Minimising the environmental impacts of anaesthetic gases in veterinary practice

This article is intended for use by qualified veterinary professionals, and should not replace clinical decision-making or discretion. Alterations to veterinary anaesthetic protocols should be overseen by a qualified veterinary surgeon working within their competency.

The greenhouse gas effects of the anaesthetic gases are significant and veterinary professionals using anaesthetic agents should demonstrate responsible stewardship. One hour of isoflurane anaesthesia in a dog is equivalent to driving 12 miles in an average European car (Jones & West 2019).

This guide aims to explain some simple steps that may be taken to reduce the environmental impact of these gases.

1. Stop using nitrous oxide

Nitrous oxide is less popular since the introduction and licensing of modern opioids, but is still used in some veterinary clinics. There are additional health and safety risks to staff working with nitrous oxide. Its low potency, combined with a significant global warming potential and ozone-depleting effect, make it a significant greenhouse gas. These factors make it of less value in the veterinary sector.

2. Avoid prolonged or unnecessary anaesthesia

Along with reducing waste gas emissions, additional benefits to the patient will arise from good planning and prompt decision making where possible.

3. Regularly check and service your anaesthetic machine, vaporiser and breathing systems

Leaks in the system can result in contamination of the working environment for staff, unstable depths of anaesthesia, and wastage of volatile inhalational agent. Regular servicing can also indicate the performance of the vaporiser and gas flowmeter at lower fresh gas flows; older equipment may require a minimum fresh gas flow to ensure accuracy of delivery by the vaporisers and anaesthetic machine flowmeters. Anaesthetic machine and breathing systems should be checked for correct function and leaks daily and before each case. Also ensure there are no leaks resulting from inadequate endotracheal tube cuff inflation or cracks in capnograph (or gas sampling) lines.

4. Use soda lime containing rebreathing systems (such as circle systems)

The safe (or actual) minimum flow which can be used with a rebreathing system will depend on the monitoring equipment available, the size of the patient, and the accuracy of delivery of both the fresh gas flowmeter and the vaporiser.

Medical anaesthetic guidelines mandate the monitoring of inspired oxygen concentrations to prevent hypoxia, end-tidal anaesthetic concentrations to prevent accidental anaesthetic awareness or recovery from anaesthesia, and capnography to assess the adequacy of ventilation (amongst other physiologic parameters).

The soda lime contained within a circle breathing system absorbs carbon dioxide, which means in theory, the fresh gas flow needs only to:

1. supply the metabolic oxygen requirement (of approximately 10ml/kg/min)
2. carry the vaporised anaesthetic agent
3. replace any losses e.g. due to gas monitor sampling (usually 150-200ml/kg/min)

IMPORTANT WARNING REGARDING USE OF LOW FLOW ANAESTHESIA: Using low flow (0.5 – 1 L/minute), minimal flow (0.25 – 0.5 L/minute) or closed circuit techniques (< 0.25 L/minute; or flow equal to metabolic oxygen consumption plus system gas leaks) will reduce volatile agent consumption, but may risk delivery of a hypoxic mixture (inspired oxygen concentrations should remain greater than 30%), accumulation of non-desirable gases such as nitrogen, or unexpected inadequate depths of anaesthesia. A hypoxic mixture is of particular risk if nitrous oxide is used, in which case inspired oxygen monitoring is highly recommended and low flow anaesthesia is not recommended. To maintain a sufficient end-tidal concentration of anaesthetic agent at lower flows, the vaporiser setting may need to be slightly increased to account for the relatively greater dilution within the breathing system. Monitoring of the end-tidal anaesthetic agent reduces the risk of accidental recovery from anaesthesia.

The fresh gas flow also needs to be higher at the start of anaesthesia for around 10-15 minutes. This is to allow the volatile agent and oxygen concentrations to increase within the breathing system, while removing the air already present in the system (de-nitrogenation). After this period the fresh gas flow can be reduced. If the anaesthetic depth needs to be changed quickly, the fresh gas flow should be increased in combination with a change in the anaesthetic vaporiser setting. At the end of anaesthesia the fresh gas flow should be increased to rapidly remove volatile anaesthetic agent from the breathing system, and 100% oxygen provided if nitrous oxide was used to avoid diffusion hypoxia.

Circle breathing systems are reliant on the soda lime and correct function of the one-way valves to eliminate carbon dioxide from the inspired breath. These should be checked daily and before each case; in particular, soda lime may appear to return to its ‘absorbent’ colour when it is actually ‘exhausted’ and unable to absorb further carbon dioxide. Use of capnography is strongly recommended to assess elimination of carbon dioxide, amongst other physiologic factors.

A last concern is the closure of the adjustable pressure-limiting valve during reduced flow anaesthesia, and accidental failure to open it when the flows are increased, which may cause pulmonary trauma and death. The rebreathing bag should be monitored to avoid over-distension.

Tip: Even reductions from 2L/min to 1L/min will more than half the volatile anaesthetic agent wasted.

Tip: Check the rebreathing bag – it should always contain enough volume for your patient to breathe in without any risk of collapsing the bag.

5. Choose non-rebreathing systems which use lower fresh gas flows

During spontaneous ventilation, a mini-lack or lack breathing system (Mapleson A systems) will require lower fresh gas flows to prevent rebreathing of carbon dioxide than a Bain or T-piece breathing system (Mapleson D, E or F systems). However, the limitations of these systems should be appreciated e.g. the Mini-lack and Lack are not suitable systems for providing sustained periods of positive pressure ventilation.

