Part 2 in the series The Veterinary Anaesthetic Machine: Made Simple thanks to Dr Gas Vet
Originally published on the Vet Anaesthesia Tips blog 29/7/16: Gas Cylinders. Scroll down to see all the posts in the series.
In Part 2 of this series of blogs about the anaesthetic machine, we will start from the beginning, with the gas supply, commencing with gas cylinders.
Purpose ….
Gas supply directly to the anaesthetic machine: cylinders may be attached to an anaesthetic machine as a sole method of fresh gas delivery, or as a backup for oxygen provision in case of pipeline failure.
Cylinder manifold: this is a bank of large cylinders stored externally to the building to provide a gas reservoir which is then piped into the building.
Safety features to avoid inadvertent supply of the wrong gas type
Colour coding
The colour of the shoulders of a cylinder indicates the type of gas it contains (the colour of the body is also standardised). This colour also matches the colour of the hose and connectors used to attach the cylinder to the anaesthetic machine.
- Oxygen: white (EU, Australia), green (North America)
- Nitrous oxide: blue
- Medical air: black and white
- Entonox (50% oxygen, 50% nitrous oxide): blue and white
Pin-index system
The pin index system further prevents the accidental supply of the wrong gas type. The cylinder valve bears two holes (or ‘nipples’) around the gas outlet. The positioning of these holes in relation to each other is specific for each type of gas.
Cylinders will be attached to a yoke (part of the anaesthetic machine that a cylinder is screwed directly into) or to a regulator separate to the machine (some machines do not have a yoke, and instead a regulator is attached to the cylinder, then a short supply pipe runs between the regulator and gas connector on the anaesthetic machine).
The yoke or regulator is specific to a particular gas type and will have two pins in a position that corresponds to the holes on the cylinder. This means that only the correct type of cylinder can be attached to a particular yoke/regulator.
Sizing
Gas cylinder sizes are alphabetical with A being the smallest, and J is the largest. Cylinders used on anaesthesia machines are usually size E or smaller. Larger cylinders (e.g. J) are used for cylinder manifolds
Safe storage
Gas cylinders are filled to high pressures, and an explosion is possible if they are stored incorrectly.
- Do not subject to extremes of temperature
- Do not store near flammable materials (e.g. oil, grease) or near a source of heat
- Prohibit smoking and naked flames from the vicinity of the storage
- Store away from corrosive chemicals
- Store in a designated area of a dry, well-ventilated room (avoid areas where they could be knocked, walked into, etc.)
- Store/rack cylinders in a way that they are secured so they cannot fall over
Cylinder valve
The main purpose of the cylinder valve is to maintain a secure seal to the contents within the cylinder.
Inappropriate handling of these valves may result in an explosion.
Cylinders come with plastic wrapping around the valve block, to protect them from dust. Leave the wrapping on until the cylinder is to be used.
It is important that lubricants such as grease and oil never come into contact with a cylinder, especially not with the cylinder valve.
How to open and close a cylinder
- Identify the contents of the cylinder (read the label, check the colour)
- Remove the plastic covering around the valve
- Prior to opening a cylinder valve, check that there is no dust or particulate matter visible around the valve.
- Hold the cylinder so that the valve opening is facing away from you (and anyone else nearby)
- “Crack” the valve by opening it very slightly prior to connecting it to the yoke/regulator in order to blow off any dust sitting on the valve, then close it again
- Ensure that the yoke/regulator sealing washer (Bodok seal) is present and intact
- Ensure that the oxygen flow of the machine the cylinder is to be attached to is turned fully off
- Ensure that the holes and pins of the pin index system are aligned so that the cylinder gas outlet sits nicely against the Bodok seal of the yoke/regulator (if it is not fitting consider whether your cylinder contains the correct gas for that yoke/regulator).
- When closing the retaining screw of the yoke/regulator ensure that you are tightening it into the conical depression located on the other side of the valve stem to the pin-index system and gas outlet, some cylinders have a pressure relief device below the conical depression that it is important not to accidentally tighten the screw against (this device looks like a screw head).
- Open the cylinder valve slowly by turning the spindle anticlockwise. The sudden opening can lead to a rapid increase in pressure which can damage the machine, and even cause an explosion or fire.
- Open the valve fully while in use.
- When the anaesthetic machine is not in use close the valve fully by turning the spindle clockwise(but avoid over-tightening). Do not leave the cylinder standing upright without support. A size E or smaller cylinder may be stored either horizontally or vertically, but must be secured.
- When removing a cylinder from the yoke/regulator; close the cylinder by turning the spindle clockwise fully, ensure flowmeters on the machine are closed and that gas within the anaesthetic machine has been purged (by pressing the oxygen flush valve with the cylinder closed until no more gas hissing can be heard), then carefully unscrew from the yoke/regulator.
Cylinder contents
Bourdon gauges for measurement of pressure within the cylinder will be found either above the cylinder yoke or regulator or on the front panel of the anaesthetic machine (newer models). These gauges should be labelled, colour coded and calibrated for the specific type of gas.
When the cylinder valve is switched off the needle of the pressure gauge should drop to zero, however as the valve is opened the needle will move to indicate the pressure within the cylinder.
The method of estimation of how much gas is contained within a cylinder depends on the type of gas. For cylinders containing gases that are present only in their gaseous form within the cylinder (e.g. oxygen, medical air) the pressure of the cylinder decreases as the amount of gas present in the cylinder decreases. This makes it easy to monitor how much gas remains in the cylinder by inspecting the pressure gauge.
For gases present partially in liquid form within the cylinder (e.g nitrous oxide) the pressure gauge will remain full until all of the liquid has evaporated, and will only start to drop when a very small amount of gas remains. The contents of a nitrous oxygen cylinder should therefore be estimated by weighing the cylinder; the weight of these cylinders will decrease proportionately with the cylinder contents.
This is intended as a simple overview of anaesthetic gas cylinders. Feel free to comment with any additional questions you may have on the topic.
Next, I will write about pipeline gas connections and pressure regulators…
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
Haha I stumbled across an oxygen picture on google and noticed it had the same flooring and walls as Murdoch clicked onto this page and seen it was your article!!
Google collects everything Brooke! 🙂
Hi Jen the author
I visited your wordpress website last year and then came up with my practice’s new Dom and torb anaesthetic charts based on your calm/ friendly and also nervous/ aggressive dog doses. My boss has asked me to support these charts now so please can you send me the link to your new webpage … or your charts and text that was on your wordpress website as I cannot find it now!!! As I need to show the info came from a specialist 🙂
Many thanks
Lucy BVSc MRCVS
Hi Lucy, Judy here – I manage Vetanswers. Thanks for the reminder, you’re right Jen’s blog has moved and you can now find it here: https://vetanaesthesiatips.com/ I’ll need to change the address on all of Jen’s blog post 🙂 Thanks for visiting the Vetanswers Blog! Regards Judy