Working towards a greener future in veterinary anaesthesia

Veterinary professionals have responsibilities to ensure they are causing minimal harm to the world they inhabit. This article will discuss the environmental science of various anaesthetic gases and quantify them in accordance with their global warming potentials (GWPs) and ozone-depleting abilities. Additional environmental impacts of anaesthetic practice will be considered, such as waste production and energy usage. Vital ways to reduce carbon footprints will also be proposed, such as low fresh gas flow anaesthesia and use of multimodal anaesthetic protocols. A couple of exciting new developments will also be introduced, illuminating the potential for a greener future within veterinary anaesthesia.

Sustainability in anaesthesia

The climate crisis is the largest and most prolonged threat to global health ever described (Costello et al, 2009). The rising temperatures documented over recent centuries are attributed largely to anthropogenic greenhouse gas (GHG) emissions and ozone depletion (International Panel on Climate Change (IPCC), 2018) — to which veterinary professionals contribute daily with our anaesthetic practices. The IPCC Special Report in 2018 concluded that humanity has less than 10 years to dramatically reduce our GHG emissions in order to limit climate change-related public health disasters (IPCC, 2018). Such disasters include mass flooding, droughts, malnutrition, pandemic diseases, habitat loss and mass extinctions, among others.

Healthcare providers in both human and veterinary medicine have potential to contribute to global warming by pollution of the atmosphere, land and waterways (McGain, 2020). The use of all anaesthetic equipment and drugs comes with a carbon footprint, either by direct environmental contamination or indirectly by requiring energy to develop, produce and transport. There is therefore an obligation to ensure that personnel are empowered with the knowledge required to make more sustainable decisions — before it is too late. The people and animals who benefit most from the provisions of healthcare are often disproportionately shielded from the most catastrophic effects of the climate crisis, and individuals must consider their contributions on a global scale.

Greenhouse gases

The sun's heat is essential for life on this planet. When the solar radiation from the sun reaches the earth's surface, it is first absorbed and then later re-emitted as infrared thermal radiation (Campbell, 2015). The remittance of this thermal radiation from the earth is essential in order to maintain stable global temperatures (IPCC, 2018). GHGs are compounds that become trapped when released into the atmosphere, and reflect infrared radiation back towards the earth. With an increasing level of GHG emissions, more and more infrared radiation is being trapped and reflected, contributing massively to the warming of the earth's surface.

Common GHGs include carbon dioxide (CO₂), methane, nitrous oxide, water and all the halogenated anaesthetic agents in common veterinary use (Campbell, 2015). The total carbon footprint of anaesthetic gases can be calculated by multiplying the total amount of the compound released into the atmosphere, by the global warming potential over 100 years (GWP100) (Andersen, 2012). The GWP100 of a compound is a direct result of how well it reflects infrared light (radiative efficiency), and how long it resides in the atmosphere doing so (atmospheric lifetime) (McGain, 2020). The radiative efficiency and atmospheric lifetime of each agent are largely dependent on the strength and position of an agent's infrared absorption bands, and how rapidly they are broken down in the atmosphere (McGain, 2020). CO₂ is used as a reference point for analysis when assigning GWP100, with CO₂ having a GWP100 of 1. Therefore, a compound with a GWP100 of 10, will have 10 times the warming potential of CO₂ over a 100-year duration (McGain, 2020).

Gaseous agents used to maintain anaesthesia are excreted in patients' expired breath, virtually metabolically unchanged (Jones, 2019). Current scavenging systems either capture these agents in activated charcoal units for later destruction (and subsequent release) or expel them directly into the atmosphere. The GWPs of certain halogenated anaesthetic agents are summarised in Table 1.

Desflurane is a volatile anaesthetic agent that has previously been used heavily in human medicine and has been included for reference, despite minimal usage in the veterinary field. The halogenated anaesthetic ethers, isoflurane and desflurane, have similar radiative efficiency; however, the atmospheric lifetime of desflurane is significantly longer (McGain, 2020). Desflurane resides in the atmosphere for around 14 years compared with isoflurane's 3 years (McGain, 2020). Sevoflurane's radiative efficiency is about 25% less than isoflurane/desflurane and has a significantly shorter atmospheric lifetime of around 1 year (McGain, 2020). Nitrous oxide (N₂O) and desflurane also have high minimum alveolar concentrations (MAC) (large amounts are often therefore used) and extortionate GWP100s (McGain, 2020).

