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The Sevoflurane Washout Profile of Seven Recent Anesthesia Workstations for Malignant Hyperthermia-Susceptible Adults and Infants: A Bench Test Study

Cottron, Nicolas MD; Larcher, Claire MD; Sommet, Agnès MD, PhD; Fesseau, Rose MD; Alacoque, Xavier MD; Minville, Vincent MD, PhD; Fourcade, Olivier MD, PhD; Kern, Delphine MD, PhD

doi: 10.1213/ANE.0000000000000208
Technology, Computing, and Simulation: Research Report

BACKGROUND: Preoperative flushing of an anesthesia workstation is an alternative for preparation of the anesthesia workstation before use in malignant hyperthermia-susceptible patients (MHS). We studied in vitro, using a test lung, the washout profile of sevoflurane in 7 recent workstations during adult and, for the first time, pediatric ventilation patterns.

METHODS: Anesthesia workstations were first primed with 3% sevoflurane for 2 hours and then prepared according to the recommendations of the Malignant Hyperthermia Association of the United States. The flush was done with maximal fresh gas flow (FGF) with a minute ventilation equal to 600 mL × 15, to reach a sevoflurane concentration of <5 parts per million. After flush, 2 clinical situations were simulated in vitro to test the efficiency of preparation: decrease of FGF from max to 10 L/min, or decrease of minute ventilation to 50 mL × 30, to simulate the ventilation of an MHS infant.

RESULTS: We report washout delays for MHS patients for previously studied workstations (Primus®, Avance®, and Zeus®) and more interestingly, for machines not previously tested (Felix®, Flow-I®, Perseus®, and Leon®). An increase of sevoflurane concentration was observed when decreasing FGF (except for flow-I® and Leon®) and during simulation of MHS infant ventilation (except for Felix®).

CONCLUSIONS: This descriptive study strongly suggests that washout profiles may differ for each anesthesia workstation. We advise the use of maximal FGF during preparation and anesthesia. Required flushing times are longer when preparing an anesthesia workstation before providing anesthesia for MHS infants.

Published ahead of print May 7, 2014

From the Department of Anesthesiology and Intensive Care, University Hospital of Toulouse, Toulouse, IFR, France.

Accepted for publication December 19, 2013.

Published ahead of print May 7, 2014

Funding: Not funded.

Conflicts of Interest: See Disclosures at the end of the article.

This report was previously presented, in part, at the SFAR annual congress (Société Française d'Anesthésie Réanimation), 2011.

Reprints will not be available from the authors.

Address correspondence to Delphine Kern, MD, PhD, Department of Anesthesiology and Intensive Care, University Hospital of Toulouse, EA 4564 MATN, IFR 150, CHRU Toulouse Purpan, place du Dr Baylac, TSA 40031, 31059 Toulouse cedex 9, France. Address e-mail to kern.d@chu-toulouse.fr.

Malignant hyperthermia (MH) is a potentially lethal inherited metabolic disorder that can be triggered by anesthetics. It is recommended that patients susceptible to MH be anesthetized using anesthesia machines free of residual volatile anesthetics.1 The preparation of anesthesia workstations for MH-susceptible (MHS) patients has been revisited because of recent publications. The established concepts were either using a dedicated “clean” machine or flushing residual volatile anesthetics from the anesthesia machine before use. According to former studies, the Malignant Hyperthermia Association of the United States (MHAUS) recommended a 20-minute flush of oxygen at a fresh gas flow (FGF) of 10 L/min.1–3 However, it has recently been demonstrated that many newer anesthesia machines may require a longer time for purging residual gases (up to 60 minutes) due to complex internal circuits containing plastic and rubber components that can absorb and subsequently release anesthetic vapor.4–12 In addition, Birgenheier et al.13 recently showed that the use of activated charcoal filters on inspired and expired limbs of contaminated Apollo® and Aestiva® anesthesia workstations (Apollo®, Dräger, Lübeck, Germany, and Aestiva®, GE Medical, Madison, WI) decreased the concentration of anesthetic vapors below 5 ppm in <2 minutes for at least 60 minutes. Also, the MHAUS had revised its recommendations and now proposes 4 alternatives to prepare anesthesia workstations:14 (1) “flush and prepare workstation according to manufacturer’s recommendations or published studies; this method requires 10 to >90 minutes,” (2) “use commercially available charcoal filters,” (3) “use a dedicated ‘vapor free’ machine,” and (4) “use an intensive care unit ventilator that has never been exposed to volatile anesthetic agent.” Guidelines for the flush of current generation anesthetic workstations for MHS patients seem to be specific for each workstation,11 and new workstations using different technologies that have recently entered the market have never been tested.15

