Vapourisers

Describe the principles and safe operation of vapourisers

Delivery of gas that is fully saturated with anaesthetic agent would result in lethal doses being administered. The use of a vapouriser allows a safe dose of anaesthetic agent to be given. Vapourisers can be divided into:

• Variable bypass vapourisers
  Air that is fully saturated with gas is mixed with a 'bypass' stream of gas, diluting the delivered concentration. Further subdivided into:
  • Plenum
    Requires supra-atmospheric pressure to operate.
    • More accurate
  • Draw-over
    Driven by the patients inspiratory effort.
    • Portable

Variable Bypass Vapouriser

Variable bypass vapourisers aim to deliver the same concentration of anaesthetic agent over a range of flows. They achieve this by:

• Flow management
  • Baffles and wicks increase the surface area of the liquid/gas interface, increasing the rate of vapourisation.
    • Excessively high flow rates may result in gas not being fully saturated with agent when it exits the vapouriser stream
    • These are less effective in draw-over vapourisers, as resistance must be minimised
• Temperature management
  The SVP of volatile agents increases non-linearly as temperature increases. Temperature changes:
  • Occur through:
    • Changes in ambient temperature
    • Loss through latent heat of vapourisation
      Liquid agent from the vapouriser will cool over the course of an anaesthetic.
  • Are managed with:
    • Temperature stabilisation
      Use of materials with both a high thermal conductivity and specific heat capacity, allowing the vapourising chamber to buffer changes in surrounding temperature.
    • Temperature compensation
      Adjusts flow into either the vapourising chamber or bypass chamber to account for changes in environmental temperature. Methods include:
      • Bimetallic strip
        Metal strip which bends in response to environmental temperature, adjusting the amount of gas entering the vapourising chamber.
      • Aneroid bellows
        Connect to a cone in the opening of the bypass chamber. As temperature decreases, the bellows contract and the cone partially obstructs the bypass channel.

Difference Between Plenum and Draw-Over Vapourisers

Plenum vapourisers are:

• More accurate
  Designed to deliver accurate agent concentrations over a wide range (0.25-15L.min^-1) of flow rates
  • Below 250ml.min^-1 the resistance of the flow splitting valve becomes more significant, causing the amount of gas in the bypass stream to be higher than intended
  • Above 15L.min^-1 gas may not be fully saturated
• Heavier
  Typically built of metals such as copper to maximise thermal stability.
• High internal resistance
  • Must be used out-of-circle
  • Must be used with positive-pressure

Draw-Over Vapourisers are:

• Less accurate
  • Less use of baffles and wicks to minimise inspiratory resistance
• Less thermally stable
  • Oxford Minature Vapouriser does not have a bimetallic strip
  • Oxford Minature Vapouriser uses glycol as a thermal buffer

Measured Flow Vapourisers

Measured flow vapourisers have a separate stream of agent-saturated gas that is added to the gas flow. This requires the device to:

• Measure fresh gas flow rate
• Adjust vapour-gas flow rate so the desired concentration is delivered

This system is used for the delivery of desflurane, as desflurane:

• Has a very high SVP
  Requires high bypass flow rate to dilute to a clinically useful concentration.
• Has a low boiling point
  Intermittently boils at room temperature, which will cause large fluctuations in delivery:
  • Excessive agent delivery during boiling
    This will lead to cooling due to the latent heat of vapourisation.
  • Cooled desflurane will have a much lower saturated vapour pressure
    Significant under-delivery will then occur.

The Tec6 vapouriser:

• Heats desflurane to 39°C
  SVP of desflurane at this temperature is 1500mmHg.
• Gaseous desflurane is then added to the fresh gas flow
  The amount added depends on:
  • Desired concentration
  • Fresh gas flow rate
    As flow increases the resistance to flow of desflurane vapour decreases.

General Safety Features of Vapourisers

Agent specificity:

• Key indexed filling
• Pin indexed safety system connectors
• Colour coding of unit and agent containers

Single agent administration:

• Interlock mechanism
  Prevents multiple vapourisers being turned on.
• Single cartridge slot (Aladdin system)

Tipping and overfilling:

• Long vapourisation chamber inflow
• Heavy construction
• Transport modes
• Side filling and overflow ports

Anti-pumping:

• Check valves and long vapourisation chamber inflow prevent entrainment of vapouriser gas in the inflow of the bypass channel

Agent depletion:

• Filling gauges
• Low pressure alarms (Tec 6)

Other Factors Affecting Vapourisers

Carrier Gas Composition:

• Nitrous oxide and air are more viscous than oxygen
• This leads to decreased flow through the vapourising chamber when FiO₂ is low
  This effect is not clinically significant.

Altitude:

• Clinical effect of volatile agent is a function of their partial pressure in tissues
• As SVP is independent of atmospheric pressure, this is unchanged at altitude

• A vapouriser set at 2% will deliver 4% gas at 0.5atm pressure, however as the atmospheric pressure is reduced the same partial pressure of vapour is delivered

• The delivered concentration of an agent at altitude is given by the equation:
  $Delivered \ Concentration \ (\%) = Intended \ Concentration \times {P_{cal} \over P_{alt}}$, where:

• $Actual \ Delivered \ Concentration$ is the concentration of agent in the gas delivered to the patient
  This must be multiplied by the atmospheric pressure to find the partial pressure of agent delivered to the patient.
• $Intended \ Concentration$ is the concentration dialed up on the vapouriser
• $P_{cal}$ is the atmospheric pressure where the vapouriser was calibrated
• $P_{alt}$ is the atmospheric pressure where the vapouriser is being used

References

1. Miller RD, Eriksson LI, Fleisher LA, Weiner-Kronish JP, Cohen NH, Young WL. Miller's Anaesthesia. 8th Ed (Revised). Elsevier Health Sciences.
2. Boumphrey S, Marshall N. Understanding vapourizers. Continuing Education in Anaesthesia Critical Care & Pain. Volume 11, Issue 6, 1 December 2011, Pages 199–203,