In terms of contextualising the severity of these emissions, it helps to compare the effects of driving a car. A study by Sherman et al (2017) concluded that even at a low fresh gas flow of 1 litre/minute, one MAC-hour (2.2%) sevoflurane produces the same carbon dioxide equivalent emissions as driving 4 miles. With the same fresh gas flow, one MAC-hour of isoflurane (1.2%) is equivalent to driving 8 miles, and desflurane (6.6%) a remarkable 190 miles. Within human medicine in the UK, anaesthetic gases contribute to around 5% of the total emissions of all healthcare facilities (Sustainable Development Unit, 2015). This figure includes all aspects of NHS functioning, including the heating of buildings and production of goods. Considering how multifactorial the NHS is, it can be derived that 5% is a significant figure. It is worth emphasising however that the use of more environmentally detrimental agents (such as desflurane and N₂O) have previously been more common practice in human medicine, and no comparable data currently exist within veterinary medicine. This should not detract from the fact that medical professionals should collectively aim for better.

Ozone depletion

The earth's ozone layer protects life from direct exposure to the sun's harmful ultraviolet rays. The ozone layer is largely composed of naturally occurring O₃ molecules, located between the troposphere and the stratosphere (National Geographic, 2021). Some environmental pollutants can exhibit a global warming effect by directly depleting the ozone layer via catalytic destruction, causing more heat from the sun to reach the surface of the earth.

When used, N₂O is often emitted in large volumes and remains in the atmosphere for 110 years, functioning as both greenhouse gas and having profound ozone-depleting effects (Royal College of Anaesthetists, 2021). N₂O has previously been described as ‘the single most important ozone-depleting emission… throughout the 21st century’ (Ravishankara, 2009: 123). Worldwide anaesthetic N₂O use is estimated to contribute 1–3% of N₂O's global emissions (Campbell, 2015). Although its usage is more common within human medicine, its usage should not be ignored by other healthcare providers. Currently conflicting information in studies regarding isoflurane and its potential for catalytic ozone depletion further promotes avoidance of its use.

Impact reduction

It is evident from the analysis of Table 1 that desflurane and N₂O should be avoided avidly from an environmental perspective, and sevoflurane is environmentally preferential because of its lower GWP100. However, along with refining their choice of anaesthetic gases, veterinary professionals should aim to reduce overall anaesthetic gas consumption.

Reduction of anaesthetic gas consumption can be done easily by using, where appropriate, rebreathing systems at the lowest safe fresh gas flow (FGF) (Jones, 2019). Low FGF anaesthesia with sevoflurane has been previously criticised, because of concerns regarding compound-A induced nephrotoxicity. Recent evidence (Duke-Novakovski et al, 2016), however, and the development of modern CO₂ absorbents that do not produce compound-A has dismissed these concerns (McGain, 2020). A lower fresh gas movement through the vaporiser during the maintenance of anaesthesia dramatically reduces the amount of volatile agent used and therefore released into the environment. Closed-system anaesthesia can be performed using adequately leak-tested rebreathing systems combined with regularly serviced monitoring equipment and anaesthetic machines. When using a closed-circle system appropriately, it is possible to maintain anaesthesia by providing only as much oxygen into the circuit as is being removed for the patient's metabolic demands. It is however always recommended to conduct low-flow anaesthesia with the aid of inspired/expired gas analysis and capnography, as well as supervision from a suitably qualified veterinarian to enable titration of oxygen delivery to the lowest, safest amount. It is important to note that when using low FGFs, a higher FGF is required initially to ensure that the circuit is filled with oxygen and that the volatile agent within the canister reaches required levels. It is also largely recommended to temporarily increase FGF when altering a patient's anaesthetic depth, to change the volatile anaesthetic agent concentration within the circuit more rapidly. Using lower FGFs has however been shown to cause more rapid exhaustion of soda lime. The disposal of soda lime and the subsequent emittance of its stored CO₂ itself comes with environmental concerns (McGain, 2020). It is a good idea to remember that reduction of anaesthetic time is therefore paramount to reducing emissions, and veterinary personnel should be proactively trying to find ways to facilitate this. Suggestions include having a surgical assistant ready to prepare the patient/open kit etc, ensuring surgeon availability before inducing anaesthesia and clipping the patient where possible following pre-medication, if well tolerated.

In order to reduce gaseous anaesthesia reliance entirely, patient-specific, analgesia-appropriate, multimodal protocols should be prioritised (Figure 1). When safe to do so, adding in minimum alveolar concentration (MAC)-sparing drugs to anaesthesia protocols will reduce overall reliance on gaseous anaesthesia and therefore reduce usage and emissions (Duke-Novakovski et al, 2016). Veterinary professionals should aim to treat patients' nociceptive responses to surgical stimulation appropriately with analgesia, rather than suppress these responses with gaseous anaesthesia. Where adequate training has been provided, use of local-regional analgesic approaches will limit nociceptive information from reaching the central nervous system and will therefore elicit a MAC-sparing response. Using local anaesthesia and appropriate systemic analgesia protocols may enable more minor procedures to be performed under sedation only, by reducing the patient's response to nociceptive stimuli. Additional MAC-sparing suggestions to discuss with the veterinary surgeon in charge of each patient include peri-operative micro-doses of alpha-2 agonists and opioid top-ups or constant rate infusions (CRIs) such as ketamine or fentanyl. Experienced veterinary surgeons could also consider whether it would be appropriate to administer total intravenous anaesthesia (TIVA), and therefore avoid using volatile anaesthetic agents entirely. This can be done using CRIs of drugs, usually including propofol or alfaxalone. It is important to provide safe, patient-appropriate anaesthetic protocols and drug additions should always be approved by a qualified and experienced veterinary surgeon overseeing the case.

