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Now we move onto vaporisers; the devices that ensure the effective and safe addition of inhalant anaesthetics to the fresh gas flow exiting the common gas outlet.
Most modern anaesthetic machines incorporate the vaporiser out-with the patient’s breathing system; “vaporiser out of circuit” (VOC). The vaporiser is placed between the flowmeter block (e.g. flowmeters for oxygen, nitrous oxide, medical air) and the emergency oxygen flush valve. This is important as it means that there is no risk of high oxygen flow from the flush valve entering the vaporiser. The vaporiser is mounted to the backbar, on older machines it may be difficult to remove and change vaporisers, but new machines use systems that allow easy placement and removal of vaporisers of a specific brand (e.g. the Selectatec for the TEC series of vaporisers). VOC vaporisers are specific to a particular anaesthetic agent, and the safest option for modern inhalant anaesthetics such as halothane, isoflurane, sevoflurane, and desflurane.
Image shows isoflurane vaporiser (Tec 3) situated on the anaesthetic machine backbar, between flow meters and common gas outlet.
Alternatively, in some older or “field” anaesthetic machines (e.g. Stephens’, Komesaroff), the vaporiser may be placed in the inspiratory limb of the breathing system; “vaporiser in circuit” (VIC). The output of these vaporisers depends not just on the dial setting selected, but upon the gas flow through them, the patient’s breathing rate/volume, ambient temperature, and the type of anaesthetic gas used. This makes it difficult to administer a known and precise amount of anaesthetic to a patient and can produce a significant risk of overdose when modern, highly volatile anaesthetic agents are used.
The remainder of this article will focus on VOC vaporisers.
Modern inhalant anaesthetics are liquids at room temperature and so must be converted to gas prior to delivery to an animal, via the process of vaporisation. During vaporisation, molecules of anaesthetic on the surface of the liquid phase escape and become vapour (gas).
This process continues until the pressure exerted by the molecules in the vapour phase is maximal (e.g. equilibrium is reached between the liquid and gas phase) – this pressure is the “saturated vapour pressure” (SVP). The SVP is different for each anaesthetic agent, and also differs according to temperature. It is the pressure of anaesthetic gas in a gas mixture (its “partial pressure”) that determines the “dose” that will reach the brain of the animal that is breathing the gas mixture. The SVP of the inhalant anaesthetics that we use in veterinary anaesthesia far exceeds the dose required to produce anaesthesia, so vaporisers must be used to ensure that a small and precise concentration of anaesthetic gas is added to the fresh gas flow (e.g. oxygen).
As you can see from the above, the most important function of a vaporiser is to ensure that a precise and constant amount of anaesthetic gas is added to the final gas mixture that exits the common gas outlet of the anaesthetic machine. In order to do this the vaporiser must account for any changes in the fresh gas flow, or temperature– as all of these will affect the amount of anaesthetic liquid that is vaporised.
If a vaporiser did not compensate for flow, low flows of carrier gas (e.g. oxygen) through the vaporiser would result in higher concentrations of anaesthetic gas being added to that carrier gas and exiting the vaporiser, and high gas flow would result in lower concentrations of anaesthetic. Vaporisers overcome this problem via one of two methods; variable bypass, or measured flow.
Most vaporisers used in veterinary practices are “plenum vaporisers” and use the variable bypass method:
Briefly, the carrier gas that enters the vaporiser is split into two streams, one that completely bypasses the anaesthetic liquid/gas, and another that passes through the vaporisation chamber and becomes fully saturated with anaesthetic. They mix together as they leave the vaporiser. The amount of gas that enters each stream is referred to as the “splitting ratio” and is adjusted by turning to the control dial of the vaporiser. Regardless of the carrier gas flow rate, the portion flowing through the vaporiser chamber will always be fully saturated with anaesthetic gas and so vaporiser output will not vary with different gas flows. In order to achieve full saturation with anaesthetic gas the gas stream in the vaporiser chamber is directed over the surface of the anaesthetic liquid and the surface area enhances with the use of wicks that dip into the liquid, or it is actually bubbled through the liquid itself.
