Development of a prototype anti-pollution filter for volatile anesthetics

Abstract

Introduction:

The use of volatile agents during cardiopulmonary bypass allows a “single drug anesthesia” and is associated with reduced peak postoperative troponin levels. Connecting the exhaust systems to the oxygenator’s gas outlet port is mandatory and allows to prevent operating room (but not atmospheric) pollution by volatile agents. The aim of this study was to create a prototype filter for volatile agents and to test its adsorption efficacy during an ex-vivo simulated conventional cardiopulmonary bypass test.

Methods:

We carried out bench tests to conceive a prototype filter that could prevent room and environment pollution without damaging membrane oxygenators. We performed the tests at the Engineering Center for Circulatory Assistance Laboratory, Dante Pazzanese Institute of Cardiology, São Paulo, Brazil. Bench tests included simulation of integral adsorption tests, filter dimensions and design, flow versus pressure curve, sizing, and tightness.

Results:

Calgon AT 410 was the best kind of activated charcoal granules for adsorption of sevoflurane, isoflurane, and desflurane. Filter dimension tests showed that a chamber of 30-cm width over 10-cm diameter filled with 200 g of the Calgon AT410 granules was the minimum required to fully adsorb sevoflurane for 90 min. Adsorption tests showed that the prototype filter fully adsorbed isoflurane in 100 ± 2.3 min, sevoflurane in 95 ± 3.4 min, and desflurane in 95 ± 4.3 min.

Conclusion:

The new version of our prototype filter adsorbed most of the volatile anesthetics agents during an ex-vivo simulated conventional cardiopulmonary bypass test.

Keywords

Filter air pollution anesthesia volatile anesthetics cardiopulmonary bypass artificial kidney apheresis & detoxification techniques

Introduction

Cardiopulmonary bypass (CPB) replaces the function of the patient’s lungs and heart during cardiac surgery, maintaining adequate oxygen and carbon dioxide levels. It includes the heart-lung machine (with all the cannula and tubing apparatus, plastic reservoirs, air aspirators, and blood pumps) and the oxygenator (which is responsible for the gas exchange and blood temperature control). Oxygenators are made mostly of polypropylene membranes that allow volatile anesthetics agents (VAA) to reach the circulating blood of the patients.¹

VAA have been used during CPB for decades, theoretically allowing a “single drug anaesthesia” technique. Evidence of their cardioprotective properties and associated reduction in mortality led to an increase in their use during CPB, since their cardioprotective properties might be related to the administration modalities.^2–5 Other benefits of using this technique are the reduction of CPB-induced inflammatory response, of the systemic vascular resistance and the improvement in cerebral oxygen saturation.^6–8

The use of VAA during CPB requires special precautions like connecting exhaust systems to the oxygenator’s gas outlet port, which are mandatory to prevent operating room (OR) pollution. Although no cause–effect relationship has been demonstrated yet, most public health authorities recommend (heterogenous) occupational exposure standards to minimize possible health risks of the operating theater personnel.^9–12 However, this does not prevent the subsequent elimination of VAA into the environment contributing to ozone depletion in the stratosphere and to the greenhouse warming in the troposphere.¹³ In this context, we created a rudimentary prototype filter essential to prevent pollution of the room and the environment at the same time.^14,15

The main aim of this study was to create a new version of the prototype filter based on the rudimentary version, and to test its efficacy in adsorbing the most commonly used VAA during an ex-vivo simulated conventional CPB.

Methods

Based on the prototype filter tested in Phase I,¹⁴ which had a rectangular shape and the external part made of plastic (dimensions: 5″×3″×9″) for the preservation of adsorbing elements, we chose a different design and compounds for the filter chamber to build our new version. The cylindrical shape made with glass fiber–reinforced polyethylene was chosen, because of (a) better dispersion of the vapor allowing for a greater contact of the vapor with the activated charcoal granules, increasing its adsorption efficiency; (b) no interaction between the chamber compounds with the vapors; (c) good seal for possible leaks; and (d) lightness (for handling). The inside of the main chamber was divided into two portions with a fenestrated internal partition to better join the charcoal granules. Moreover, a polypropylene cartridge with 5-µm porosity (10-mm thickness) was placed inside the filter chamber before the gas outlet extremity to work as a dust filter (Figure 1).

