Inhalational anaesthetics: an assessment of agent delivery and capture

Inhalational anaesthetic agents are potent greenhouse gases, which account for 5% of a typical hospital's carbon emissions [1], 2% of all healthcare emissions [2] and up to 63% of the carbon footprint of surgical procedures [3]. The NHS long-term plan sets targets for 40% reductions in anaesthetic gas emissions [4]. Campaigns such as the desflurane reduction project [5] and NHS desflurane reduction toolkits aim to reduce the use of the most environmentally damaging inhalational anaesthetics. Desflurane was removed from use at Guy's and St. Thomas' NHS Foundation Trust in 2021, reducing our department's annual inhalational agent carbon footprint by 1956 tonnes CO2 equivalent (CO2e). Sevoflurane remains in use, which contributes 115.9 tonnes CO2e, but remains valuable for clinical indications when total intravenous anaesthesia is not used. New opportunities, such as inhalational agent waste capture technology, are therefore important for further reductions. This study's aim was to assess the efficiency of a capture system in vitro. The accuracy was assessed by testing the relationships of delivered anaesthetic agent volume recorded by the anaesthetic machine with the mass of actual vaporiser-delivered anaesthetic agent. We report the findings using SID-Dock capture system (SageTech Medical Ltd., Paignton, UK). The system comprises four parallel capture vessels (‘SID-Cans’) connected between the anaesthetic machine and the active scavenging system, designed to trap and store waste inhalational agent. The system was certified for use with our anaesthetic machines (Draeger Primus, Draeger Medical UK Ltd., Hemel Hempstead, UK). Flow rates and sevoflurane concentration were varied over 23 tests, using concentrations between 2 and 8 vol. % and 0.5–15 l.min−1 flow rates. The system was purged after each test. The machine was set to volume-control ventilation with a tidal volume of 500 ml, respiratory rate of 12, inspiratory:expiratory ratio of 1:2 and 5 cm H2O positive end-expiratory pressure. A 4 m circle breathing system (Intersurgical®, Wokingham, UK) was attached and a 2 l reservoir bag used as a lung analogue. Minimum test run times were calculated to ensure logbook records of delivered sevoflurane volumes exceeded the allowable error of our equipment, so any discrepancies seen were likely to be real. The SID-Cans and vaporiser weights were measured and recorded at the start and end of each of the 23 tests, using scales with an absolute error of 0.1 g. Reported delivered sevoflurane volumes from the anaesthetic machine logbook were recorded. All measurements were in grams using a specific weight value of 1.53 g.ml−1 for sevoflurane [6]. After testing, the contents of the SID-can were extracted and reported by the manufacturer. Our protocol is available on request. Overall, the capture efficiency of volatile anaesthetic gas was 94.8%. Extraction data showed that 362 g of the 382 g sevoflurane delivered across the tests was captured, together with 27 g water. Changes in mass and capture efficiency are shown in Table 1. At high flow rates, the mass of sevoflurane delivered was consistently higher than that captured, but performance improved as flow rates reduced. Our study showed that the capture efficiency using this technology was high. Although performance deteriorated at higher flow rates, most cases will run in the range of 0.5–2 l.min−1 [7]. At lower flow rates, the mass change of the capture canisters was consistently greater than the mass of sevoflurane delivered. Extraction data showed that 27 g of water was acquired during the tests and this may explain the extra mass. It was, however, impossible to calculate the efficiency for each flow rate or concentration as we did not have the separated extracted data for each. This would have only been possible if extraction of canisters occurred after every test, but this is unlikely to be practical or feasible given the very small mass changes at low flow rates and concentrations. Our study of the relationship between sevoflurane mass delivered by the vaporiser and the related logbook record show that for further studies of volatile capture efficiency, the vaporiser mass change will need to be used rather than logbook records due to the lack of a reliable relationship with these samples. The tolerances of the equipment and logbook report error, while they may appear large, are not used in clinical practice and therefore are not of clinical significance because patient parameters and end-tidal anaesthetic agent concentration/minimum alveolar concentration values are generally used. The error may be of more importance if extracting data from logbook records for future carbon reporting; however, with large enough samples, the error may approximate to the true value. In conclusion, this study demonstrates that SageTech SID-Can have high baseline efficiency when applied in vitro. We believe we can now assess this technology within a clinical setting, with an aim to study any inefficiency brought about by the provision of anaesthesia to patients. MV and RK contributed equally to this article. The authors would like to thank Mr T Wright, lead equipment manager. The London Sustainability Network Desflurane Toolkit 2021 is available by email from [email protected]. Presented in part at the Institute of Physics and Engineering in Medicine (IPEM) Science, Technology and Engineering Forum (STEF) 2023 Glasgow, UK, March 2023. No competing interests declared.