration when using the 500 ml jar. Neither time nor liquid volume were significant factors when using 1000 ml jar. At all liquid volumes and time points, the WAG concentration associated with using the 500 ml container was marginally to significantly greater than that for the 1000 ml jar.

Isoflurane is the preferred gas anesthetic agent in many animal research facilities. This agent typically is delivered to large animal species by using a precision vaporizer; an alternative technique often used for rodents in research is the open-drop method. This method is inexpensive and simple to use, requiring only a container (such as a jar with a removable top), absorbent material (for example, cotton), and an apparatus to separate the animal physically from the liquid agent. The liquid agent is placed on the absorbent material, and the material is placed in a closed container with the animal while the liquid vaporizes, thereby anesthetizing the patient. The principle limitation with this delivery technique is that it is suitable only for procedures of short duration, such as blood collection.

Use of the open-drop method requires careful considerations relevant to patient safety. The liquid agent is highly irritating to mucous membranes^8,15 and therefore cannot contact the animal. At our institution, cotton gauze or paper towel are placed inside a histology tissue cassette to prevent contact (Figure 1 A). Alternatively, some scientists use a gas-permeable physical barrier (such as screening) to separate the animal from the isoflurane-soaked medium. In addition, users need to be aware of the gas concentration that can be generated by a delivered volume of liquid. Guidelines regarding the amount of liquid isoflurane necessary to achieve a desired, safe gas concentration when used at standard temperature and pressure have been published;³ however, the use of excessive liquid could result in gas saturation and concentrations as high as 32%,⁵ a concentration that can be lethal to the patient. Furthermore, the recommendations for safe liquid anesthetic volumes are based on the assumption that isoflurane behaves as an ideal gas. Although this assumption may be accurate, isoflurane gas generated from a known volume of liquid is 2.96% ± 0.29% higher than that predicted by ideal-gas laws.⁶ These facts warrant careful patient monitoring when using the open-drop technique.

Figure 1.

(A) The open-drop jar arrangement used in this study. An absorbent medium (for example, paper towel) is placed inside a histology tissue cassette to ensure that the patient will not contact the liquid agent, and liquid isoflurane is added once the cassette is in place. The lid is placed on the jar while the isoflurane vaporizes. (B) The 2 jars used in this study. The 500-ml, 8.89-cm (3.5-in.) jar is on the left and the 1000-ml, 16.51-cm (6.5-in.) jar on the right.

Finally, as with any use of a volatile anesthetic, minimizing waste anesthetic gas (WAG) exposure is important for personnel safety. The Occupational Safety and Health Administration has not established permissible exposure levels for isoflurane, nor has the National Institute of Occupational Safety and Health (NIOSH) developed specific recommended exposure levels.⁷ This is because Criteria for a Recommended Standard Occupational Exposure to Waste Anesthetic Gases and Vapor¹⁰ was published prior to the advent of isoflurane.⁷ This being the case, the NIOSH recommended exposure level of 2 ppm over a 60-min period for other halogenated anesthetic agents such as halothane occasionally is cited as a safe exposure limit for isoflurane.¹⁰ However, whether this limit is valid is debatable because isoflurane is generally considered to have fewer negative health consequences compared with the agents evaluated by NIOSH in 1977.^2,9,14 Specific guidelines for isoflurane are likely forthcoming,⁷ but in the meantime, the 2-ppm threshold is perhaps the best available information.

When using standard vaporizer delivery systems, WAG scavenging usually is accomplished either actively through the use of vacuum and exhaust systems or passively with activated charcoal canisters. These controls are not an inherent part of using the open drop method because of the simplicity of that apparatus used. One engineering control that is recommended to minimize WAG pollution is a chemical fume hood.¹ This measure is only a recommendation, and use of the open-drop method on the laboratory bench without WAG exposure control remains common practice (Per obs). Usage in this way creates the potential for personnel exposure to waste isoflurane. The level of exposure, however, has not been previously reported. The current study was designed to determine the concentration of WAG to which personnel could be exposed when using the open-drop technique without exposure controls to administer isoflurane anesthesia.