Figure 1.

Diagram illustrating the final version of the anti-pollution filter.

A cross section of the device is presented in Figure 2, showing the gas flow inside the filter and the barriers that the gases have to overcome to exit.

Figure 2.

A cross section of the device.

Bench tests

After choosing chamber design and compounds, bench tests were carried out in the Engineering Center for Circulatory Assistance Laboratory, Dante Pazzanese Institute of Cardiology, São Paulo, Brazil.

During tests, VAA percentage was measured with the gas analyzer of a Dräger Primus ^® anesthetic machine (component: AMO ILCA2 PCB). We considered the VAA fully adsorbed only when the gas analyzer measured zero. All joints were checked and painted with soapy solutions to make gas-tight connections, ensuring there was no leakage. Measurements of pressure gradient were made using the pressure transducer of the anesthetic workstation—Dräger Primus ®. The activities required were based on our clinical experience and focused on overcoming the technical and scientific challenges we encountered at each stage in a sequence, as follows.

Bench tests of different activated charcoal

To choose the best charcoal to fill the internal part of the filter, we tested Calgon AT 410 and Calgon WS 480 granules (Calgon Carbon Corporation, Pittsburg, PA, USA) and FBC 4x10 granules (Fábrica Brasileira de Catalizadores, Contenda, Brazil). Arbitrarily, five different sizes of prototypes were machined, assembled, and filled with the granules: size 1 (20 g), size 2 (120 g), size 3 (200 g), size 4 (270 g), and size 5 (300 g). They were connected to a 3-L/min gas flow mixed with 2% sevoflurane vaporized in a 100% oxygen, for 100 min. These tests were done to determine the best granule suitable to fully adsorb the sevoflurane according to amount and time.

Bench tests to determine filter dimensions

The best prototype obtained on the previous phase was connected to a Dräger Primus ^® anesthetic workstation. At this stage, we used a fixed concentration of 3% sevoflurane vaporized in a 3-L/min flow of 100% oxygen for 90 min. Measurements of sevoflurane were made after the outlet gas port of the filter. When the sevoflurane was fully adsorbed for at least 90 min, we reduced the amount of the charcoal and chamber’s size by 10%. We continuously repeated the test, until we reached the minimum amount of charcoal inside the chamber that was needed to fully adsorb the sevoflurane for at least 90 min. Every time we changed the amount of charcoal granules in the filter and reduced the chamber’s size, we positioned the cylinder with its axis of symmetry vertical to prevent channeling.

Prototype filter adsorption tests

Based on our experience and daily clinical practice, we simulate a situation of conventional CPB and tested the filter to adsorb the most commonly used VAA (sevoflurane, isoflurane, and desflurane) along the time. We predefined an evaluation form with different concentrations of the used VAA vaporized in different gas flows, varying the exposed time (Evaluation form 1—Supplemental Material), and follow its sequence during the tests. These tests were performed connecting the filter to a Dräger Primus ^® anesthetic workstation, using different concentrations of the vaporized VAA in 100% oxygen, varying gas flows, and the exposed time. Measurements of the VAA (sevoflurane, isoflurane, and desflurane) were done right after the outlet gas port of the filter. We tested five filters for each VAA used (Figure 3).

Figure 3.

Simulation of the prototype filter integral adsorption tests. A. Outlet gas port of anesthesia machine, B. Filter inlet port, C. Filter tested, D. Filter outlet port, and E. Gas reader connection.

