Anaesthetic machine: Vapourizers Flashcards
Vapourisers
Definition, basic design
= a device used to add a specific, controlled and predictable concentration of an inhalational agent, in the form of a vapour, to the fresh gas flow
Amount delivered is expressed as a percentage of saturated vapour added to the gas flow.
Basic design
* Vapourising chamber containing liquid anaesthetic agent
* FGF passing through the vaporizing chamber picks up the anaesthetic vapour, and is mixed with anaesthetic-free gas bypassing the chamber
* Proportion of vapour-containing gas and bypass gas is controlled by a dial controlling the bypass channel
In pictures: bottom left: bypass channel closed. Bottom right: bypass channel open
Functional characteristics of the ideal vaporizer
Unaffected by (5), additional features (5)
Should be unaffected by:
* Changes in fresh gas flow
* Volume of liquid agent
* Ambient temp and pressure
* Decrease in temp due to vaporization
* Pressure fluctuation due to mode of respiration
In addition:
* Low resistance to flow of gas
* Lightweight, small liquid requirement
* Safety features to prevent accidental delivery of excessively high concentrations
* Economical, minimum servicing
* Corrosion and solvent-resistant
Types of vaporizer
Several different types, can be classified according to their location in the breathing system
* Draw-over vaporizer: e.g. Goldman vaporizer, Oxford miniature vaporizer
* Plenum vaporizer
Draw-over vaporizer
Examples, mechanism, advantages and disadvantages
Examples: Goldman vaporizer (see image), Oxford miniature vaporizer
Situated inside the breathing system
Very low resistance to gas flow
Gases are driven by patient’s respiratory efforts or by a self-inflating bag
Advantages:
* Small, simple and lightweight
* Agent non-specific: allowing use of any volatile agent
* Inexpensive
-> used in the ‘field’ or other difficult environments
Disadvantages:
* Not as efficient as plenum vaporizers
* Performance affected as temperature of anaesthetic agent decreases due to loss of latent heat during vaporization
Plenum vaporizer
Location, uses, resistance, calibration
The type of vaporizer used in most modern anaesthetic machines
- Situated outside the breathing system
- Gases are driven through the high-resistance and unidirectional vaporizer by gas supply pressure
- Calibration of each vaporizer is agent-specific
- Highly accurate and reliable in delivering desired inhalational anaesthetic concentration, despite changes in fresh gas flow or temperature
Plenum vaporizer mechanism
Fresh gas flow effect on inhalational agent concentration
Fresh gas flow is split into 2 streams immediately on entry to plenum vaporizer
* One stream flows through bypass channel, other, smaller stream, flows through vaporizing chamber
* Two streams reunite as gas leaves vaporizer
* Adjustment of percentage control dial of vaporizer alters the amount of gas flowing through each chamber
Gas in vaporization chamber is fully saturated with vapour before rejoins the bypass gas stream
* Surface area of contact between carrier gas and anaesthetic agent is increased either by having wicks saturated by the inhalational agent, a series of baffles, or bubbling gas through the liquid
* Full saturation should be achieved independent of any changes in FGF
In modern vaporizers, inhalational agent concentration supplied by vaporizer is virtually independent of FGFs between 0.5-15L/minute
Plenum vaporizer vs draw-over vaporizer
Location, resistance to gas flow, driving force, specificity, efficiency
- Draw-over situated inside breathing system, plenum outside
- Draw-over has very low resistance to gas flow, plenum has high resistance
- In draw-over, gases driven by patient effort or self-inflating bag. In plenum, gases driven by gas supply pressure
- Draw-over vaporizer is agent non-specific, plenum vaporizers are agent-specific
- Draw over vaporizers are cheap, lightweight, simple. Plenum vaporizers are more efficient.
Vapourizer design: What material are vaporizers made of?
Why
Copper
Copper has high density, high specific heat capacity and very high thermal conductivity.
Acts as a heat sink, readily giving heat to the anaesthetic inhalational agent and maintaining its temperature
Why is loss of latent heat of vaporization a problem for vaporizers
As the inhalational agent evaporates, its temperature decreases due to the loss of latent heat of vaporization.
A cold liquid is less volatile than a hot one so lowering the temperature of the inhalational agent makes it less volatile and the concentration carried by the FGF decreases.
-> can compensate for this using temperature compensating valves
Temperature compensating valves
Why required? Due to loss of latent heat of vaporization
* As the inhalational agent evaporates, its temperature decreases due to the loss of latent heat of vaporization.
* A cold liquid is less volatile than a hot one so lowering the temperature of the inhalational agent makes it less volatile and the concentration carried by the FGF decreases.
Temperature-sensitive valve within the body of the vaporizer automatically adjusts the splitting ratio of FGF and inhalational agent. As temperature of inhalational agent decreases -> flow into vaporizing chamber is increased to maintain full saturation of gas leaving it
Temperature valves incorporate either:
* Bellows design: as temperature decreases, bellows contract -> restricting flow of fresh gas through narrowed valve channel, thus more flow through vaporizing chamber - see picture
* Bimetallic strip: two stips of metal with different coefficients of thermal expansion, bonded together. As temperature of inhalational agent decreases, strip bends -> allowing more flow into the vaporising chamber
Note bimetallic strips used in Tec Mk 2,3,4,5, vaporizers
Temperature compensating valves in Tec Mk 2 vs Tec Mk 3 vaporizers
Bimetallic strip used in both
* Tec Mk 2: positioned inside vaporization chamber (see picture)
* Tec Mk 3 (and 4,5): located putside the vaporization chamber
Problem with Tec 2 design:
* preservatives e.g. thymol in halothane can cause bimetallic strip to stick, adversely affecting its function.
