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Veterinary Anaesthetic Machine Made Simple - Backbar & Common Gas Outlet

Posted in Operations @ May 4th 2017 - By Dr Jen Davis, BVMS Masters Vet Anaes. MANZCVS (Anaesthesia & Critical Care) Dip ECVAA
Veterinary Anaesthetic Machine Made Easy Part 7 Backbar Common Gas Outlet

Part 7 in the series - The Veterinary Anaesthetic Machine Made Simple, thanks to Dr Gas Vet.

The final components of the anaesthetic machine, and of the low pressure area of the machine, are the backbar, pressure relief valve, and common gas outlet. (Note that we haven’t covered vaporisers yet; these are not technically considered to be components of the anaesthetic machine, more separate pieces of equipment in their own right).


The backbar is the area of the anaesthetic machine that holds/supports the flowmeters and vaporisers, and provides pathways for gases to travel between them. On older machines the back bar is obvious as a bar running across the back of the machine to which these components attach (see image above). However, on newer machines the “bar” is often encased within the plastic outer shell of the machine.

Flowmeters will be permanently attached to the backbar, upstream (e.g. closer to the gas supply and further from the patient) to the vaporisers.

Vaporisers may be permanently attached to the backbar, or easily detachable via a quick release system. On older anaesthetic machines (that are still commonly found in veterinary clinics) it is common to have a single vaporiser permanently fixed to the back bar. A disadvantage of this system is that the anaesthetic machine is then limited to only providing one type of inhalation anaesthetic (usually isoflurane), and if there is a problem with the vaporiser or it needs servicing the entire anaesthetic machine is out of action.

Vapouriser fixed permanently to backbar of an older machine

Newer systems for connecting vaporisers to the backbar allow vaporisers to be easily and quickly removed or added to the anaesthetic machine, while maintaining a gas tight seal. One of the most common of these systems is the “Select-a-tec” for Datex Ohmeda’s “Tec” series of vaporisers. This system consists of ‘male’ protruding docking valves attached to the backbar, and ‘female’ recesses on the vaporiser that specifically match the male valves. The vaporiser must be seated correctly over the valves, then locked in place before it can be used. Drager have a similar mounting system on their machines, that works with their series of vaporisers.

Anaesthetic vaporisers mounted next to each other using the Select-a-tec system on a GE Datex Ohmeda machine

These mounting systems usually allow the placement of more than one vaporiser on an anaesthetic machine. On older machines this introduces the risk that more than one vaporiser may be turned on at a time, potentially causing an anaesthetic overdose. Newer machines have an ‘interlocking pin system‘ to prevent this. When one vaporiser is mounted next to another, and one it turned on, a pin protrudes from that vaporiser into the vaporiser next to it, and prevents that vaporiser from being turned on.

Pressure Relief Valve

The backbar should have a high-pressure relief valve (check valve) attached. This is a valve that opens if high pressures (usually 30 – 40 kPa) occur in this low-pressure area of the anaesthetic machine. Opening of the valve allows gas to escape and so reduces the pressure to avoid damage to components such as the flowmeters and vaporisers. High pressures might occur when certain ventilators are used as they may apply back pressure into the anaesthetic machine. Note that these valves are present to protect the anaesthetic machine, not the patient attached to the machine (valves in the breathing system protect the patient from high gas pressures).

High pressure relief valve on the backbar of an anaesthetic machine

Common Gas Outlet

The common gas outlet is an important component of the anaesthetic machine to identify prior to its use. This is where the gases (from the flowmeters) and anaesthetic vapours (from the vaporisers) exit the anaesthetic machine. It is here that breathing systems (or ventilators, or oxygen supply systems) are attached in order to administer these gases to the patient. The outlet may be fixed in position, or swivel.


Most anaesthetic machines have just one common gas outlet, usually positioned downstream of the vaporiser(s) and often close to the oxygen flush valve. However, some newer machines with an in-built circle breathing system may have a separate common gas outlet for use with other types of breathing systems. If this is the case it is important to clarify that the correct outlet is receiving the gas flow before an patient is attached.

This common gas outlet on a newer anaesthetic machine (GE Datex Ohmeda) is only used with a non-rebreathing system, as the circle system is built into the machine. This means it requires adjustment of the knob to the right of it to ensure gas will be delivered to the correct outlet depending on type of breathing system used.

 The common gas outlet is a standard size: 22mm male / 15mm female. It is useful to note that, as well as allowing the attachment of all types of breathing system, this size of fitting also allows the attachment of any endotracheal tube connector (the end that fits into the breathing system when you attach the patient) as these are also the same standard size. This makes it easy to make your own oxygen supplementation device: an ET tube connector can be attached to any type of tubing and can then be used to insufflate oxygen from the anaesthetic machine into a mask or even directly into an animal’s trachea via a urinary catheter should intubation be difficult or not possible (e.g. during bronchoscopy). This can be especially useful when using older anaesthetic machines that do not have a separate oxygen outlet for this purpose.

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

Click here to read The Veterinary Anaesthetic Machine Made Simple Part 5: The Oxygen Flush Valve

Click here to read The Veterinary Anaesthetic Machine Made Simple Part 6: Flowmeters

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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