Vaporizers Flashcards
Define a vaporizer
A device for adding clinically useful concentrations of anaesthetic vapour to streams of carrier gas
Why is a vaporiser required and list the SVP for all of the volatile agents
A vaporiser is required because the SVP of the volatile anaesthetic agents at room temperature is many times greater than that required to produce anaesthesia
Agent SVP (Boiling Point)
ISO 33 (48.5) HALO 32 (50.2)
SEVO 23 (58.5) ENF 22 (56.5)
DES 89 (22.8)
Extra info:
N2O 5200
Xenon N/A
Distinguish a Plenum vaporiser from a draw-ver vaporiser
Plenum vaporiser –> positive pressure generated upstream of vaporiser and pushed through into two channels 1. bypass vaporiser chamber and 2. flows through vaporiser chamber.
Draw-over vaporiser –> negative (subatmospheric) pressure generated distal to the vaporiser chamber drawing the carrier gas over the volatile anaesthetic agent.
What type of vaporiser is the Boyle’s Bottle? Describe this device
It is a plenum vaporiser
- Glass bottle with volatile agent
- Rotatory valve controlling proportion of carrier gas flowing into vaporiser chamber.
- Bypass channel permitting passage of flow not in contact with volatile gas simultaneously controlled by rotatory valve.
- Anaesthetic gas concentration dependent on flow, a plunger or cowl allows carrier gas to either be bubbled through liquid or variably directed close or far away from surface of liquid.
What are the MAJOR PROBLEMS with the Boyle’s Bottle plenum vaporizer
Volatile anaesthetic agent concentration is dependent on both
- FLOW
- TEMPERATURE VARIATION due to variable changes (latent heat of vaporization)
How can the problem of FLOW dependence of volatile agent concentration in anaesthetic gas be overcome in modern vaporizers?
Arrange it so: ALL GAS EMERGING FROM VAPORIZER IS 100% saturated with volatile. The final % of anaesthetic gas delivered can then be set using a SPLITTING. The splitting valve functions to split the flow between the bypass channel and vapourizer chamber. With specific splitting ratios, the splitting valve can be calibrated to deliver specific concentrations of volatile independent of the flow rate of the carrier gas.
E.g. ISO SVP 32 therefore max ISO would be 32% if no splitting valve existed and flows were high.
If splitting valve exists and only 5 % of flow is permitted into vaporiser chamber then (according to Dalton’s law), the concentration of ISO in the carrier gas would be 5 % of 32 % = 1.6% provided that:
- Temperature kept constant
- Enough surface area in vaporiser to maintain SVP
What is required to ensure the splitting valve delivers a constant concentration of anaesthetic gas into the carrier gas flow
- Constant temperature
- Enough Surface Area for vaporisation to ensure ISO reaches its SVP
(The Splitting valves are calibrated to these variables made constant by the device)
How is adequate surface area achieved inside the vaporisation chamber of a plenum vaporizer
- Metal or fabric wicks in the chamber (surface tension draws the vaporiser liquid up the wicks and increases surface area for vaporization
- Bubble the gas through the volatile liquid by means of a sintered disc (glass or metal) Multiple small holes.
Why are vaporizers made of metal and why were boyle’s bottles surrounded by water reservoirs
Metal and water have high specific heat capacities. This means that they are both able to effectively conduct heat from the environment into the vaporiser chamber and volatile agent as the volatile vaporisers consuming heat for the latent heat of vaporisation.
Furthermore, the metal/water is an effective store of heat for the same purpose as the above.
This means as the latent heat of vaporisation is consumed it is replaced into the system efficiently by the metal or water reservoir meaning that SVP does not decrease with decreasing temperature of the system.
What is the definition and an example of an adiabatic process
No heat is exchanged between the environment and the system. This does not mean that there is no temperature change, it just means that there is no heat exchange.
E.g. Rapid increase in temperature when rapidly increasing the pressure of a gas –> fire hazzard
How do modern vaporisers adjust the splitting ratio automatically according to the temperature of the vaporiser device and liquid?
If the volatile temperature falls then the splitting ratio needs to be increased to ensure the delivery of set concentration of anaesthetic gas. This adjustment to the splitting ratio can be done automatically or manually. Modern vaporizers have mechanisms which do this automatically:
- Bimetallic strip (two metals with different co-efficients of thermal expansion) varies the exit orifice diameter from vaporiser chamber
- Bellows containing fluid (with a high coefficient of thermal expansion) within the volatile chamber expands with increasing temp and occludes avalve, regulating the splitting ration automatically
- Metallic rod valve expands or contracts with temp which opens/closes valve which regulates splitting ratio
- Liquid anaesthetic added straight to gas stream
–> high pressure gradient –> low flow and more liquid anaesthetic into stream (Page 121 kenny and davis for diagram)
–> High flow –> increased pressure gradient more liquid volatile added in line with carrier gas
–> Low flow –> lower pressure gradient less liquid volatile added in line with carrier gas
This system is independent of both flow and temperature
Describe the function of a heated chamber pulsed vaporizer
PV=nRT
n/V = P/RT
The number of moles per unit volume is fixed if the pressure and temperature are kept constant.
A heater heats the volatile to a specific temperature within a chmabe rat constant pressure. A valve is then manually opened to allow a specific calibrated quantity of volatile agent into the fresh gas flow.
NB is n/V = P/RT
FGF is measured and informs the solenoid valve frequency of opening and closing which ensures the correct number of moles of anaesthetic gas enters at various FGFs
Why does desflurane need its own special vaporizer
Due to the unique physical properties of Desflurane compared to other contemporary volatile agents:
- High SVP of 89.2 kPa
- -> This means a HIGH number of molecules are present above the volatile liquid exerting a high vapour pressure. This high number of molecules would require a very high FGF to achieve clinically useful concentrations of desflurane. Not economical. - HIGH Latent heat of vaporization
- -> Much faster cooling of volatile liquid –> this affects the consistency/precision of vaporization making it more difficult to ensure a constant and controlled delivery of anaesthetic agent to the patient. As the liquid cools the amount of vaporization comes down.
Hence, irrespective of the conductivity of the container, Desflurane requires additional heat (electric heater) as the cooling occurs at a rapid pace
Explain how a desflurane vaporiser works
- Des heated to 39 deg C which establishes a constant SVP of 200 kPa.
- Pressure transducer measures the FGF and transmits this via an electronics module to an electronically controlled valve.
- The electronically controlled valve opens/closes releasing variable quantities of desflurane (at constant SVP 200 kPa).
- This desflurane quantity released must still pass through the output concentration control (set by anaesthetist) to join the fresh gas flow.
Overall desflurane is heated to a constant temperature and hence SVP.
And the amount of desflurane released is automatically dependent on FGF via the electronic module and electronically controlled valve.
A consistent quantity of desflurane can then be introduced into the fresh gas flow despite its high SVP and high latent heat of vaporization
How can back pressure by the ventilator into the vaporiser chamber influence the concentration of delivered volatile agent. How is this prevetned
If the volume in the volatile chamber is larger than the bypass conduit then back flow of gas can enter the bypass conduit increasing the delivered concentration of volatile anaesthetic to the patient.
Prevent
- Pressure reducing valve downstream of volatile chamber outlet
- Equal volume volatile chamber and bypass conduit
- Long inlet tube to the vaporizing chamber so that retrograde flow does not enter the volatile chamber