Closed circle anesthesia - NYSORA

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Closed circle anesthesia

Learning objectives

  • Describe the purpose and composition of closed circle anesthesia
  • Describe the technique and safety precautions of closed circle anesthesia
  • Describe the advantages and disadvantages of closed circle anesthesia


  • The current inhalational anesthetic agents with low blood solubility sparked renewed interest in closed circle anesthesia, due to their cost and environmental concerns
  • The circle system’s components are arranged in a circular manner
  • The system allows rebreathing of anesthetic agents while avoiding rebreathing of CO2
  • This allows the use of low or minimal fresh gas flow

System composition

  • The circle system consists of:
    • Fresh gas flow (FGF)
    • Inspiratory and expiratory unidirectional valves
    • Inspiratory and expiratory tubing
    • Y-piece connector
    • APL valve
    • Reservoir bag
    • CO2 absorber
  • The most efficient arrangement:
    • FGF enters the system before the inspiratory unidirectional valve
    • The APL valve and reservoir are situated between the expiratory valve and the CO2 absorber
    • The CO2 absorber is situated before the FGF entry point


  • Closed circle anesthesia: 
    • FGF is just sufficient to replace the volume of gas and vapor absorbed by the patient (Basal oxygen requirements, volatile agent uptake, and nitrous oxide uptake if used)
    • No gas leaves through the APL valve
    • Exhaled gases are rebreathed after CO2 absorption
  • Low flow anesthesia
    • FGF < alveolar ventilation (usually < 1.5 L/min)
    • Excess gases are eliminated via the APL valve
  • Ultra-low flow anesthesia
    • FGF < 0.5 L/min
    • Excess gases are eliminated via the APL valve


closed circle anesthesia, vaporizers, spontaneous breathing, fgf, frc, time constant, ventilator, oxygen, mac, vapor, ed95, ventilation-priming dose, nitrogen, co2, soda lime, calcium hydroxide, sodium hydroxide, silica, zeolite, baralyme, barium hydroxide, barium octahydrate, sevoflurane, carbon monoxide, enflurane, isoflurane, desflurane, moisture, condensation, hydrophobic filter, n2a, tidal volume, monitoring


  • Monitor inspired oxygen, end-tidal carbon dioxide, and inhalational agent
  • Prevent unidirectional valves from sticking due to water vapor condensation
  • Resistance to breathing is increased during spontaneous ventilation due to the unidirectional valves
  • Prevent compound A formation: Newer designs of soda lime claim less or no production of compound A, baralyme is worse than soda lime, and Amsorb© is the safest
  • Methane, acetone, ethanol, and hydrogen can accumulate but generally do not become clinically significant
  • Uneven filling of the canister with soda lime leads to channeling of gases and reduced efficiency
  • There is an increased potential for leads and disconnection due to the many connections

Advantages & disadvantages

Economy of gases and inhalational agentsPoorer regions may not have the necessary resources
Humidification of the inspired gasesAccumulation of unwanted gases (less of a problem with modern systems)
Reduced atmospheric pollution

Suggested reading

  • Pollard BJ, Kitchen, G. Handbook of Clinical Anaesthesia. Fourth Edition. CRC Press. 2018. 978-1-4987-6289-2.
  • Herbert L, Magee P. Circle systems and low-flow anaesthesia. BJA Education. 2017;17(9):301-5.

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