How to pressure check an anaesthetic machine

02 November 2015
Practical
7 mins read
02 November 2015
Volume 6 · Issue 9

Abstract

Pre-anaesthetic checks, including those focusing on the anaesthetic machine, form an important part of patient safety and have become an integral part of the peri-anaesthetic checklists currently being introduced into veterinary medicine. Careful testing of the anaesthetic machine involves pressure testing the various components, allowing detection of any defective workings prior to use. Implementing a step by step guide ensures that no important stages are omitted from pressure checking and prevents patient safety from being compromised. Visual checks of the gas cylinder connections, vaporiser(s), flow meters and common gas outlet should be followed by testing of the low and high pressure systems within the anaesthetic machine and then the vaporiser and its attachment to the back bar. Any leaks detected should be identified and resolved if possible or repairs organised and alternative equipment utilised.

Patient safety is an area of veterinary medicine that is rapidly increasing in awareness amongst anaesthetists, nurses and practitioners. Anaesthetic and surgical safety checklists are gaining in popularity in veterinary medicine after their successful implementation into human medicine. An important step prior to induction of anaesthesia is to test the anaesthetic machine, ensuring it is correctly set up, functioning normally and therefore safe to use. The anaesthetic machine itself should be pressure tested at least once daily, preferably prior to first use and the breathing system selected should be leak tested prior to each and every use.

Anaesthetic machines used in veterinary medicine are found in a variety of designs and are often ex-hospital machines (Figure 1). It is important to be aware of any particular features when planning pre-use testing, as the machine set up may vary depending on the design. Anaesthetic machines may be supplied by one or more gas cylinders (e.g. oxygen, nitrous oxide, medical air) or pipelines (Figures 2a and b), and have vaporisers fixed within the machine or positioned on a selectatec bar (Figure 3a and b). It is important to be aware of where connections such as the common gas outlet (CGO) are positioned and which, if any, safety features are fitted to the machine being used. Safety features may consist of an oxygen low-pressure alarm, hypoxia alarm if nitrous oxide is used, and a gas on/off switch. Cylinder connections onto the machine will be dependent on the size of cylinder. Size E and J cylinders will have a yoke attachment (Figure 4), whereas size F cylinders will have a screw thread (bullnose) attachment (Figure 5) with the first stage regulator and pressure manometer attached.

Figure 1. Modern multi-gas anaesthetic machine with a selectatec vaporiser back bar and integrated mechanical ventilator.
Figure 2a. Yoke attachment for a size J cylinder with integrated pressure regulator and gauge, suitable to feed into a pipeline system.
Figure 2b. Cylinder bank and manifold for pipeline system.
Figure 3a. Selectatec manifold ‘back bar’ for vaporiser attachment
Figure 3b. Vaporiser fitted to selectatec manifold with locking mechanism visible. A second selectatec space is visible to the right.
Figure 4. Size E cylinder yoke attachment with pressure gauge.
Figure 5. Size F cylinder with screw threaded (bullnose attachment) pressure regulator and gauge.

All anaesthetic machines have a high and a low-pressure section, both of which need to be checked for leaks. The high-pressure system consists of the cylinder or pipeline, cylinder yoke (if present), gas lines up to the first stage regulator, and ends at the second stage regulator (Figure 6). The reduction in pressure at the second stage allows for safe, consistent gas delivery to the flow meters. From the flow meters onward it is a low-pressure system.

Figure 6. Second stage pressure regulator situated on the underside of this anaesthetic machine

When performing pressure and safety testing prior to anaesthesia a step-by-step approach should be utilised to ensure no section is omitted and therefore detect any faults present.