Creating different sizes of prototype filter

Based on the amount of charcoal and the dimensions of the filter that we found on test 2, we increased or reduced the amount of the activated charcoal and filter’s chamber size by 30%, creating different filter sizes. We considered the last prototype version obtained on test 2 as “medium size,” those with 30% more charcoal as “large size,” and those with 30% less as “small size.”

Prototype filter pressure versus flow tests

First, we randomly predefined an evaluation form with different gas flows over time (Evaluation form 2—Supplemental Material) to be followed during the tests. Measurements of pressure gradient at different flows were made across the prototype filters, using the pressure transducer of the anesthetic workstation—Dräger Primus ^®. We connected different sizes of the filter to a 100% oxygen gas flow, increasing it every 10 min to a maximum of 15 L/min. These tests were performed in groups of ten filters of the three different sizes.

Prototype filter tightness tests

At this stage, the components and connections of the filter chamber were tested for leakage. We connected each different filter size to a high-pressure airflow. Pressures as high as 500 mmHg were created inside the chamber immersed in liquid, observing the appearance (or not) of air bubbles.

Statistics: Data were stored electronically and analyzed using STATA software, version 16 (StataCorp, College Station, TX), and the level of significance adopted was p <0.05. For comparison of pressure versus flow and adsorption time tests, repeated measures analysis of variance (ANOVA) test was used. Descriptive data are shown as median and inter-quartile range (IQR), mean and standard deviation (m ± SD), or number and percentage (n (%)) as appropriate.

Results

During the bench tests of different activated charcoal (test 1), the Calgon AT410 granules present in the prototype filter size 5 had the best efficiency to fully adsorb the sevoflurane (size 5: 300-g AT 410 = 100 min; 300-g WS 480 = 50 min; 300-g FBC 4×10 = 60 min) (Figure 4).

Figure 4.

Bench tests of different activated charcoal to fully adsorb the sevoflurane according to amount and time.

Filter dimension tests showed that a chamber of 30-cm width over 10-cm diameter filled with 200 g of the Calgon AT410 granules was the minimum required to fully adsorb sevoflurane for 90 min.

Adsorption tests showed that the prototype filter fully adsorbed isoflurane in 100 ± 2.3 min, sevoflurane in 95 ± 3.4 min, and desflurane in 95 ± 4.3 min (Table 1). No statistical difference was found when comparing adsorption time between isoflurane and desflurane (p = 0.13), sevoflurane and desflurane (p = 0.99), and isoflurane and sevoflurane (p = 0.49).

Table 1.

Prototype filter adsorption tests.

Volatile anesthetic agent	Fully adsorption time(Medium size filter)(m ± SD) (min)
Sevoflurane	95 ± 3.4
Desflurane	95 ± 4.3
Isoflurane	100 ± 2.3

Dimensions tests created three different filter sizes: small—22-cm width over 10-cm diameter filled with 140 g of activated charcoal; medium—30-cm width over 10-cm diameter filled with 200 g; large—38-cm width over 10-cm diameter filled with 260 g.

Tests of pressure versus flow showed that the chamber’s pressure increased when the filters were exposed to higher flows. Pressures created by the different flows were statistically different among the three different filter sizes (Table 2).

Table 2.

Prototype filter tests of pressure versus flow.

Time(min)	Gas flow (L/min)	Filter’s chamber pressure (mmHg) (m ± SD)			p value
Time(min)	Gas flow (L/min)	Small filter(n = 10)	Medium filter(n = 10)	Large filter(n = 10)	p value
0–10	1	0.73 ± 0.35	0.73 ± 0	0.15 ± 0.31^a	0.0004
11–20	3	2.57 ± 0.87^b	0.91 ± 0.71	1.62 ± 0.31	0.003
21–30	5	4.26 ± 1.58	3.97 ± 2.06	2.20 ± 1.10^a	0.008
31–40	7	6.39 ± 2.60^b	5.29 ± 1.76	3.60 ± 2.23^a	0.009
41–50	9	8.21 ± 2.73^b	7.06 ± 2.80	5.07 ± 2.34^a	0.01
51–60	11	10.00 ± 3.77	9.34 ± 3.64	6.69 ± 2.65^a	0.03
61–70	13	12.28 ± 4.01^b	11.10 ± 4.14	8.23 ± 3.50^a	0.02
71–80	15	15.00 ± 5.50^b	13.60 ± 5.17	10.29 ± 4.10^a	0.04

The p values refer to the comparison among the three groups (analysis of variance (ANOVA) test).