6. Use end-tidal capnography to find the lowest fresh gas flow which will prevent rebreathing of carbon dioxide when using a NON-rebreathing system

When using a non-rebreathing system, the fresh gas flow is needed to push exhaled carbon dioxide away from the patient. Without capnography, the fresh gas flow must be calculated based on the breathing system used, the patient’s body weight, and assumptions about the patient’s current ventilation depth and rate. With capnography, the fresh gas flow can be adjusted to just prevent inspiration (or rebreathing) of exhaled carbon dioxide.

The flow required to prevent rebreathing may vary during the course of the anaesthetic, as the patient changes their ventilation rates and depths. This is easily detected using the capnography trace. Other potential causes of rebreathing of exhaled carbon dioxide include; exhausted soda lime and faulty rebreathing system valves (for rebreathing systems only), and excessive equipment dead space (for both rebreathing and non-rebreathing systems).

Note: this point does not apply for rebreathing systems (with soda lime) – see point 4 for rebreathing system use.

Tip: Use capnography to identify the minimum flow required to prevent rebreathing of carbon dioxide during the inspiratory period

7. Use volatile agent-sparing drugs and techniques

More information on drug choices is available in the BSAVA Manual of Canine and Feline Anaesthesia (see references below). Check national regulations for licensing and doses in individual countries. Options for pharmaceutical techniques, which may reduce the requirement for volatile anaesthetic agents, include:

1. Using appropriate premedication and analgesia when possible
2. Systemic drugs
   • Boluses
   • Partial Intravenous Anaesthesia (PIVA)
3. Regional anaesthetic techniques
   • Extradural or spinal anaesthesia
   • Peripheral nerve blocks
   • Local infiltration or wound catheters
   • Splash blocks

 

8. Consider use of total (TIVA) or partial (PIVA) intravenous anaesthesia

Three anaesthetic drugs are available for injectable anaesthesia in small animals; propofol, alfaxalone and ketamine. At time of writing, their licensing conditions (e.g. species, dose, duration) are limited and should be reviewed for each product before use. TIVA should be provided using the manufacturer’s recommendations for the drugs. Intermittent bolus and continuous infusion techniques are available. Endotracheal intubation, an intravenous catheter and oxygen should always be provided, and manual ventilation may be required.

More information on TIVA is available in the BSAVA Manual of Canine and Feline Anaesthesia (see references), and for alfaxalone infusion see https://www.alfaxan.co.uk/resources. Advice from an RCVS Specialist in Veterinary Anaesthesia and Analgesia should be sought if required.

9. Consider using sevoflurane rather than isoflurane

Sevoflurane is a less potent anaesthetic agent than isoflurane, but is has significantly less greenhouse gas effect due to a lower global warming potential (sevoflurane GWP₁₀₀ 130; isoflurane GWP₁₀₀ 510).

Due to sevoflurane’s lower potency compared with isoflurane, the sevoflurane vaporiser dial needs to be set higher to achieve the same depth of anaesthesia. Details on species-specific settings can be found in the BSAVA Manual of Canine and Feline Anaesthesia (see references).

It is highly recommended that users of sevoflurane should be guided by national regulatory body recommendations for minimum fresh gas flows. At the time of writing, in the USA there is a datasheet warning for sevoflurane regarding use of a low flows, following studies showing nephrotoxicity in rats due to the accumulation of Compound A (the likely risk of nephrotoxicity in humans or other animals with low flow and sevoflurane has not been established). Currently, the FDA recommends “Sevoflurane exposure should not exceed 2 MAC-hours at flows of 1 to 2L/min. Fresh gas flows <1L/min are not recommended.”. The UK datasheets for SevoFlo state that “Long-duration, low-flow sevoflurane anaesthesia should be avoided due to the risks of Compound A accumulation.”

Look out for recapture technologies which may become commercially available to veterinary professionals in the near future. These can capture anaesthetic agents in passive scavenging systems, and redistill the anaesthetic agent ready for reuse. These systems are currently in trials for sevoflurane within the NHS. The cost of sevoflurane is likely to reduce when recaptured medical product becomes available.

Tip: Recommendation for using sevoflurane with low flow anaesthesia – always maintain fresh gas flow rate of >1L/min and maintain anaesthesia for a maximum duration of 2 hours at less than 2 L/min.

10. Engage your staff

Communicate and support your changes clearly to all staff engaged in veterinary anaesthesia, and seek specialist support from an EBVS®, RCVS, ECVAA or ACVAA Specialist in Veterinary Anaesthesia and Analgesia if in doubt.

Further information is available at:

• Jones, RS & West, E (2019) Environmental Sustainability in Veterinary Anaesthesia. Veterinary Anaesthesia and Analgesia 46 (4) 409-420, and the accompanying podcast 
• https://ava.eu.com/managing-the-isoflurane-shortage/
• https://www.bsava.com/News/ArticleID/2535/Isoflurane-supply
• BSAVA Manual of Canine and Feline Anaesthesia and Analgesia (3^rd Edition)
• Brattwall M, et al. (2012) Brief review: theory and practice of minimal fresh gas flow anesthesia. Canadian Journal of Anesthesia 59, 785-797
• Feldman JM (2012) Managing fresh gas flow to reduce environmental contamination. Anesthesia and Analgesia 114, 1093-1101.
• Gregorini P (1992) Effect of low fresh gas flow rates on inspired gas composition in a circle absorber system. Journal of Clinical Anesthesia 4, 439- 443
• Gibson S (2020) Low Flow Anaesthesia. WebinarVet seminar (due Thursday 25^th June 2020)

 

With thanks to Sarah Gibson and Louise Clark for review of this article.