Initial machine checks

Intial machine checks should comprise:

  • Check gas cylinders are attached correctly (Figures 4 and 5) labeled full and in use respectively (Figures 7a and b), or pipeline gases are attached
  • Turn on cylinders or gas pipeline and check pressure gauges (Figure 8) on cylinders (13700 kPa when full) or pipeline (4 bar) (Figure 9) to ensure sufficient pressure in the system. If any audible leaks are present then the supply should be switched off and the seals on the cylinder yokes or screw valves checked and re-attached. Replace any seals or re-tighten screw valves if necessary
  • Check oxygen supply (safe use: two oxygen cylinders attached/available or a pipeline supply and one spare cylinder attached) +/- other gases, e.g. nitrous oxide or medical air if used
  • Ensure the flow meter(s) moves freely across all rates (Figure 10)
  • Ensure the vaporiser(s) is fitted correctly and locked securely into place (if using a selectatec system)
  • Ensure the vaporiser is filled and the dial moves freely (make sure you do this with the O2 flow meter turned off).
    • Figure 7a. Cylinder labeled ‘in use’.
    Figure 7b. Cylinder labeled ‘full’.
    Figure 8. Full oxygen E-size cylinder with pressure gauge reading 13700 kPa.
    Figure 9. Pipeline pressure gauge showing normal operating pressure of 4 bar (gauge on left). Cylinder pressure gauge to the right.
    Figure 10. Single flowmeter or rotameter with bobbin set to 2 litres/min.

    Next move on to checking the high and low-pressure sections of the anaesthetic machine in more detail.

    High pressure leak testing

    The high-pressure system checks test the machine from the cylinders or pipeline connections to the second stage regulator.

  • Turn all flow meters to zero (Figure 11) and turn machine switch (Figure 12) to ‘off’, if present (new and ex-hospital machines often have an on/off switch to prevent a patient being connected to the machine with no gas flow)
  • Open and then close both oxygen cylinders, allowing a short period of time to fill the reservoir tank (if present)
  • Check gauges of cylinders. A falling pressure indicates a high-pressure leak. Leak testing around the cylinder yoke should then be performed if necessary
  • Turn machine switch ‘on’, open flow meters and drain the machine again
  • The low oxygen pressure alarm should become audible (if present)
  • This may be repeated for any other gases in use, e.g. nitrous oxide.
    • Figure 11. Flowmeter set up on a multiple gas anaesthetic machine. This machine is fitted with an on/off switch, which maintains the gas flow at 0.3 litres/min when turned on.
    Figure 12. Anaesthetic machine fitted with an on/off switch.

    Additional tests

    The following additional tests should be performed if there is a pipeline supply in addition to a cylinder supply:

  • All flow meters should be left open throughout testing (if using oxygen plus other gases, e.g. nitrous oxide)
  • Insert oxygen pipeline probe into the ceiling or wall socket (Figures 13 and 14) (Shrader outlet) and tug test (one firm pull on the probe to make sure it is firmly connected to the outlet)
  • Confirm pipeline oxygen gauge reads approximately 4 bar (Figure 9)
  • Check that the O2 flow meter bobbin spins freely at both low and high flows and the movement is smooth (older flow meters are susceptible to static electricity build up and may stick)
  • Disconnect O2 wall connection and drain machine of oxygen using the emergency ‘flush’ button
  • The low oxygen pressure alarm should become audible (if present)
  • The above may also be used to test any additional pipeline gases available, e.g. nitrous oxide.
    • Figure 13. Ceiling mounted schraeder socket.
    Figure 14. Wall mounted schrader socket with oxygen probe inserted.

    Vaporiser checks

    Now move on to check for any faults associated with the vaporiser(s), e.g. leaks associated with the vaporiser and its connection to the machine:

  • Set the oxygen flow to 2–4 litre/min
  • Ensure scavenging system is connected and a breathing system attached with a sealed patient end to prevent environmental contamination
  • Adjust the vaporiser from 0 to 1% slowly. The oxygen flow should not decrease by more than 1 litre/min
  • Repeat this test on a second vaporiser if present.

    • To check the vaporiser(s):

  • Turn off oxygen flow meter (and any other gases being used)
  • Check the contents of each vaporiser (Figure 15)
  • Check filling ports of each vaporiser are closed (Figure 15)
  • Check the control dial turns freely
  • If using a selectatec bar with more than one vaporiser mounted, test the lock out system for each vaporiser to ensure only one vaporiser can be switched on at a time (Figure 16)
  • Turn the machine switch (if present) to ‘off’.
    • Figure 15. Isoflurane vaporiser being filled with a key-filler attachment
    Figure 16. Two vaporisers fitted onto a selectatec manifold.

