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Veterinary Anaesthetic Machine Made Simple - The Oxygen Flush Valve

Posted in Operations @ Mar 16th 2017 - By Dr Jen Davis BVMS Masters Vet Anaes. MANZCVS (Anaesthesia & Critical Care) Dip ECVAA
Veterinary Anaesthetic Machine Made Easy Part 5 The Oxygen Flush Valve Blog

Part 5 in the series - The Veterinary Anaesthetic Machine Made Simple - thanks to Dr Gas Vet

Originally published on the Vet Anaesthesia Tips blog. Scroll down for links to Parts 1, 2, 3 and 4 in the series.

What is the Oxygen Flush Valve?

The oxygen flush valve delivers oxygen at a high pressure (~400kPa) and flow rate (30 – 70 L/minute).

This oxygen comes directly from the oxygen cylinder or pipeline, so bypasses the anaesthetic agent vaporiser and gas flowmeters.

How do I find it and activate it?

The oxygen flush valve is usually well marked, with “emergency O2” or simply “O2”. It is usually a large button that is either elevated or recessed.

It is often located next to the common gas outlet.


When the button is pressed, oxygen at a high pressure and flow rate will be delivered to the common gas outlet until pressure is removed from the button.

What is it for?

It can be used to rapidly purge the anaesthetic machine of any other gases and anaesthetic agents.

This is why it is often referred to as the “emergency” oxygen flush. 

Some examples of when this might be required include:

  • During CPR when an animal arrests during anaesthesia, and an anaesthetic machine is used to provide ventilation
  • If an animal is suspected to develop raised intra-cranial pressure or malignant hyperthermia during anaesthesia (e.g. inhaled anaesthetic agents may worsen the situation)
  • If the machine is to be used to provide flow-by oxygen to a conscious patient (e.g. isoflurane has a pungent odour and is not well tolerated via mask)

When should it not be used?

The oxygen flush valve should never be pressed when a patient it attached to the anaesthetic machine using a non-rebreathing system (e.g. Bain, Lack, T-piece). This is because this high pressure and flow of oxygen will be directly transmitted to the animal’s airways and lungs with the potential to cause barotrauma.

Use of the oxygen flow valve when a patient is attached by a rebreathing system (e.g. circle) is less likely to cause trauma if it is used carefully (e.g. only to fill up the rebreathing bag, or ventilator bellows). However, the risk of barotrauma still exists so this should be avoided where possible.

If you need to use the oxygen flush valve to purge the machine while a patient is attached, it is safest to simply disconnect the animal from the breathing system momentarily. If your reason for using the flush valve is to fill up the rebreathing bag, or change the concentration of anaesthetic agent in the system rapidly, consider whether this might more safely be achieved by turning up the oxygen flowmeter.

Next time: flowmeters…

Any questions for Jen? Ask them in the Comments section below...

Click here to read The Veterinary Anaesthetic Machine Made Simple: Part 1

Click here to read The Veterinary Anaesthetic Machine Made Simple Part 2: Gas Cylinders

Click here to read The Veterinary Anaesthetic Machine Made Simple Part 3: Pipeline Gas Supply

Click here to read The Veterinary Anaesthetic Machine Made Simple Part 4: Pressure Gauges & Regulators

About Jen

Dr Jen Davis (@Dr GasVet) is a European Specialist in Veterinary Anaesthesia and Analgesia. She is currently undertaking a PhD at Murdoch University, investigating the early diagnosis of acute kidney injury induced by anaesthesia-related hypotension.

Jen also works part-time as registrar in veterinary anaesthesia at The Animal Hospital at Murdoch University, where she administers sedation, anaesthesia, and analgesia to all species of animal, as well as teaching undergraduate students and resident vets studying to become anaesthesia specialists.

A summary of Jen’s research, and open access to her published work can be found on ResearchGate.

For more excellent posts on veterinary anaesthesia vist Jen's blog: Vet Anaesthesia Tips and register to receive notifications of new posts by email

You can also follow Jen on:

Twitter: Vet Anaesthesia Tips  |  Facebook: Veterinary Anaesthesia Tips


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