Indicates that the large filter values are significantly different from both the small and medium filters (t test).

Indicates that small and medium filters values are significantly different (t test).

Tightness tests showed no leakage when large and medium filters were exposed to a high-pressure airflow into the chamber (500 mmHg). Small filters presented leakage when exposed to 120 mmHg or higher.

Discussion

The new version of the prototype filter adsorbed most of the VAA during an ex-vivo simulated conventional CPB.

Ideal VAA filters connected to membrane oxygenators during CPB should not create an overpressure that could force the gas through the membrane fibers and the blood, potentially causing air embolism. They also have to adsorb efficiently the three most commonly used VAA at different concentrations, different mix of gas flow, and have to last for the length of time of a conventional CPB. At the end of the procedure, the main chamber and all its internal contents should be recycled. Activated charcoal filters are already available for absorption of anesthetic vapors (e.g. the Cardiff Aldasorber; Shirley Aldred & Co Ltd, Brough, UK). However, they are exclusively designed to be connected to the exhaust gases of anesthetic workstations. They were not designed, not tested, and should not be connected to the membrane oxygenators.

Activated charcoal is a form of carbon processed that increase the surface area available for adsorption or chemical reactions. Due to its high degree of microporosity, just one gram has a surface area of more than 500 m².¹⁶ Activated charcoal filters are used to remove residual VAA from anesthetic workstations in patients susceptible to malignant hyperthermia, although they last for a short period of time.¹⁷ In our study, the prototype filter filled with 200 g of Calgon AT410 granule was efficient to adsorb the three most commonly used VAA (sevoflurane, isoflurane, and desflurane) for at least 90 min, the length of a conventional CPB (Table 1).

The American National Standards Institute (ANSI) addressed the anesthetic gases elimination systems applied to either oxygenators or anesthesia devices (standard ANSI Z79.11). According to this regulation, the gas scavenger pattern of membrane oxygenators should not generate positive pressures higher than 10 cm of water (7.4 mmHg) or negative pressures lower than −0.5 cm of water (−0.37 mmHg).¹⁸ These precautions ensure that the gas withdrawal system does not adversely affect membrane oxygenators. The backpressure can force the gas through the membrane fibers into the blood flowing through it, causing air embolism. On the other hand, a strong aspiration with excessive negative pressure may result in poor oxygenation of the blood. Our results demonstrated that in the three different filter sizes analyzed during pressure versus flow tests, the mean pressure created inside the prototype filter tested was below 7.4 mmHg, even under inflow conditions of 7 L/min (large size = 3.60 ± 2.23 mmHg; medium size = 5.29 ± 1.76 mmHg; small size = 6.39 ± 2.60 mmHg—Table 2). This is the maximum inflow recommended by the majority of membrane oxygenators’ brands since pressure build-up could create hazard in gas mix and even bubbles trickling when used during conventional CPB. Therefore, our prototype is within the established safety specifications to avoid overpressure inside the membrane that could force the gas through the membrane fibers and the blood, potentially cause air embolism.