It is important to remember that all anaesthetic drugs and processes come with a carbon footprint because of production, packaging, transportation and energy use. A total mindset change is needed to prioritise the reduction of waste and energy consumption in every way possible. The incorporation of adequate recycling practices is vital, and pharmaceutical wastage can be reduced by reviewing dispensing policies (e.g. how much propofol is drawn up for a patient pre-emptively). All equipment should be turned off when not in use, and pharmaceuticals must be disposed of appropriately to prevent soil and waterway pollution.

Attention must be brought to manufacturing companies, to ensure adequate sustainability policies are in place. Pressure can be applied by simple online enquiries and prioritising purchases from more environmentally sustainable corporations where possible. Practices can also shop around for energy companies to supply their hospitals, prioritising those that have a renewable energy focus. Direct correspondence with companies regarding their sustainability policies can highlight to businesses that practices are actively checking their efforts to work with them collaboratively, for a greener future. These responsibilities can be overseen by an allocated employee. Appointing a ‘green team’ leader within every practice to serve at the forefront of changing protocols and to provide education is a fantastic initiative to increase cooperation, alongside job satisfaction and development for that individual. Management can also work with employees by encouraging staff members to commute to work, offering cycle storage facilities, cycle-to-work schemes and contributions to public transport costs.

Future ideas and research

Medical professionals should aspire to a future where current practice causes no damage to the world they inhabit, and the lives of others. There are many exciting developments on the horizon within this field, promising a greener future for veterinary anaesthesia. Much of the current focus is on gas recapture and reuse, largely with the development of metal organic frameworks (MOFs).

MOFs can be compared with sponges, made up of structured frameworks of bridged metal ions that create open pore-like networks (McGain, 2020). These pores entrap specific guest molecules, holding them for later release. The future use of MOFs could include using them as part of scavenging systems, to capture metabolically unchanged anaesthetic gases for reuse-preventing wastage and atmospheric pollution. There is still work required in the field of recapture and reuse, and the veterinary profession should be receptive to new products and information.

An environmentally friendly gaseous anaesthetic agent, known as xenon, can be produced by fractional distillation of air as a byproduct of oxygen production, with no GHG or ozone-depleting effects. Xenon is thought to exert anaesthetic action by non-competitive inhibition of N-methyl-D-aspartate (NMDA) receptors, with little effect on GABA A receptors or non-NMDA glutamatergic receptors (Franks et al, 1998). In humans, xenon has demonstrated exceptional cardiovascular stability and reduced recovery times. However, the production process of xenon is currently both energy-intensive and expensive (Franks et al, 1998). Its usage may however be viable in the future, if stringent recapture and recycling technologies (such as MOFs) are employed (Jones, 2019).

Conclusions

It can be concluded that gaseous anaesthetic agents can contribute to the warming of the planet, via the greenhouse gas effect and by potential ozone depletion. By prioritising a volatile anaesthestic agent with a lower GWP100 (e.g. sevoflurane) and reducing total gas usage (via lower FGFs, multimodal anaesthetic approaches and reducing total anaesthesia times), veterinary professionals can greatly reduce daily environmental impact. They must, however, ensure that we are reducing waste and energy consumption in all other aspects of veterinary practice. To ensure that personal efforts are not in vain, the veterinary industry should be receptive to new developments and continue to apply pressure to corporations and governments.

KEY POINTS

• The global warming potentials (GWPs) of anaesthetic gases vary greatly — with sevoflurane being the most environmentally friendly.
• Nitrous oxide has been linked to direct ozone depletion, suggesting its use should be avoided.
• Veterinary professionals are encouraged to implement multimodal anaesthesia protocols, reducing the reliance on volatile anaesthetic agents. Other suggestions include the safe implementation of low-flow anaesthesia and total intravenous anaesthesia.
• Individual practices should appoint an environmental leader and apply pressure to production companies, alongside creating practice incentives to encourage staff to live more sustainably.
• New developments within sustainable anaesthesia include metal organic frameworks for gas recapture and reuse, as well as the development of an environmentally friendly volatile agent, xenon.