Improving the efficiency of sevoflurane delivery during general anaesthesia by educating and motivating anaesthetists to utilise the Volatile Efficiency Ratio

Abstract

Volatile anaesthetic agents such as sevoflurane contribute to greenhouse gas emissions, and selecting low fresh gas flows on anaesthetic machines minimises their waste. Facilitating improvements in sevoflurane use requires the education, motivation, and standardised evaluation of anaesthetists. There is currently no standard of practice related to the efficiency of anaesthetic gas delivery per case. We conducted a multi-component study termed ‘Low With The Flow’ (LWTF) to directly address these requirements by educating and motivating anaesthetists to reduce fresh gas flow and thereby sevoflurane use. We introduced a novel metric, the ‘volatile efficiency ratio’ (VER), able to be calculated on Draeger Primus™, the Draeger Atlan™ family and Draeger Perseus A500™ machines, to audit sevoflurane use in a case-by-case fashion, and assess whether the intervention could achieve a set VER target. The LWTF intervention significantly improved the efficiency of sevoflurane delivery (VER 0.46 pre-intervention (n = 518) versus VER 0.57 post-intervention (n = 531), 95% confidence interval 0.092 to 0.129, P < 0.0001) resulting in a calculated average of 1.3 kg carbon dioxide equivalent emissions reduction and approximately AUD 3.50 saving per case. Consequently, the financial and environmental outcomes from sevoflurane delivery were considerably reduced. Our LWTF intervention provides a valuable model for other anaesthetic departments to investigate and address the global environmental and financial burdens related to their volatile anaesthetic use. For anaesthetists using anaesthesia machines that do not facilitate calculation of VER, an approach using components of our LWTF intervention may still reduce the environmental and financial impacts associated with administration of volatile anaesthesia.

Keywords

Anaesthesia sustainability volatile anaesthesia behaviour sevoflurane

Introduction

Anaesthesia-related greenhouse gas emissions can be reduced by minimising waste of anaesthetic gases. Anaesthetic gases such as sevoflurane, desflurane, and nitrous oxide contribute directly to climate change as greenhouse gases. Nitrous oxide, in addition, is a major contributor to ozone depletion.¹ Apart from the direct greenhouse gas effects of inhalational anaesthetics, energy is required in the collection, production, and manufacturing of all gases, including oxygen.

In response to the environmental crisis, many institutions have recommended avoidance of desflurane and nitrous oxide due to their significant environmental impact.^2–4 The National Health Service Scotland and the Western Australian Government, as well as many individual public hospitals across Australia have ceased desflurane use altogether.^5,6 Furthermore, in the Australian private sector, St John of God Health Care has set a precedent for other private healthcare networks in removing desflurane from its formulary.⁷

For cases where inhaled anaesthetic agents are chosen, anaesthetists have a responsibility to administer them in a manner that minimises unnecessary waste. Reducing waste anaesthetic gases can be achieved by practising low flow anaesthesia (LFA). LFA is generally defined as a ventilator fresh gas flow (FGF) of less than 1 L/min during the maintenance phase of anaesthesia.⁸ LFA is encouraged by the Australian and New Zealand College of Anaesthetists and is a safe, simple and reliable method to reduce waste anaesthetic gases.^9–12 Beyond reducing the carbon footprint, LFA offers additional benefits including preservation of mucociliary function by maintaining the temperature and humidity of the inspired gas.^13,14 Regarding cost savings, the University of Wisconsin Hospital estimated that minimising waste gases can save USD 2000 per operating room per annum.¹⁵ Avoidance of LFA is frequently defended by the outdated concern for Compound A-induced nephrotoxicity and has driven unnecessary waste of sevoflurane.^10,16,17

One method to reduce consumption involves computerisation to automate LFA via the ‘end-tidal control’ function.¹⁸ The environmental and financial cost of replacing machines to those that enable end-tidal control of anaesthetic gases is very high. In anaesthetic departments that do not have this technology, FGF must be manually selected at different stages of anaesthesia to achieve and maintain the desired end-tidal oxygen and sevoflurane concentrations.

Several departments have adopted sustainable anaesthesia strategies, and targeted education has resulted in reduced FGF and reductions in the purchasing of volatile anaesthetic bottles.^19,20 However, measurement of the use of anaesthetic agents via the purchasing of bottles is not a direct measure of use per case and cannot be targeted to the individual anaesthetist or case.

