2016A Question 02

Draw and label a circle breathing system. Explain its advantages and disadvantages.

Examiner Report

89% of candidates achieved a pass in this question

Main Points expected for a pass:

  • A legible and accurate diagram of a circle breathing system.
  • Correct components in the correct order.
  • A reasonable list of advantages and disadvantages. Additional points could be gained for:
  • Further detail on how components were arranged.

Common problems:

  • This question was generally well handled.
  • Some candidates spent too much time on one half of the question.
  • There was confusion regarding the location of the APL valve.
  • Illegible handwriting
  • Poor layout and structure and evidence of poor time management
  • The main reason for failing this question was insufficient core knowledge

Model Answer


  • Define
  • Diagram
  • Pros
  • Cons


Circle system is a semi-closed, re-breathing anaesthetic circuit.



Factor Description
Main features

- Volatile anaesthetic: Accurate, minimal waste

- O2: Accurate, can deliver 100%

- CO2: Efficient removal by:

 - Scavenger (earlier in expiratory limb – to spare CO2 absorber))

 - CO2 absorber (later in expiratory limb)

Secondary features

- Minimal dead space: Only tubing distal to Y piece and HME

- Low resistance

- Humidification and warming by CO2 absorber


- Sodalime remote from airway hence minimal dust inhalation

- ↓ Waste

- ↓ O2 pollution


Factor Description
Main features

- Equilibration between FGF and circuit-patient gas composition is dependent on FGF rate

- FiO2 can fall below 0.21 if low fresh gas flow rate and low fresh gas PO2 (since semi-closed)

- Sodalime degradation products: CO, Compound A

Secondary features

- Higher resistance than Mapleson E and F due to valves and longer tubing

- Possibility of injury if dysfunctional valves, scavenging system


- Expensive

- Complicated

- Bulky

- Atmospheric pollution from scavenging

Last updated 2021-08-23

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