Prompted by these observations, we completed a bench test of 7 recent anesthetic workstations in common use to report the sevoflurane washout profile in preparation for MHS patients, as recommended by MHAUS, except that we used the maximal FGF allowed by each machine.

After washout, the simulation of 2 clinical scenarios evaluated the preparation of anesthesia workstations by the measure of potential release of volatile gas. The first one evaluated whether FGF can be safely decreased from maximal to 10 L/min with adult ventilatory patterns. The second one evaluated whether minute ventilation (MV) can be safely decreased from adult to pediatric ventilatory patterns, to assess if the preparation is effective for MHS infant ventilation.

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METHODS

This experimental prospective study was performed in vitro (Fig. 1) on 7 different models of current anesthesia machines. After standardized priming, workstations were prepared as recommended by the MHAUS. Early washout phases assessed the time to reach a sevoflurane concentration of <5 ppm, the threshold that theoretically does not trigger an MH crisis,16 with the maximal FGF allowed by each workstation. A late washout phase rebound effect was then defined as an increase in sevoflurane concentration greater than 5 ppm and occurring after this early washout phase. In the adult experiment, this rebound effect was identified after reduction of FGF from maximal to 10 L/min. In the pediatric experiment, this rebound effect was identified after reduction of the MV from a typical adult to a typical pediatric ventilatory pattern (Fig. 2).

Figure 1

Figure 1

Figure 2

Figure 2

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Anesthesia Workstations and Circuit

The 7 anesthesia workstations Primus® (Dräger, Lübeck, Germany), Zeus Infinity® (Dräger, Lübeck, Germany), Perseus® (A500, Dräger, Lübeck, Germany), Avance® (GE Datex-Ohmeda, Munich, Germany), Felix AInOC® (Taema, Air liquide, Antony, France), Flow-i® (Maquet, Solna, Sweden), and Leon® (Heinen and Löwenstein, Bad Ems, Germany) had undergone maintenance by the manufacturers before the trials. Compliance and leak test were checked before each experiment. An adult or pediatric circle breathing circuit was used, depending on the experiment (Int ‘Air Medical, Bourg-en-Bresse, France, volume = 1.2 L or 0.6 L, respectively) and connected to a 1-L lung model (Test Lung 190, Maquet, Solna, Sweden), a 2-L breathing bag (Maquet, Solna, Sweden), and an adult (Hydro-guard, MK2, Intersurgical Ltd, Wokingham, United Kingdom) or a pediatric (Humid-Vent, Filter-Pedi, Teleflex Medical, High Wycombe, United Kingdom) respiratory filter.

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Sevoflurane Priming

The same sevoflurane vaporizer (Sevotec 5, GE Healthcare, Chalfont St Giles, Buckinghamshire, United Kingdom) was used for all anesthesia machines, after refurbishing and calibration, except for the Zeus®, Perseus®, and Flow-i® that were outfitted with their own vaporizer. To be certain that all workstations received the same total amount of sevoflurane during priming, 1 preliminary calibration procedure was performed on each workstation: the concentrations of sevoflurane delivered by vaporizers were measured on the Y-piece with an external gas monitor (S/5, GE Healthcare, Chalfont St Giles, Buckinghamshire, United Kingdom, accuracy of ±0.15 vol% and of 5% of the reading and a detection threshold of 0.15 vol%), and the concentration set on the vaporizer was adapted to be measured at 3%. Thereafter, workstations were primed for 2 hours with an effective 3% sevoflurane concentration. The fresh gas was set at 30/70 O2/Air with FGF at 3 L/min in controlled ventilation mode with a tidal volume of 600 mL and a respiratory rate of 15 bpm (MV = 9 L/min).