The volatile efficiency ratio – a metric to assess the efficiency of sevoflurane delivery

The World Federation of Societies for Anaesthesiologists has called for anaesthetists to develop measurement tools to determine their own carbon footprint.²¹ A tool to easily evaluate the waste of volatile anaesthetic agents on a case-by-case basis is needed in order to identify, audit and reduce the environmental impact of anaesthesia.

While attempts to reduce waste by encouraging use of LFA have been successful in previous studies,²² we feel that audits of efficiency using average FGF rates may be misleading. This is because average FGF calculations may include high flows for long periods in the absence of volatile agent delivery, usually at the commencement and completion of a case.

At the completion of a procedure, some anaesthetic machines allow a review of anaesthetic gas usage, including the mix of gases used, total oxygen, air, and volatile agent. The Draeger Primus™, the Draeger Atlan™ family and Draeger Perseus A500™ machines (Draeger AG & Co. KGaA, Lubeck, Germany) display the total volume of volatile uptake by the patient as well as the total volume consumed by the machine. These parameters are calculated via continuous measurement of the volatile agent concentration in the inspiratory and expiratory limbs. This calculation has been clinically assessed as accurate and is a useful estimate for use of volatile anaesthetic delivery in routine practice.²³ From these parameters, the ratio between uptake and consumption can be independently calculated and we have termed this the ‘volatile efficiency ratio’ (VER).

VER = \frac{\begin{matrix} volume of volatile anaesthetic agent \\ taken up by the patient (mL) \end{matrix}}{volume of volatile anaesthetic agent consumed (mL)}

A higher VER reflects more efficient use, with the uptake of volatile closely matching the total volatile consumption or delivery. A ratio of 0.5 represents that 50% of sevoflurane consumed by the machine was delivered (uptake) to the patient in that specific surgical procedure (and therefore 50% did not contribute to anaesthesia). Calculating a VER target of 0.5 is an easy calculation for the provider. The VER is directly related to the FGF rate, impacting the amount of gas scavenged and delivered incidentally to the operating room. Conversely, high FGF in excess of requirements leads to an undesirably low VER. Figure 1 illustrates the data available on anaesthetic machine logbooks after completion of a case performed on a Draeger Primus anaesthetic machine.

Figure 1.

Photographs of Draeger Primus^TM anaesthetic machine logbooks comparing (a) an above standard volatile efficiency ratio of 0.87 (20/23) and (b) a below standard volatile efficiency ratio of 0.27 (6/22).

By utilising this individualised efficiency metric alongside an intervention, we aimed to determine whether clinicians could strive towards a new standard of practice and provide a more efficient approach to anaesthetic gas administration. This would reduce waste of anaesthetic gases, contribute to the reduction of greenhouse gas emissions and have economic benefits.

Methods

Ethical approval and patient selection

This study received approval from the Human Research and Ethics Committee at St Vincent’s Hospital, Melbourne under approval number QA22017. A total of 1049 patients in operating rooms at St Vincent’s Hospital Melbourne undergoing general anaesthesia with sevoflurane as the maintenance agent were included in the study.

Data collection and blinded analysis

Logbooks from Draeger Primus anaesthetic machines were retrospectively identified for 1049 (518 pre-intervention and 531 post-intervention) patients who underwent general anaesthesia. Anaesthetists were blinded to involvement in the study. All case logbook summaries available on anaesthetic machines were collected manually at the end of each day over a 2-month period prior to the intervention, and then over another 2-month period following the intervention. Cases involving gas induction or utilising a combination of propofol and sevoflurane for maintenance were excluded from the study. Procedures with open airway techniques resulting in a high circuit leak were also excluded and anaesthetists were encouraged to use total intravenous anaesthesia (TIVA) as an alternative in these cases. Patient demographics, volatile anaesthetic consumption, volatile anaesthetic uptake, surgical specialty, and derived values such as average FGF, anaesthetic gas uptake to consumption ratios, and cost were recorded, and carbon dioxide (CO₂) equivalents were calculated.

Intervention components

The ‘Low With The Flow’ (LWTF) intervention conducted over a 4-month period comprised five key components:

Encouragement to utilise the VER to measure and standardise practice: anaesthetists were encouraged to achieve a minimum VER of 0.5 as a target standard per case.

Encouragement of LWTF strategies: intraoperative strategies were promoted to minimise waste, including avoidance of excessive gas flow during preoxygenation, avoidance of co-induction with sevoflurane by using divided doses of propofol on induction, initiation of low FGF before volatile administration, reduction of FGF to less than 1 L/min during the maintenance phase, and complete cessation of FGF on the anaesthetic machine while on cardiopulmonary bypass.