Materials and Methods

Two screw-top glass jars were used as representative open-drop induction chambers; the small jar was 8.89 cm (3.5 in.) tall and 500 ml in volume, whereas the large jar was 16.51 cm (6.5 in.) tall and contained a volume of 1000 ml (Figure 1 B). The diameters of the openings were the same for each jar and measured 8.25 cm (3.25 in.). For each experiment, a standard histology tissue cassette containing a piece of paper towel measuring approximately 2.5 × 2.5 cm (1 × 1 in.) was placed in the jar (Figure 1 A). Liquid isoflurane at a volume of 0.1, 0.2, or 0.3 ml was withdrawn from the stock by using a tuberculin syringe and placed onto the paper towel, and the lid was placed immediately on the jar. The jar was kept closed for 1, 2, or 3 min, after which time the lid was removed. These liquid volumes and time points were chosen because, in our experience and on the basis of calculated values in the literature,³ they represent a reasonable array of conditions for anesthesia induction using the open-drop method. After removal of the lid, the tip of an ambient gas analyzer sampling probe (MIRAN SapphIRE, Foxboro, MA) was placed directly over the center of the jar opening. Gas concentration was measured for 30 s, a period sufficient to allow gas concentrations to peak and then recede. This procedure was repeated 3 times for each jar volume, volume of liquid isoflurane, and time period. Between replications, the gas concentration inside the chamber was measured and verified to be 0. This procedure then was repeated with the gas sampling probe suspended 20 cm above the jar opening center immediately after lid removal, and again at 40 cm above the jar opening, heights which were considered to reasonably represent an operator's breathing zone. During all experiments, the room temperature was approximately 21 °C with 10 to 13.8 air changes hourly. The average atmospheric pressure at our resident elevation of 1010 feet is 0.96 atmosphere. All work with isoflurane was done in accordance with Emory University policies; under the conditions of the experiment, only nitrile gloves, a laboratory coat, and protective eyewear were required. In addition, experiments were not conducted in a fume hood because doing so would compromise the results.

Statistics.

All data were analyzed statistically by using Graphpad Prism software (Graphpad Software, La Jolla, CA). To determine the effect of the volume of liquid isoflurane and time allowed for vaporization on WAG concentration, 2-factor ANOVA was performed. To analyze for differences in WAG concentration between the 2 jar volumes at each time point and liquid volume, data were analyzed by using the Student t test. Finally, to determine whether distance from the jar opening affected WAG concentration, 1-way ANOVA was performed. A P value less than 0.05 was considered to indicate a significant difference in all tests.

Results

At the level of the chamber openings, average WAG concentration ranged from a low of 125.67 ppm in the 500-ml jar and 43.87 ppm in the 1000-ml jar when exposure to 0.1 ml for 1 min was used to a high of 662.6 ppm in the 500-ml jar and 122.77 in the 1000-ml jar when 0.2 ml was used for 3 min (Figure 2 A, B). The use of 0.3 ml for 3 min in the 500- and 1000-ml chambers resulted in WAG concentrations of 490.47 and 115.63 ppm, respectively, slightly lower concentrations than those seem when using 0.2 ml. At the vessel opening, time was the only significant (P < 0.0001) factor that affected WAG concentration when the 500-ml jar was used. Neither time nor liquid volume was a significant factor when the 1000-ml jar was used. The WAG concentration was always greater when the 500-ml jar was used than when the 1000-ml jar was used (Figure 2 A, B). The differences did not reach statistical significance (P = 0.057) when 0.1 ml was used for 3 min but were significant (P < 0.05) for all other liquid volumes and time points. Although the effect of time and liquid anesthetic volume was not always statistically significant, a clear trend of higher WAG concentrations when greater liquid volumes were used and when more time was allowed for liquid volatilization was evident.

Figure 2.

WAG concentrations measured at the level of the opening of the (A) 500-ml jar and (B) 1000-ml jar. Data analysis using 2-factor ANOVA showed that time was the only significant (P < 0.0001) factor affecting WAG concentration when the 500-ml jar was used; neither time nor liquid volume significantly effected WAG concentration during use of the 1000-ml jar. There is, however, a clear trend toward higher WAG concentrations with greater time and liquid volume in all cases. A Student t test showed that using the 500-ml jar generated WAG concentrations that were significantly (P < 0.05) greater than those from the 1000-ml jar at all time points and liquid volumes except 0.1 ml for 3 min (P = 0.057). Bars represent SEM.

WAG concentrations at the level of the chamber opening and at the operating distances of 20 and 40 cm at each time point with a given liquid volume were measured. WAG concentrations at both operating distances were nearly always 0 ppm with the following exceptions: For the 500-ml jar, trace concentrations of 1.33 ppm occurred at 20 cm when 0.3 ml was used for 2 min, and 1.1 and 0.33 ppm at 20 and 40 cm, respectively, were obtained when 0.3 ml was used for 3 min. For the 1000-ml jar, 0.467 and 0.033 ppm were found at 20 and 40 cm, respectively, at a volume of 0.3 ml and time of 3 min (Data not shown). The WAG concentration was always significantly (P < 0.005) greater at the level of the jar opening compared with either operating distance. The difference between WAG concentrations at 20 and 40 cm was not significant at any time point or liquid volume.