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Workstations Preparation

At the end of priming, ventilation was stopped; the vaporizer was switched off and then removed from the machine. The carbon dioxide (CO2) absorber was changed. The test lung and breathing bag were exchanged with fresh replacements or models that had been washed in an autoclave at 134°C for 10 minutes. Adult or pediatric circle breathing circuits were replaced by a new circuit according to the experiment.

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Early Washout Phases: Adult and Pediatric Experiments

Early washout phases were similar in the 2 experiments except the use of adult breathing circuits in adult experiments and pediatric breathing circuits in pediatric experiments. After priming and workstation preparation, the workstation was reconnected to the test lung, the FGF was set initially at the maximum allowed by the machine, and the controlled ventilation was repeated with the same MV. Early washout was assessed by measuring the time until the concentration of sevoflurane reached 5 ppm.

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Late Washout Phases

Adult Experiment: FGF Decrease

After the early washout phase, as the concentration of sevoflurane reached 5 ppm, the FGF was reduced to 10 L/min. If a rebound effect was noticed, defined by a measured sevoflurane concentration >5 ppm, the maximal concentration of sevoflurane was reported as Cmax, and late washout was assessed by measuring the time until the concentration of sevoflurane reached 5 ppm again.

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Pediatric Experiment: MV Decrease

After the early washout phase, as the concentration of sevoflurane reached 5 ppm, the anesthesia machine was considered ready to ventilate an MHS infant. The controlled ventilation was set with a tidal volume of 50 mL and a respiratory rate of 30 bpm (MV = 1.5 L/min) to simulate pediatric ventilation, without changing the FGF. If a rebound effect was noticed, the maximal concentration of sevoflurane was reported as Cmax, and late washout was assessed by measuring the time until the concentration of sevoflurane reached 5 ppm again.

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Measurements

Residual sevoflurane concentrations were measured in the Y-piece of a circle breathing circuit with the Photoacoustic Field Gas-Monitor–INNOVA 1412 (Lumasense®) during the washout phases. This device is able to measure a wide range of chemical substances including anesthetic vapors using infrared spectroscopy. It was calibrated for sevoflurane with a detection limit of 0.01 ppm. The gas analyzer was zeroed before experiments according to the manufacturer’s recommendation. The data were collected in a computer every minute via a RS232 cable. Adult and pediatric experiments were repeated 3 times for each workstation.

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Statistical Analysis

The purpose of this descriptive study was to assess the sevoflurane washout time of 7 anesthesia machines before ventilation of MHS patients. In this way, no comparisons were performed between workstations. For every washout, 3 independent datasets were obtained per workstation and per condition (adult or pediatric experiment). Data were reported in minutes for early and late washout and in parts per million for sevoflurane concentration, reported as median [min–max].

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RESULTS

Early Washout Phases: Adult and Pediatric Experiments

The concentration of sevoflurane decreased exponentially in the circuit for all workstations during the early washout phase as illustrated in Figure 3 and 4. Washout profiles are summarized in Tables 1 and 2.

Table 1

Table 1

Table 2

Table 2

Figure 3

Figure 3

Figure 4

Figure 4

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Late Washout Phases

Adult Experiment: FGF Decrease

When the FGF was decreased from maximal to 10 L/min, a rebound effect was observed for 5 workstations (Felix®, Primus®, Avance®, Zeus®, and Perseus®) with a maximal sevoflurane concentration and a time to reach 5 ppm again illustrated in Figure 5. Washout profiles are summarized in Table 1.

Figure 5

Figure 5

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Pediatric Experiment: MV Decrease

During late washout, when the MV was decreased to simulate the anesthesia of an MHS infant, a rebound effect was observed for 5 workstations: Flow-i®, Leon®, Primus®, Perseus®, and Avance®, with a maximal sevoflurane concentration and a time to reach 5 ppm again illustrated in Figure 6. One rebound to 5.3 ppm was measured in 1 run for Zeus®, decreasing to <5 ppm in 2 minutes. Three additional runs were repeated in this model, none of which demonstrated another instance of rebound effect. Washout profiles are summarized in Table 2.