A departmental education session: an education session was conducted to introduce the intervention strategies, outline the environmental impacts of healthcare and emphasise the role of anaesthetic gases in contributing to global warming. Anaesthetists were informed about utilising the anaesthetic machine logbook to assess sevoflurane usage per case, calculate the VER and align practices with the department standard.

Visual prompts: visual prompts in the form of posters and stickers were placed in the anaesthetic department and on anaesthetic machines to remind providers of the LWTF strategies and encourage behavioural change.

Individualised feedback: anaesthetists received personalised feedback on their adherence to the intervention approach. The VER target facilitated the identification of particular anaesthetists whose practices were not sustainable, allowing for corrective actions.

Data analysis

A power analysis was based on data from the first 100 pre-intervention cases to determine the likely standard deviation of the data. The mean VER of the initial 100 cases was 0.46, with a standard deviation of 0.131. We aimed to achieve 80% power to detect a 5% difference in our primary endpoint, VER (i.e. change in VER of 0.023). The number required to achieve this was 511 in each group.

All statistical analyses were performed using Stata^TM v15 (Stata Corp LP, College Station, TX, USA). The distribution of average FGFs was not normally distributed so was evaluated using a Mann–Whitney U test. Other data were normally distributed and compared using t-tests. The calculation of the CO₂ equivalent in kilograms was derived from previously published methodology utilising the equation: volume of sevoflurane (mL) × density of 1.5 to convert to mass (mL/kg) × 130 (global warming potential of sevoflurane).²⁴ To calculate cost, the unit cost of sevoflurane was calculated according to USD 85 per 250 mL bottle as described previously.²⁵

Results

One thousand and forty-nine cases were included in the study: 518 cases before and 531 cases after intervention. The age of the patients was similar between groups (mean age 53.3 years pre-intervention versus 54.9 years post-intervention). There was no significant difference between the median duration of anaesthesia (130 min pre-intervention versus 132 min post-intervention, P = 0.681).

Volatile waste was reduced post-intervention with a significant increase in the mean VER of 0.111 (0.456 pre-intervention versus 0.567 post-intervention, 95% confidence interval (CI) 0.093 to 0.130, P < 0.0001) (Figure 2). There was also a significant difference in average FGF per case (median 3.16 L/min pre-intervention versus 2.53 L/min post-intervention, P < 0.0001) (Figure 3). Post-intervention, every surgical specialty met the proposed target VER of 0.5 (Table 1).

Figure 2.

Volatile efficiency ratios per case (a) pre-intervention and (b) post-intervention.

Figure 3.

Median fresh gas flows per case (a) pre-intervention and (b) post-intervention.

Table 1.

Mean volatile efficiency ratio and median fresh gas flow pre- and post-intervention across surgical specialties.

Specialty	Pre-intervention			Post-intervention
Specialty	Mean hourly sevoflurane consumption (mL/h)	Median fresh gas flow (L/min)	Mean volatile efficiency ratio	Mean hourly sevoflurane consumption (mL/h)	Median fresh gas flow (L/min)	Mean volatile efficiency ratio
Cardiothoracic	n = 33			n = 38
Cardiothoracic	13.8	2.22	0.36	10.5	1.45	0.51
Ear, nose & throat	n = 21			n = 42
Ear, nose & throat	21.1	3.45	0.43	17.5	2.78	0.54
General surgery	n = 149			n = 137
General surgery	20.8	3.62	0.45	16.2	3.05	0.56
Maxillofacial	n = 4			n = 9
Maxillofacial	22.1	3.99	0.48	13.7	2.61	0.50
Neurosurgery	n = 25			n = 40
Neurosurgery	16.6	2.47	0.49	11.8	2.52	0.52
Orthopaedic	n = 123			n = 114
Orthopaedic	18.6	2.80	0.48	15.0	2.22	0.62
Plastics	n = 55			n = 52
Plastics	25.9	3.92	0.45	18.6	2.56	0.56
Urology	n = 54			n = 53
Urology	22.7	3.27	0.44	16.5	2.71	0.59
Vascular	n = 15			n = 27
Vascular	15.7	2.87	0.52	11.7	2.45	0.59
Other	n = 39			n = 19
Other	17.2	2.70	0.50	16.6	2.90	0.53
Total	n = 518			n = 531
Total	19.8	3.16	0.46	15.3	2.54	0.57

Environmental and financial savings were evident following the LWTF intervention. The mean CO₂ equivalent emission of sevoflurane was notably reduced by 1.3 kg per case, accompanied by a statistically significant mean cost reduction of approximately AUD 3.50 per case (Table 2).

