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Originally published on the Vet Anaesthesia Tips blog. Scroll down for links to Parts 1, 2 and 3 in the series.
For the next blog in this series about the anaesthetic machine, we will look at the final components of the high pressure system of the anaesthetic machine: pressure gauges, and pressure regulators.
(Remember that gas pressure within the anaesthetic machine moves from high – intermediate – low as we move from the areas where gas is delivered to the machine to where gas is delivered from the machine to the patient).
Gas cylinders, and pipeline connections, should always have a corresponding pressure gauge. For cylinders containing gases that are all gas at room temperature (e.g. oxygen, medical air) the pressure gauge allows assessment of how much gas remains in the cylinder. However, remember that this does not apply to nitrous oxygen cylinders (check my first blog on gas cylinders). For pipelines, the gauge enables assessment of whether the pipeline is connected and open to the anaesthetic machine.
These pressure gauges are often “bourdon gauges” – these are simple, inexpensive and robust gauges that can withstand high pressures. They consist of a curved flattened tube that expands when pressure is applied causing it to straighten out. The straightening of the tube then triggers a mechanism to move a dial pointer on the “face” of the gauge.
Gauges are often colour coded according to what gas pressure they are measuring (e.g. white for oxygen, blue for nitrous oxide).
When using a cylinder supply for a gas that is all gas at room temperature (e.g. oxygen), you would expect the reading on the dial of the pressure gauge to drop slowly. This should be used to indicate when it is time to change the cylinder for a new one.
When using a pipeline supply you would expect the reading on the dial of the pressure gauge to remain fairly steady when in use, as pipeline pressure should not fluctuate much.
You are most likely to look at these gauges when checking and setting up an anaesthetic machine prior to use.
Be aware that if a cylinder has been turned off, or a pipeline closed, but not disconnected from the machine the dial of the pressure gauge may not read zero and it could look as if the gas supply is appropriate.
However, when a flow meter was opened the dial reading would quickly drop to zero as the gas left in the pipelines/machine was quickly depleted. This demonstrates the importance of performing anaesthetic machine checks prior to each use. Also, a good way to avoid this confusion is to purge the machine of gas by pressing the oxygen flush valve once you have turned off the machine gas supply at the end of the day.
Note that you might see other similar-looking pressure gauges attached to your anaesthetic machine: for instance, a manometer attached to the ALP valve of a circle system (to indicate what pressure is being applied to an animal’s lungs during ventilation), or on active scavenging system (to indicate the degree of suction). You should be able to identify which are gas supply pressure gauges by their location on the machine (e.g. near cylinder or pipeline attachment), and they will usually have the type of gas they are measuring marked.
Pressure regulators may also be referred to as “pressure-reducing valves”. These are placed between gas cylinders and the anaesthetic machine. Anaesthesia machines should have a regulator for each medical gas supplied to the machine.
The pressure within a gas cylinder will vary depending on how full it is and the temperature. The pressure in a full cylinder is relatively high (2000psi in an oxygen cylinder). Thus the purpose of the pressure regulator is: (a) to reduce the pressure of gas entering the anaesthetic machine from the cylinder, and (b) to maintain a constant pressure of gas delivery to the anaesthetic machine as the contents of the cylinder become depleted.
The consequences of unchecked, high pressure delivery of gas to the anaesthetic machine include:
As well as those connected to each cylinder, additional regulators may be positioned in the anaesthetic machine to smooth out fluctuations in pressure supply. Safety relief valves are often fitted further downstream in the anaesthetic machine to allow the escape of gas if the regulators fail.
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 5: The Oxygen Flush Valve
Click here to read The Veterinary Anaesthetic Machine Made Simple Part 6: Flowmeters
Click here to read Veterinary Anaesthesia – Vaporisers
Click here to read Benefits of using a co-induction technique in veterinary anaesthesia
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.
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Such a great article this can really help a lot of people to decide what is the best for pressure equipment.
Thanks for your question Christa – I’ll see what Jen has to say 🙂 Regards Judy
Hello which part is the pressure reducing valve in the photo please?
Hello Jen, great articles. I have a question regarding the set up of our oxygen tanks (Size NG x 2). In the attached photo of our set up, I assume the 2 dials at the very top are pressure regulators to reduce the pressure from the tanks to a more usable pressure (the other gauges above the tanks tell us how much gas in in the tanks).
With regards to the top ones, am I correct that the one on the right is the pressure in the tanks at 2000 psi and then drops it down to about 25psi as shown in the dial on the left as it goes into the building? What I do not understand is the black dial under the gauges “Gascon systems – increase and decrease” are we meant to adjust this at all? It does not seem to affect the pressure in the gauges? Thank you so much Anna