Figure 6

Figure 6

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DISCUSSION

A review by Kim et al.15 strongly suggested the need of a comprehensive study of the washout profile of potent inhaled anesthetics for new anesthesia workstations. We report washout delays for MHS adults for previously studied workstations (Primus®, Avance®, Zeus®) and more interestingly, for machines not previously tested (Felix®, Flow-I®, Perseus® and Leon®). A rebound effect was observed in the 2 tested conditions, that is, after decreasing FGF and MV to a pediatric ventilatory pattern. This strongly suggests that the maximal FGF used during flush should be maintained at maximal during MHS patients’ ventilation. The major finding of this study is that flush time is affected by MV and that required flush times can be extreme when the MHS patient is an infant.

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EFFECT OF DECREASING FGF

Previous studies have shown that anesthetic concentration in the circuit is inversely related to FGF rate.1,4–11,13 Increasing FGF from 10 to 18 L/min decreased washout delay for Primus® primed with isoflurane 1.5% (63.6 ± 5.1 vs 48.6 ± 2.7 minutes)9 and for Zeus® primed with sevoflurane 2% (85 ± 6 vs 16 minutes).10 Nevertheless, the maximal FGF provided by the machines we have tested varies from 11 to 18 L/min, with specificities for 2 machines. The maximal FGF allowed by the Flow-i® is restrained to 2 L/min above MV. To explain, during inspiration, FGF is instilled directly into the inspiratory part of the circuit on the Flow-i®, in the absence of a fresh gas decoupling valve that redirected the FGF in the reservoir bag.17 The adjustment of the FGF to the MV is automatically applied by the device to avoid unexpected changes in tidal volume or airway pressure when FGF is suddenly increased to perform a rapid change in inspired sevoflurane or oxygen concentration. In the same way, the maximal FGF allowed by the Perseus® is restrained in case of small tidal volume or high respiratory rate. In this way, washout delays were investigated with the maximal FGF allowed by each workstation.

The rebound effect observed after decreasing FGF is null or minimal for workstations that have the lowest maximal FGF (Flow-i®, 11 L/min, Felix® and Leon®, 12 L/min). In return, the rebound effect is higher for machines for which maximal FGF is higher, reflecting the release of anesthetics absorbed during priming in plastic and rubber components of the internal circuit of the workstation.

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EFFECT OF DECREASING MINUTE VENTILATION

Our results show that decreasing MV to simulate clinically relevant pediatric ventilation produces a rebound effect, although FGF remains at a maximum flow rate. This rebound effect in pediatric ventilation has not been previously described, and anesthesiologists should be aware before administering anesthesia in MHS infants because of the risks related to this condition.18 Indeed, in our clinical practice, we currently provide anesthesia care for children suspected with neuromuscular disorders. Using muscle biopsy, the risk of MH or rhabdomyolysis is estimated to be 1% for these patients.18

This pediatric rebound effect might be explained by the reconcentration of anesthetics eluting from plastic and rubber and/or to the recirculation of pockets of volatile anesthetic trapped in the compartmentalized internal breathing circuitry. Surprisingly, we did not observe any rebound effect for the Felix®. One hypothesis is that the prolonged early washout delay of the Felix® was sufficient to definitively wash from the machine “visible and nonvisible” volatile anesthetic. This was not the case of the Leon® where chemical and design features might be responsible for significant rebound in pediatric ventilation.

For the Zeus®, a minor rebound observed in 1 run with pediatric ventilation was not reproduced on 3 other runs. This result might have been due to the “Turbovent” technology as the source of pressure, added to a constant flow inside the workstation. Despite using the same technology, a small rebound effect was observed with the Perseus probably due to an automatic decrease of FGF from 15 to 10 L/min when MV is reduced.