Diagram illustrates a variable bypass vaporiser
Measured flow vaporisers are more likely to be found on more modern anaesthetic workstations (e.g. GE Healthcare) that veterinary clinics might acquire second-hand from human hospitals. This technique is also used in most desflurane vaporisers. In these vaporisers, only a small portion of the carrier gas flow actually enters the vaporiser, and rather than being split this portion becomes fully saturated with anaesthetic prior to exiting the vaporiser and joining the carrier gas flow.
GE Healthcare Aladin cassette vaporisers – function as variable bypass and measured flow
Heat is required, and used, during the process of vaporisation. This leads to cooling of the liquid anaesthetic and, if not compensated for, would lead to slowing down of vaporisation over time, a fall in SVP, and so a reduced concentration of anaesthetic gas leaving the vaporiser. Vaporisers compensate for this by being made from materials with good heat conductivity, and also incorporating mechanisms that alter how much carrier gas flows through the vaporisation chamber depending on temperature changes.
In addition to flow and temperature compensation, modern vaporisers also compensate for changes in pressure through the anaesthetic machine (e.g. back-pressure produced by the use of a ventilator).
Desflurane requires a specialised vaporiser because it has a boiling point close to room temperature. This means that it would not reliably stay as a liquid in the vaporisation chamber at room temperature and so the output would be highly variable. To overcome this problem, the chamber containing the desflurane liquid is heated to 39 degrees Celsius via an electrical filament, and a measured amount of vapour is then added directly to the fresh gas flow. These vaporisers must therefore be connected to a power source, and incorporate various alerts and alarms to ensure they are functioning properly.
Drager D-vapor desflurane vaporiser
Exposure of anaesthetic liquid to room air via an open bottle or spilling of liquid poses an occupational health hazard as anaesthetic vapour may be breathed in by personnel in the room. Also, it is important to ensure that the correct anaesthetic agent is added to the correct vaporiser. These problems are overcome by agent-specific filling devices.
Most commonly, the “keyed-filler” is used in halothane and isoflurane vaporisers.
Isoflurane keyed filler
Sevoflurane and desflurane bottles are designed to fit specifically onto the appropriate vaporiser.
Sevoflurane filling system
If the wrong agent is inadvertently added to a vaporiser then all liquid should be drained from the vaporiser, it should be flushed with oxygen or air at 5L/min with a dial set at 5% for 30 minutes, then should be filled with the correct agent and left for 2 hours prior to use.
It is important that only one type of anaesthetic gas, from one vaporiser, is delivered to an animal at a time. Some older machines allow for multiple vaporisers fitted to the back bar in series to be used concurrently. To minimise risk when using these machines it is preferable to place the vaporisers in order of anaesthetic gas potency and vapour pressure: e.g. sevoflurane upstream closest to the flowmeters, then isoflurane –> halothane –> desflurane downstream nearest to the common gas outlet. Newer machines use an interlocking system on the back bar that ensures only one vaporiser can be used at a time, generally via a device that locks the control dial of the vaporiser/s not in use.
If plenum vaporisers are accidentally tipped (e.g. during transport from one machine to another, or on an anaesthetic trolly) there is a risk that anaesthetic liquid can enter the bypass channel leading to a higher than expected anaesthetic output. Even though some newer vaporisers incorporate safety systems to minimise this risk, liquid can still enter the bypass chamber if tilted more than 30-45 degrees. If a vaporiser is accidentally tipped, the emptying and flushing procedure detailed above (for dealing with a vaporiser filled with the wrong agent) should be performed.
Vaporisers should be checked for leaks as part of a daily anaesthetic machine check prior to use:
Additionally, vaporisers should be regularly serviced by properly qualified technicians.
Any questions for Jen? Add 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.
Connect with Dr Jen:
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