* chemically active bimetallic strip was liable to corrode in the oxygen/inhalational agent mixture, even for agents that do not contain preservatives (enflurane, isoflurane, sevoflurane, desflurane)
Desflurane
Boiling point, SVP, implications for and temp/pressure of vaporizer
Extremely volatile
Boiling point of 23.5 degrees C at atmospheric pressure
Saturated vapour pressure is 664mmHg at 20 degrees C
-> normal variable-bypass type vaporizer cannot be used.
Completely different vaporizer design: Tec 6 vaporizer
Desflurane vaporisation chamber is
* 39 degrees (i.e. above boiling point)
* SVP >1550mmHg
Desflurane vaporizer mechanism
Temp, pressure, FGF, control of output concentration
Tec 6 vaporizer
Temperature and pressure
* Desflurane vaporisation chamber is electrically heated to 39 degrees and SVP >1550mmHg.
* Will not function at lower temp or pressure, has 10-50 minute warm-up period.
FGF
* Fresh gas flow does not enter the vaporization chamber (unlike other vaporisers) but enters the path of the regulated concentration of desflurane vapour. Resulting gas mixture is delivered to the patient.
* Fresh gas flow is restricted by a fixed orifice so that pressure of the carrier gas within the vaporiser is proportional to the gas flow.
Ensuring that output concentration is independent of FGF rate
* Differential pressure transducer measure pressure of FGF at the orifice on one side and pressure of desflurane vapour upstream of pressure-regulating valve on other side
* Pressure transducer adjusts pressure-regulating resistor (R1) at outlet of vaporisation chamber so that pressure of desflurane vapour upstream of resistor = pressure of FGF at orifice
* -> therefore** flow of desflurane out of the vaporising chamber is proportional to FGF**, and enabling output concentration to be independent of FGF rate
Control of output concentration
* Percentage control dial with a rotary valve adjusts R2 resistor, which controls flow of desflurane vapour into the FGF, and thus the output concentration
* Dial calibration is from 0-18%, with 1% graduation from 0-10% and 2% graduation from 10-18%.
Other points
* Vaporizing chamber is sealed from the atmosphere, so there is a rotating filling port at the front of the device
* When in use, is mounted on the Selectatec system
Identify components of the vaporizer
Vaporiser safety systems
- Selectatec system to prevent more than one vaporizer being switched on at once.
- Anti-spill mechanism to prevent liquid anaesthetic entering bypass channel.
- Pressure controls to prevent damage to vaporizer/flowmeters and prevent retrograde flow into vaporizing chamber or bypass channel (hence risk of increase in inspired concentration of agent)
- Agent-specific filling devices to prevent adding wrong agent to wrong vaporizer. Geometrically and colour coded to fit safety-filling port of correct vaporizer and anaesthetic agent.
Selectatec system
Functions (2), type of vaporizer, disadvantage
Plenum vaporizers are mounted on the back bar of the anaesthetic machine using the interlocking Selectatec system.
* Pins in manifold are linked to the control dial
* Locking lever of system has to be engaged before percentage control dial can be moved -> therefore FGF only enters vaporizer when it has been switched on
* Also, interlocking extension rods extend and prevent more than 1 vaporizer being switched on at any one time -> preventing contamination of a vaporizer positioned downstream
For example: if control dial of one vaporizer is still on, you will not be able to move the percentage dial of a different vaporiser
Disadvantage: increased potential for leaks. If O-rings on vaporizer mount accidentally adhere to one vaporizer, will be a leak when another vaporizer is positioned -> need to check condition of O-rings of Selectatec system.
Emergency oxygen flush
Flow, pressure, safety features, risks
- Supplies pure oxygen from outlet of anaesthetic machine
- Flow bypasses the flowmeters and the vaporizers
- Flow 35-75L/min at pressure of ~400 kPa
Safety features
* Non-locking button, self-closing valve
* Recessed in a housing to prevent accidental depression
Risks
* Barotrauma
* Dilution of anaesthetic gases mixture and possible awareness
* Should not be activated while using minute volume divider ventilator -> ventilator will not function at these high flows
Oxygen supply failure alarm
Trigger, actions
Failure of oxygen supply (detected as O2 pressure <200 kPa)
-> Nitrous oxide supply automatically switched off as safety measure
-> Air (with 21% oxygen) delivered to the patient
Pressure-activated, requires no electrical supply.
Triservice anaesthetic apparatus
- Suitable for use in remote areas where the supply of compressed gases and vapours is unreliable.
- Has two Oxford miniature vaporizers (OMVs, i.e. draw-over vaporizers), a self-inflating bag and a non-rebreathing valve.
- Suitable for both spontaneous and controlled breathing (via self-inflating bag).
- Oxygen supplement cylinder can be connected upstream of the vaporisers
Oxford miniature vaporizer
- Draw-over vaporizer
- Capacity of 50ml of anaesthetic agent
- Ethylene glycol jacket acts as a heat sink and stabilises vaporizer temperature.
- However no temperature compensation. NOT suitable for prolonged use at high gas flows. Vapour concentration decreases as temperature decreases.
- Can be used with different inhalational agents (calibration scales on the Oxford miniature vaporizers can be detached allowing the use of different inhalational agents)
- Spill-proof when turned off but not during use