    Low-pressure leak testing

    The low-pressure system checks test the anaesthesia machine from the flow meter(s) to the CGO (Figure 17). They check the section of the machine that is downstream from any fitted safety devices except an oxygen analyser (if one is fitted to the anaesthetic monitor). The components within this area are often the ones most likely to break or leak due to their fragile construction. The flow tubes with the rotameter set up are one of the most delicate components of the machine, and are susceptible to cracking or breaking. An anaesthesia machine may have up to 16 O-rings (rubber ‘seals’ used to ensure no leaks at connections) (Figure 18) in the low-pressure circuit. Leaks can occur at the interface between the glass flow tube and the manifold, and at the O-ring junction (Figure 3) between the vaporiser(s) and its manifold (selectatec system). Loose filler cap seals (Figure 19) on vaporisers are also a common source of leaks. Performing the appropriate low-pressure leak test before every case should be considered as part of a preanaesthestic safety check.

    Figure 17. Common gas outlet (CGO) and oxygen flush position on this wall mounted anaesthetic machine.
    Figure 18. ‘Bodok’ seal used at various connections throughout the anaesthetic machine and attachments. The rubber seal may perish over time and its failure is a common reason for leaks. They should be replaced on a regular basis.
    Figure 19. ‘Isoflurane vaporiser showing viewing chamber and filler cap to bottom right for insertion of filling key. Incorrect re-insertion or failure to replace the filler cap may result in leaks.

    Completing the low-pressure safety check

    This may be completed with a suction bulb (if available), which may be a more accurate way to test this section of the machine. If a bulb is not available please use the section following this instead.

  • Turn the flow meters on to 2 litres/min
  • Test the leak test device by deflating bulb and occluding connector end of device and ensure the bulb does not re-inflate
  • Connect leak test device (suction bulb) to the CGO
  • Compress and release the bulb until it stays deflated. The vacuum causes the bobbin to fall but it should then settle. If the bulb reinflates there is a low-pressure leak
  • Test each vaporiser set at 1% repeating the above steps
  • Turn all flow meters off.

    • If you do not have access to a self-inflating bulb then complete the low-pressure testing using the following method:

  • Turn the flow meters on to 2 litres/min (Figure 10) and check bobbin position
  • Occlude the CGO with the palm of your hand
  • The bobbin should fall briefly and then settle
  • Release the occlusion of the CGO and the bobbin should return to its original position
  • If the bobbin does not fall, settle and then return to its original position then there is a leak in the low-pressure section
  • Repeat the test with the vaporiser(s) set to 1%.

    • Conclusion

    Incorporating pressure testing of the anaesthetic machine into your standard pre-anaesthetic preparation makes for good clinical practice. It ensures that any leaks or faults are detected prior to induction of anaesthesia and therefore any compromise to patient or staff safety. Differentiating between leaks within the high and low-pressure sections of anaesthetic machine helps in troubleshooting problems and in deciding whether an engineer is required to repair any faults. Producing a step-by-step guideline provides an aide memoire to ensure no important checks are missed and the safety checks become part of normal clinical practice.

    Key Points

  • Use of peri-anaesthetic checklists in practice is an easy step to implement to ensure good case preparation and improve patient safety.
  • An anaesthetic machine should be checked a minimum of once daily, prior to first use to ensure they are working correctly. The selected breathing system should be checked prior to each and every use.
  • Anaesthetic machines can vary in design, emphasising the importance of being familiar with the set up of the machine(s) available and their safety features.
  • Thorough completion of both high and low pressure system checks are important to ensure any leaks are detected and either corrected or alternative equipment used.
  • Vaporisers and their attachments are a common place to identify leaks and should be checked thoroughly as part of the pre-anaesthesia process.
    • patient safety
    pressure testing
    anaesthetic machine
    safety checklist