Vaporisers

Describe the principles and safe operation of vaporisers

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

  • Variable bypass vaporisers
    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 Vaporiser

Variable bypass vaporisers 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 vaporisation.
      • Excessively high flow rates may result in gas not being fully saturated with agent when it exits the vaporiser stream
      • These are less effective in draw-over vaporisers, 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 vaporisation
        Liquid agent from the vaporiser 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 vaporising chamber to buffer changes in surrounding temperature.
      • Temperature compensation
        Adjusts flow into either the vaporising 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 vaporising 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 Vaporisers

Plenum vaporisers 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 Vaporisers are:

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

Measured Flow Vaporisers

Measured flow vaporisers 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 vaporisation.
    • Cooled desflurane will have a much lower saturated vapour pressure
      Significant under-delivery will then occur.

The Tec6 vaporiser:

  • Heats desflurane to 39°C
    SVP of desflurane at this tempeature 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 vaporisers being turned on.
  • Single cartridge slot (Aladdin system)

Tipping and overfilling:

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

Anti-pumping:

  • Check valves and long vaporization chamber inflow prevent entrainment of vaporizer gas in the inflow of the bypass channel

Agent depletion:

  • Filling gauges
  • Low pressure alarms (Tec 6)

Other Factors Affecting Vaporisers

Carrier Gas Composition:

  • Nitrous oxide and air are more viscous than oxygen
  • This leads to decreased flow through the vaporising 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 vaporiser 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:
    , where:

  • is the concentration of agent in the gas delivered to the patient
    This must be multipled by the atmospheric pressure to find the partial pressure of agent delivered to the patient.
  • is the concentration dialed up on the vaporiser
  • is the atmospheric pressure where the vaporiser was calibrated
  • is the atmosopheric pressure where the vaporiser 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 vaporizers. Continuing Education in Anaesthesia Critical Care & Pain. Volume 11, Issue 6, 1 December 2011, Pages 199–203,
Last updated 2017-09-23

results matching ""

    No results matching ""