Circle System

This covers the circle breathing system. A general overview of anaesthetic breathing systems is covered under anaesthetic circuits.

The circle breathing system is a highly efficient system which:

  • Has several key advantages
    • Preserves anaesthetic gases making volatile anaesthesia cost-effective
    • Preserves medical gases (oxygen) which is useful in resource-limited settings (e.g. prehospital)
    • Preserves heat and moisture
    • Reduces fire risk
      Particularly with older agents.
  • Requires re-breathing of expired gases
    CO2 is actively removed.
  • Is a closed-circuit system
    • The only gases which must be replaced are those:
      • Consumed by the patient
        • Oxygen
        • Absorbed and metabolised volatile agents
      • Lost via leak


A circle circuit consists of:

  • A Y-piece, connecting the circuit to the patient
  • Expiratory and inspiratory valves, ensuring unidirectional flow
  • A means of generating pressure
    In most systems this consists of both a ventilator and a reservoir bag with APL valve attached, with a bag/vent switch to swap between circuits.
    • These are typically placed on the expiratory limb so that gas can be removed via scavenging prior to passage through soda lime
      This reduces soda lime consumption, as some CO2 will be scavenged.
  • Soda lime
    To absorb CO2.
  • Fresh gas flow
    • Includes oxygen, air and nitrous oxide
    • Oxygen enters the back-bar last
    • When the vapouriser is out-of-circuit, all fresh gas flow will pass through the vapouriser prior to entering the circle
  • A separate high-pressure high-flow oxygen flush, which bypasses the vapouriser

Soda Lime

Soda lime:

  • Consists of granules of:
    • 81% Ca(OH)2
    • 4% NaOH
    • 15% H2O
    • Silicates
      Hardens granules.
    • pH indicator
      Visual representation of uptake of CO2 by soda-lime.
      • Phenolphthalein
        Red to white.
      • Ethyl violet
        White to purple.
  • Granules are 4-8 mesh in size
    • Will pass through a mesh with 4 holes per square inch, but not 8
    • Balance between surface area (speed/efficacy of reaction) and resistance to flow
  • Absorbs CO2 by following reaction:

    • This increases the pH of the soda lime, causing the pH indicator to change colour
  • 100g of soda lime can absorb ~26L of CO2


  • Cheaper to operate
  • Conserves gases, heat, and moisture
  • Low dead space
  • Reduced greenhouse effects


  • Gas mixture settings are not delivered to the patient
    Settings affect the fresh gas flow mixture, whilst the patient respires gas from the circuit. These are not identical, especially at low flows.
  • Nitrogen may build up in the circuit during low-flow anaesthesia, and potentially lead to delivery of a hypoxic gas mixture
  • Less portable than open-circuit systems
  • Increased circuit resistance
  • Requires soda lime, which can be toxic
    • Produces Compound A-E from sevoflurane
    • Produces carbon monoxide from desflurane, isoflurane, and enflurane
    • Dangerous if aspirated


  1. Aston D, Rivers A, Dharmadasa A. Equipment in Anaesthesia and Intensive Care: A complete guide for the FRCA. Scion Publishing Ltd. 2014.
Last updated 2021-08-23

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