Some of our results may seem paradoxical but are actually related to the technical specificities of each of the anesthesia workstations tested in this study. For example, we did not observe any rebound effect for the Flow-i® workstation in the adult experiment, while the rebound effect was important in the pediatric experiment. Indeed, in the adult experiment, the effective FGF was decreased from 11 L/min ((600 mL × 15) + 2 L/min) to 10 L/min, which is probably not enough to observe a rebound effect. However, lowering MV in the pediatric experiment automatically decreases FGF to (50 mL × 30) + 2 L/min, that is, 3.5 L/min. However, the Flow-i® workstation does not allow use of a high FGF with low MV because FGF is automatically restrained when MV decreases. This information has been communicated to the manufacturer of the Flow-i® workstation. We think that the exchange of the breathing system by an autoclaved one might be a complementary action, but this would have to be evaluated.5

After the late washout phases, an additional technical maneuver, which consisted of switching from mechanical to manual/spontaneous ventilation mode using the reservoir bag, was performed on all on workstations except Leon® (this was not described in the initial experimental protocol). It is interesting to note that a last rebound effect was observed in all workstations during this maneuver, suggesting that an additional flush in manual/spontaneous ventilation mode before anesthesia, whether the breathing bag is connected in parallel or sequentially, may be warranted. This observation, however, requires further investigation to be fully validated.

Because preparation with flushing of the anesthesia workstation might be particularly time-consuming, alternatives have been proposed. One solution is to autoclave the workstations’ components (Fabius®, Primus®)5,11 or to replace the Advance Breathing System (ABS®) (Avance® or Amingo carestation®) with a laundered component.4 Using a dedicated “clean” anesthesia machine is possible, but that would require additional cost and more maintenance.6,9 Another solution is to use an intensive care unit ventilator, but such a device is not always available, and washout time is unknown for these machines.15

We have noted the recent emergence of activated charcoal filters designed to prepare anesthesia workstations for MHS patients in a matter of minutes. Gunter et al.6 first described this procedure with the QED® (Anecare Laboratories, Salt Lake City, UT), and another study demonstrates the efficiency of Vapor-Clean® filters (Dynasthetics, LLC, Salt Lake City, UT) placed on contaminated workstations (Aestiva® and Apollo®).13 MHAUS confirms the use of Vapor-Clean® filters as an alternative or in addition to flush.14 Nevertheless, several questions about these devices remain unresolved, including duration of action,19 and the decrease of MV has not been tested, so there is no evidence of their efficiency in pediatric anesthesia. Despite these limitations, their use can be particularly beneficial in 2 particular cases: during an actual MH crisis13 and to provide ventilation of an MHS patient in an emergency that cannot wait for standard washout delay. This device could also be useful for anesthesia workstations with longer washout delay.

Our study has several limitations. The threshold of 5 ppm should be discussed. We will probably never know the minimal triggering concentration, because human studies are unethical, and penetrance and expressivity of MH is variable.20 Nevertheless, no MH crisis has been reported during MHS anesthesia after workstation preparation to 5 ppm or less. According to other studies5–7,9–11,13 and in reference to an MH swine model,16 we chose this threshold, which is commonly accepted to be “safe.” Moreover, one must remember that using a contaminated anesthesia workstation in MHS patient may trigger a crisis.21

Because sevoflurane is the most commonly used volatile anesthetic for pediatric anesthesia and because of its partition coefficients for plastics and rubber, we decided to use it for the priming of the anesthesia workstations. The partition coefficient for plastics and rubber is critical, because the more soluble the gas, the more difficult it is to remove the agent from the workstation. The following order is assumed: halothane >isoflurane >sevoflurane >desflurane.22–24 Most studies were done with sevoflurane, but a recent study demonstrated longer washout time for desflurane on Aestiva® and Aisys® workstations (GE, Healthcare, Helsinki, Finland).7 We assume that our results should not be extrapolated to other gases, and further experiments are necessary to complete washout recommendations.

Even if our results are reproducible on each experiment and compatible with washout delays previously observed,7,9,10,13 many factors could influence washout profiles. Indeed, clearance of volatile anesthetic probably depends on many factors: internal volume, composition and geometry of a breathing system, FGF, sometimes MV, and obviously the type and amount of volatile anesthetic that contaminated the machine. The age of the machine or the delay from the last maintenance could also be responsible for accumulation of water or fine particles of CO2 absorbent that can absorb and release volatile anesthetics.7 This potential variability is illustrated by the results of the Zeus®, for which 3 runs showed no rebound while 1 run showed a small rebound measured at 5.3 ppm. In addition, only 1 device was tested per anesthesia workstation, and our experimental protocol did not include all FGF and MV. Based on these considerations, we recommend preparing anesthesia workstations with flushing times longer than those observed, and we recommend continuing the flush until the anesthetic is provided to avoid an increase of gas eluting from the machine in standby mode.