Table 2.

Environmental and financial savings pre- and post-intervention.

	Pre-intervention	Post-intervention	P-value
Mean sevoflurane use standardised to case duration of 130 min	35.12 mL per case	28.40 mL per case	<0.0001
Mean CO₂ equivalent emissions from sevoflurane	6.88 kg per case	5.57 kg per case	<0.0001
Estimated mean cost of sevoflurane, AUD	18.29 per case	14.80 per case	<0.0001

CO₂: carbon dioxide; AUD: Australian dollars.

Discussion

Improved environmental and financial outcomes of sevoflurane use after the LWTF campaign

This study has demonstrated the effectiveness of the LWTF intervention in significantly reducing sevoflurane waste at a tertiary hospital. The intervention campaign focused on educating, raising awareness, and encouraging anaesthetists to employ more sustainable intraoperative practice with anaesthetic gases. This low-cost, resource-efficient, 4-month campaign reduced the median FGF from 3.16 L/min to 2.54 L/min, leading to a significantly improved VER from 0.46 to 0.57. This reduced the calculated mean CO₂ equivalent emissions by 1.3 kg CO₂ per case and an approximate AUD 3.50 saving in sevoflurane use per case. Importantly, these numbers only reflect savings related to sevoflurane. By reducing overall FGF, there is also an inherent reduction in the waste of oxygen, contributing further to the environmental and economic benefits of this intervention.

VER as a metric for efficiency of sevoflurane delivery

While previous interventional studies have reduced volatile anaesthetic agent use within the operating theatre,²⁰ this study pioneered the VER to identify and monitor sevoflurane waste in a case-by-case fashion. Anaesthetic machines capable of quantifying sevoflurane uptake and consumption permit a straightforward calculation of the VER. By encouraging anaesthetists to calculate the VER at the completion of the case, an easy assessment can be made to determine whether they have met a chosen efficiency target. This fosters continuous improvement in their manner of volatile agent delivery and allows comparison between individual anaesthetists and departments. Consistently low VER values enable identification of variables that contribute to waste. The accepted standard of a VER of 0.5 was chosen as an initial target but with further education, this target could be increased.

It is important to note that the FGF reported is for the duration of the case, and is not limited to the duration of sevoflurane delivery. High FGF during prolonged preoxygenation and during the washout phase at the end of surgery may have contributed to a mean FGF that was higher than expected post-intervention. Our inability to differentiate between the FGF selected at different times during a case is a limitation of our study. However, this also highlights that the VER is superior as a metric to assess the efficiency of sevoflurane delivery, as it specifically evaluates the period in which sevoflurane is delivered. The rate of sevoflurane consumed in millilitres per hour may be skewed by long periods without volatile agent delivery. Furthermore, larger patients, for instance, require more sevoflurane to achieve the same target concentration as in smaller patients, making the total millilitres of sevoflurane consumed on its own a less reliable reflection of efficiency.

The effect of duration of anaesthesia and the importance of induction to improve efficiency of sevoflurane delivery

The duration of anaesthesia influences the capacity to achieve a high VER. The longer the duration of anaesthesia, the easier it is to achieve the target ratio of greater than 0.5. Despite this, if methods to reduce waste on induction are employed, the target VER is achievable. For example, the shortest case in our study of 21 min duration had a VER of 0.5 and another general anaesthetic case of 30 min duration had a VER of 0.86. This reduction in waste is reinforced in the literature, with studies showing that changes made during induction can result in large reductions in volatile consumption.^17,26 Moreover, lower FGF has been shown to reduce agent consumption without compromising time to induction or depth of anaesthesia in a paediatric study.²⁶

Improvement during cardiothoracic surgery by ceasing fresh gas flow during cardiopulmonary bypass

The pre-intervention findings highlighted the high waste potential associated with cardiothoracic procedures involving cardiopulmonary bypass. Specifically, cardiothoracic cases had a pre-intervention mean VER of 0.36 (markedly lower than observed in other surgical disciplines). This discrepancy was attributed to the near-complete loss of volatile agent from the breathing circuit during apnoea while on cardiopulmonary bypass. Alongside the other interventions, complete cessation of FGF and the maintenance of a closed circuit during cardiopulmonary bypass resulted in a noteworthy improvement in the mean VER, increasing to 0.51 post-intervention. This represents a unique recommendation, applicable during cardiopulmonary bypass procedures.

Barriers to employing low flow anaesthesia and utilising the VER

Warnings from drug manufacturers.

