Low Flow Anesthesia in a Veterinary Practice: Key Do's and Don'ts

The Benefits of Low-Flow Anesthesia for Veterinary Practices.

There was once a time when anesthetic was all about just keeping the patient ‘asleep’ so that a procedure could be carried out.

If we are being responsible practitioners and business owners, we should be striving to reduce the use of some of these inhalants.

Using injectable anesthetic protocols eliminates the volatile gas element altogether, but there is still a carbon footprint associated with the production of the drug, the electricity needed to power the syringe driver, drug wastage, and so on.

We also need to be careful not to compromise patient safety.

Low-flow anesthesia helps to provide the compromise we need, allowing us to continue with inhalants but at lower levels than we are traditionally used to.

What is low-flow anesthesia?

Simply put, low-flow anesthesia is the minimum safe gas flow you can use during the maintenance phase of a patient’s anesthetic, that still supplies them with enough oxygen to continue with normal metabolic functions.

It is defined as a fresh gas flow (FGF) of 0.5-1 L/min. 

Minimal flow (which is 0.25 – 0.5 L/minute) or closed-circuit techniques (< 0.25 L/minute) also exist, but care must be taken to ensure your patient doesn’t become hypoxic.

This is made riskier if nitrous oxide is used, so this would not be recommended. It’s also important to remember that many general-purpose vaporizers do not operate reliably below 0.5L/min.

Many practices have conventionally used comparatively high FGF rates, usually much more than is actually required for our patients.

This has many downsides, including being wasteful of oxygen and inhalational anesthetics. This not only impacts the environment but also costs money. There is an increased risk of unnecessarily exposing the veterinary team to anesthetic agents with this method too.

For our patients, there is also the downside of having lots of unnecessary cooling gases entering the airways, which is not ideal for a small animal under anesthesia. Hypothermia is a real risk, particularly for animals under 10kg, so re-breathing warm gases in low-flow anesthesia can be very helpful.

What are the dos and don’ts of low-flow anesthesia in a veterinary clinic?

Most of the problems faced in low-flow anesthesia are due to inappropriate equipment which we will cover here.

Dos:

Do use a veterinary vaporizer that can deliver precise fresh gas flows down to 200ml/min and make sure that it has been calibrated and tested for low flow rates.

Despite claims, some vaporizers struggle with high resistance, an inability to respond quickly to setting changes, and inaccuracies in the delivery of inhalants (Kelly and Kong, 2011).

Don’t hesitate to speak to the manufacturer if you require specific advice about individual vaporizers.

Do use a precise oxygen flow meter which can be easily read and used to monitor fresh gas flow rates down to 200ml/minute.

Do use a low volume / low resistance circle absorber which can rapidly respond to changes in vaporizer settings at 200ml/min fresh gas flow. It should respond within 5 breaths ideally.

Some can be very slow (up to 20 minutes). Increasing your FGF during vaporizer setting changes can help, but may negate some of the benefits of using a low-flow system.

Do always leak-test your system before use. The soda lime in any rebreathing circuit needs daily checking too, as once exhausted the indicator can revert back to its original color if left in situ which can be misleading, severely compromising its carbon dioxide absorbing abilities.

Do use a higher FGF (2L/min) and vaporizer settings at the start of your anesthetic. Around 10-15 minutes is required to allow the anesthetic agent and oxygen concentrations to build to a sufficient level in the breathing system (Feldman, 2012).

Similarly at the end of your anesthetic increasing your FGF can help to remove any volatile agent more rapidly.

Do use appropriate analgesia and volatile agent-sparing techniques.

This includes using systemic analgesia as well as regional anesthesia techniques like epidurals, peripheral nerve blocks, and splash blocks. You could also consider total or partial intravenous anesthesia.

Do use capnography whenever you’re doing a low-flow protocol. It gives us a measure of how well a patient is ventilating and is the only reliable early measure of hypoxemia.

Don’ts

Don’t use repurposed human equipment for low-flow anesthesia in small animals. It is unlikely to be accurate enough for the patient sizes you will be dealing with.

Don’t try and use non-rebreathing systems that require high gas flows to expel expired CO2, such as Bain circuits or Ayres T-pieces.

Don’t use low-flow when nitrous oxide is being delivered unless there is in-circuit oxygen concentration monitoring. There is a risk of hypoxia in these patients.

Don’t expect staff to feel comfortable with new techniques without appropriate training, invest time in your team.

Don’t worry if you don’t have access to all the patient monitoring tools.

However, your patient will need to be observed very closely.

Most practices have a pulse oximeter though, which you should use as a minimum.

This tells us about hemoglobin saturation, which is essential in low-flow anesthesia.

Gas/anesthetic agent monitoring is ideal as it monitors inspired O2, as well as inspired and expired anesthetic agent concentrations, but this is expensive so many practices do not have access to this.

Summary

There are multiple benefits to low-flow anesthesia, and it should be considered by all veterinary hospitals. Ensuring you have the right equipment in place is key, as is spending time on staff training so that you can be sure you are providing the very best care for your patients.

References

Feldman, Jeffrey M. MD, MSE. Managing Fresh Gas Flow to Reduce Environmental Contamination. Anesthesia & Analgesia 114(5):p 1093-1101, May 2012. | DOI: 10.1213/ANE.0b013e31824eee0d

Jones, RS & West, E (2019) Environmental Sustainability in Veterinary Anaesthesia. Veterinary Anaesthesia and Analgesia 46 (4) 409-420

Kelly, J.M. & Kong, K.-L. Accuracy of ten isoflurane vaporizers in current clinical use. Anaesthesia, 2011; 66: 682–688

‘The whys and hows of low flow: an introduction to safe low-flow anesthesia’ by Simon Wheeler and Colin Dunlop, 14th June 2023

‘Low Flow Anaesthesia: Frequently Asked Questions, April 2020’ – Davies Veterinary Specialists, April 2020

‘Reducing anesthetic gas for environmental benefit’ - Ellie West MA Cantab VetMB CertVA DipECVAA AIEMA MRCVS, Clinical Anaesthetist, and Sustainability Lead at Davies Veterinary Specialists, 30th January 2020