Despite these limitations, we believe our study is clinically useful. Indeed, before our study was conducted, there was no data about washout delays for Felix®, Flow-i®, Perseus®, or Leon® during adult ventilation and absolutely no data about washout delays for any anesthesia workstation during pediatric ventilation.

To conclude, this descriptive study strongly suggests that sevoflurane washout profiles may differ among various models of modern anesthesia workstations. It suggests that maximal FGF used during flush should be maintained during MHS patients’ ventilation. It highlights the importance of additional flushing time before ventilation of an MHS infant to avoid an increase of sevoflurane concentration due to a decrease of MV. Further studies are still required to complete guidelines for preparation of modern anesthesia workstations before anesthesia of MHS patients.

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DISCLOSURES

Name: Nicolas Cottron, MD.

Contribution: This author helped design and conduct the study, analyze the data, and write the manuscript.

Attestation: Nicolas Cottron has seen the original study data, reviewed the analysis of the data, and approved the final manuscript.

Conflicts of Interest: The author has no conflicts of interest to declare.

Name: Claire Larcher, MD.

Contribution: This author helped design the study and write the manuscript.

Attestation: Claire Larcher has seen the original study data and approved the final manuscript.

Conflicts of Interest: Claire Larcher reported a conflict of interest with Dräger.

Name: Agnès Sommet, MD, PhD.

Contribution: This author helped design the study, analyze the data, and write the manuscript.

Attestation: Agnès Sommet has seen the original study data, reviewed the analysis of the data, and approved the final manuscript.

Conflicts of Interest: The author has no conflicts of interest to declare.

Name: Rose Fesseau, MD.

Contribution: This author helped design the study.

Attestation: Rose Fesseau approved the final manuscript.

Conflicts of Interest: The author has no conflicts of interest to declare.

Name: Xavier Alacoque, MD.

Contribution: This author helped design the study.

Attestation: Xavier Alacoque approved the final manuscript.

Conflicts of Interest: The author has no conflicts of interest to declare.

Name: Vincent Minville, MD, PhD.

Contribution: This author helped design the study, analyze the data, and write the manuscript.

Attestation: Vincent Minville has seen the original study data, reviewed the analysis of the data, and approved the final manuscript.

Conflicts of Interest: The author has no conflicts of interest to declare.

Name: Olivier Fourcade, MD, PhD.

Contribution: This author helped design the study.

Attestation: Olivier Fourcade approved the final manuscript.

Conflicts of Interest: The author has no conflicts of interest to declare.

Name: Delphine Kern, MD, PhD.

Contribution: This author helped design and conduct the study, analyze the data, and write the manuscript.

Attestation: Delphine Kern has seen the original study data, reviewed the analysis of the data, approved the final manuscript, and is the author responsible for archiving the study files.

Conflicts of Interest: Delphine Kern reported a conflict of interest with General Electric.

This manuscript was handled by: Maxime Cannesson, MD, PhD.

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ACKNOWLEDGMENTS

The authors wish to thank all manufacturers for the loan of anesthesia workstations and their technical assistance during bench test. GE Healthcare (GE Datex-Ohmeda, Munich, Germany) supports this study by providing the external gas monitor (S/5, GE Healthcare), and support half the rent of the Photoacoustic Field Gas-Monitor. Dräger (Lübeck, Germany) and Maquet (Solna, Sweden) also supported half the rent of the Photoacoustic Field Gas-Monitor. Maquet (Solna, Sweden) provides breathing bags and test lungs. Sevoflurane was provided free by Abbot (Ultane). The authors gratefully acknowledge the technical assistance of S. Daydou (School of Anesthesiologists' Nurses) and G. Visnadi (Department of Medical Engineering) from the University Hospital of Toulouse.

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