The reluctance to adopt LFA is often rooted in concern for renal injury arising from the formation of metabolic by-products such as Compound A. This compound results from the interaction of volatile agents with CO₂ absorbents. Despite this concern, no paper has revealed any evidence of renal injury in humans, including a recent randomised controlled trial comparing sevoflurane administration at minimal versus high FGF in a population at high risk of renal injury.²⁷ Recent recommendations call for the removal of any restriction to LFA from drug manufacturers’ data sheets.¹⁷ By encouraging drug companies to update warnings, we anticipate a positive shift in the perception and motivation of healthcare providers toward the adoption of this environmentally sustainable practice.

2. High-leak procedures.

Procedures involving open airways, such as bronchoscopy, result in significant circuit leak. Low FGF leads to insufficient circuit volume required for ventilation. The high FGF necessary during these procedures results in very high levels of volatile agent waste. In these cases, the advantages of TIVA become even more profound, offering a compelling advantage where volatile waste is likely to be considerable.

3. Longstanding practices of anaesthetists.

Motivating anaesthetists to adopt LFA poses a challenge. Achieving change requires a comprehensive approach encompassing problem identification and explanation, elucidation of the benefits tied to the proposed alteration, outlining the methods for implementation, and the provision of continuous encouragement and feedback – a concept often referred to as ‘nudges’.²⁸ Acknowledging the significance of auditing and the value of continuing education meetings add layers of strategic insight to this multifaceted approach.^29,30

We advocate for the integration of LWTF practices into the curriculum for anaesthesia trainees. Recognising the responsibility of anaesthetists and empowering the trainee with strategies to reduce waste will improve the carbon footprint of daily practice. With clear metrics (such as the VER), a trainee can easily measure and audit the environmental impact of their own practice under supervision. Trainees have an opportunity to absorb, implement and disseminate changes in long-accepted practice. This has already been exemplified by organisations such as the Trainee-Led Research and Audit in Anaesthesia for Sustainable Health, who are already leading change in sustainable practice in healthcare.³¹

Changing the practice of individual anaesthetists was not directly assessed in our study and represents a limitation, but the VER facilitates rapid identification of those anaesthetists who do not meet chosen targets. Moreover, as the study was conducted over consecutive years and due to the nature of new senior staff hiring, there was a small population of providers that did not receive the education surrounding the intervention.

4. Availability of anaesthetic machines with the ability to measure and display uptake and consumption.

The context of this study is limited to anaesthetic machines that calculate and display volatile agent uptake and consumption. This is possible via Draeger Primus, the Draeger Atlan family and Draeger Perseus A500 anaesthesia machines. We believe providers benefit from the availability of software that provides this important information. It may be prohibitive to replace machines due to financial and environmental reasons; however, manufacturers should continue to expand their role in sustainable anaesthesia. For those manufacturers that have not done so already, it would be ideal if they could expand analysis capabilities to enable metrics such as the VER. For those providers who administer anaesthesia on machines that are not capable of measuring the VER, a LWTF approach to volatile administration may still be of benefit.

Conclusion

This study has demonstrated a successful and cost-effective reduction in sevoflurane waste through the LWTF intervention. The introduced quantitative metric, the VER, enables evaluation of the efficiency of volatile administration at the patient level when using the Draeger Primus, the Draeger Atlan family and Draeger Perseus A500 anaesthesia machines. The VER is a metric that can be used to compare individual anaesthetists or anaesthetic departments. It has the potential to be used as an aid to determine compliance with sustainability framework targets. Our LWTF intervention provides a valuable model for other anaesthetic departments to investigate and address the global environmental and financial burdens related to their volatile anaesthetic use. For anaesthetists using anaesthesia machines that do not facilitate calculation of the VER, an approach using components of our LWTF intervention may still reduce the environmental and financial impacts associated with administration of volatile anaesthesia.

Footnotes

Author contribution(s)

Samuel T Costello: Conceptualization; Data curation; Formal analysis; Investigation; Methodology; Project administration; Supervision; Validation; Visualization; Writing – original draft; Writing – review & editing.

Blake J Vorias: Data curation; Formal analysis; Investigation; Methodology; Writing – original draft; Writing – review & editing.

Roman Kluger: Formal analysis; Writing – review & editing.

Declaration of conflicting interests

The author(s) declared no potential conflicts of interest with respect to the research, authorship, and/or publication of this article.

Funding

The author(s) received no financial support for the research, authorship, and/or publication of this article.

ORCID iDs

Samuel T Costello

Roman Kluger

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