There is a well-established benefit with the use of inhalational anesthetics in cardiovascular surgery, especially during CPB, and this is an incentive to the use of this technique.^4–8 However, the risk of polluting the OR is still a matter of discussion among physicians. McNulty et al. demonstrated that there is a potential risk of room contamination from VAA even when used only immediately before CPB. To minimize the risk, routine oxygenator scavenging can be used as an alternative to simply avoiding high concentrations of inhalation anesthesia before initiating CPB.¹⁹ Most of the centers that use VAA during CPB have exhaust gas systems that emit surplus gases into the atmosphere to avoid OR pollution. De Simone et al. presented an innovative technique for VAA suction from the CPB circuit. This original device collects and disposes of any VAA present in the exit stream of the oxygenator, preventing its dispersal into the operating theater environment and adaptively regulates pressure of oxygenator chamber in the CPB circuit.²⁰ The hazard of membrane oxygenator’s damage due to suction and pollution of the OR would therefore be minimized; however, the risk of pollution of the environment would continue to exist.

Greenhouse gases emissions from hospitals cannot be ignored, despite most of the global climate changes are primarily due to CO2 emission from industry and vehicles. Waste anesthesia gases (WAG) contributes about 0.25% to the global greenhouse gases, and cardiac ORs have an extra load of CPB WAG contributing to OR pollution.²¹ The use of these filters may have a positive impact to the OR personnel as well as reduce environmental pollution.

Limitations of the study

The gas analyzer measured the residual VAA filtered waste. Unfortunately, it gives a percentage of the VAA present as waste and not parts per million per se, partly compromising the accuracy of the tests. As authors, we know that even trace amounts of vapor can cause pollution of the room. Our filters are disposable and when they are disconnected from the CPB machine, they basically behave like a sponge, and slowly release the VAA back to the atmosphere. During tests of pressure versus flow, dry gases were used to measure resistance, while in conventional CPB moisture and condensation could also be present, leading to a large effect on resistance to flow. The charcoal disposal is a concern about used filters. Generally, they are disposed off in regular landfills which results in hazardous materials, thus polluting underground and water sources. They have also been destroyed by incinerating at a high temperature. However, this type of treatment releases toxic gases.

Conclusion

The new version of our prototype filter adsorbed most of the VAA during an ex-vivo simulated conventional CPB test. Further experimental studies in animals should be performed to confirm their efficacy before using them in humans and considering them as alternative anti-pollution system for VAA when connected to the membrane oxygenators.

Supplemental Material

Evaluation_form_1_-_Supplemental_material – Supplemental material for Development of a prototype anti-pollution filter for volatile anesthetics

Supplemental material, Evaluation_form_1_-_Supplemental_material for Development of a prototype anti-pollution filter for volatile anesthetics by Caetano Nigro Neto, Aron Jose Pazin Andrade, Edir Leal, Francisco Jose Lucena Bezerra, Vladimir Lomivorotov and Giovanni Landoni in The International Journal of Artificial Organs

Footnotes

Acknowledgements

The authors would like to thank all people who helped and provided any contribution to the study. We are also indebted to all technicians from Engineering Center for Circulatory Assistance Laboratory who helped for the care provided to the tests of these filters.

Authors’ Note

Part of this work was presented at the SCA 2018 “SCA 40th Annual Meeting & Workshops Phoenix, AZ – USA,” Scientific Program Poster Presentations: Session 1. Date and Time: Sunday, April 29, 2018, from 12:30 to 1:30 p.m. Abstract title: “Improvements introduced in adsorption filter for volatile anesthetic agents for cardiopulmonary bypass.” Poster Number: SCA74.

Declaration of conflicting interests

The author(s) declared the following potential conflicts of interest with respect to the research, authorship, and/or publication of this article: Patent—C Nigro Neto. “Introduced in Adsorption Filter for Inhaled Halogenated Anesthetics for Cardiopulmonary Circulation Bypass.” Patent US9861927B2, USA, 2018.

Funding

The author(s) disclosed receipt of the following financial support for the research, authorship, and/or publication of this article: This study was supported entirely by departmental funds.

ORCID iD

Caetano Nigro Neto

Supplemental material

Supplemental material for this article is available online.

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