Discussion

The purpose of this study was to determine the WAG concentration to which personnel are exposed when the open-drop technique is used without exposure controls to administer isoflurane anesthesia. The only WAG scavenging system that is practical when using the open-drop method is a chemical fume hood or a flexible ‘snorkel’ hood system, and the use of a fume hood is the general recommendation within the veterinary profession.¹ This practice is only a recommendation, and no national policies or regulations mandate the use of such control measures. Therefore, the open-drop technique frequently is performed on an open bench in general procedural space primarily for purposes of convenience or because no appropriate scavenging equipment, such as a chemical fume hood, is readily available. This practice clearly creates a situation where operator exposure to WAG could occur. While most data to date show no negative health effects secondary to isoflurane waste gas inhalation,⁴ it is nevertheless prudent to minimize exposure. We showed that WAG concentrations in the operator breathing zone were low, suggesting that in typical scenarios, the exposure is minimal.

At reasonable working distances of 20 or 40 cm from the chamber opening, WAG concentrations were at trace levels or 0 ppm. This result generally agrees with other similar studies of WAG in laboratory animal facilities. For example, 1 study showed that when maintaining isoflurane anesthesia in rats using a mask, WAG concentrations were often greater than 100 ppm at the mask opening, but concentrations ranged from 0 to 6.5 ppm when measured 25 cm from the mask, the distance designated as the anesthetist's breathing zone.¹² When isoflurane anesthesia was maintained in rabbits by using a laryngeal mask airway, similar findings were reported: WAG at the oral commissure averaged 8.4 ppm but was 0 ppm in a 45-cm breathing zone.¹³ The findings of the current study along with other reported data show clearly that WAG concentration near the emission point can be high while concentration in the breathing zone and consequently, operator exposure, is negligible.

Although the current study shows that WAG concentrations in the breathing zone low, different work conditions could increase personnel exposure. As already discussed, the distance between operator and the source of gas emission is important: working closer to the source will likely increase exposure. Although not examined as a variable in the current study, the ventilation rate within the area of operation is also important.¹² When all conditions were identical but the number of air changes per hour increased from 30 to 42, WAG in the breathing zone dropped from levels as high as 6.5 ppm to 0 ppm.¹² Presumably, the converse is also true, and using the open-drop method in a laboratory space with lower air turnover might increase exposure. In this study, using the smaller, 500-ml chamber resulted in greater WAG concentration at the opening. This finding probably reflects a combined effect of the greater volume and taller height of the 1000-ml jar. Given that isoflurane is approximately 6 times heavier than air (according to the manufacturer's Material Safety Data Sheet), the vapor will settle in the bottom of the chamber, and less would be expected to escape from a tall compared with short chamber. The use of other chamber configurations could result in greater WAG exposure. Finally, as the time allowed for liquid volatilization increases, so does WAG concentration. For example, with 0.3 ml in the 500-ml jar, WAG concentration increased from 351.17 ppm at 1 min to 515.5 ppm at 2 min at the jar opening. In our experience, an adequate plane of anesthesia typically is achieved by about 1 min, but this amount of time may occasionally be exceeded, thereby increasing WAG concentration. In addition, at higher temperatures or lower atmospheric pressure, liquid isoflurane will vaporize more rapidly, creating higher WAG concentration. Therefore, although the open-drop technique was studied here under controlled, representative conditions, personnel may use this method under conditions that vary widely, and WAG concentration could vary accordingly.

The question remains whether the degree of WAG exposure presents a significant health risk. If the NIOSH recommended exposure level of 2 ppm for other halogenated gas anesthetics is used as a benchmark, then the data from this study suggest that the open-drop technique is reasonably safe to perform without exposure controls. However, low concentrations of WAG in the breathing zone were apparent under some conditions. This being the case, the risk for health effects is something greater than 0. In simplest terms, risk characterization involves an assessment of dosage and duration of exposure.¹¹ Personnel might perform the open-drop technique literally hundreds of times over years of work, so that although the dosage is small, as shown in this study, the duration of exposure and cumulative dose could become substantial over time. Formal risk assessment is beyond the scope of this study, but these issues should be considered should any policy regarding use of this anesthesia delivery method be formulated at an institution.

The results of this study suggest that use of the open-drop technique to administer isoflurane without scavenging of WAG represents a minimal health risk to the operator. We make the following recommendations to maximally mitigate the small occupational risk that does exist: minimize time for vaporization, use a feasibly large and tall chamber, maintain a reasonable working distance from the chamber opening when the top is removed, and work in an area with good air circulation. The alternative is to perform work in chemical fume hood, the safest option. Following these recommendations will create the safest possible work environment when using the open-drop technique.