Circle breathing system: components and function

What it is / why we use it

A rebreathing system with unidirectional gas flow and a CO₂ absorber, allowing low-flow anaesthesia while preventing CO₂ rebreathing.
- Primary functions: deliver O₂/anaesthetic, remove CO₂, enable controlled/assisted/spontaneous ventilation, conserve heat/humidity, reduce theatre pollution with low flows.
Key concept: CO₂ is removed chemically (absorber), so rebreathing of other gases (O₂, N₂O, volatile) is permitted and can be advantageous.

How it works (functional overview)

Inspiratory phase: gas flows from FGF + reservoir bag/ventilator through inspiratory unidirectional valve → patient.
Expiratory phase: exhaled gas passes via expiratory unidirectional valve → CO₂ absorber → back to reservoir/ventilator for the next inspiration.
Excess gas is vented via the APL valve (manual/spontaneous modes) or ventilator spill valve (ventilator mode) to the scavenging system.

Core components (what to list in a viva)

Fresh gas inlet (FGF) into the circle (often near absorber or inspiratory limb depending on machine design).
- Function: provides O₂/air/N₂O and volatile, determines wash-in/wash-out and achievable low-flow technique.
Inspiratory limb (corrugated tubing).
- Function: conducts gas to patient, contributes to compliance/volume loss and potential for kinking/disconnection.
Expiratory limb (corrugated tubing).
- Function: conducts exhaled gas away from patient to absorber, site for water condensation and obstruction risk.
Unidirectional valves: inspiratory and expiratory (disc/leaflet).
- Function: ensure one-way flow, prevent mixing of inspired/expired gas streams.
- Failure modes: sticking open/closed, misassembly, moisture/condensate, foreign body, can cause rebreathing or obstruction.
CO₂ absorber canister containing soda lime (or equivalent).
- Function: removes CO₂ to permit rebreathing, produces heat and water, reduces inspired CO₂.
- Includes: canister(s), granules, seals, bypass prevention, and often a sight glass/indicator for colour change.
Reservoir bag (manual/spontaneous ventilation) and/or ventilator bellows/piston.
- Function: gas reservoir, compliance buffer, allows manual ventilation, visual/tactile monitoring of ventilation.
APL (adjustable pressure limiting) valve (in manual/spontaneous circuit).
- Function: limits circuit pressure, vents excess gas to scavenging, determines ease of manual ventilation and risk of barotrauma if closed.
Patient connection (Y-piece) with catheter mount and often HMEF.
- Function: interface to airway device, HMEF adds humidification/filtration but increases dead space and resistance.
Scavenging interface connected to APL/ventilator spill valve.
- Function: removes waste anaesthetic gases, prevents theatre pollution, must not impose excessive positive/negative pressure on breathing system.
Pressure monitoring (airway pressure gauge/transducer) and gas monitoring (FiO₂, agent, capnography).
- Function: detect disconnection/obstruction, rebreathing, inadequate FGF, exhausted absorber, valve faults.

CO₂ absorber: function and key points

Soda lime composition (typical): Ca(OH)₂ majority, with small amounts of NaOH/KOH (activators) and water, hardeners (e.g., silica) may be present.
Overall reaction: CO₂ + H₂O → H₂CO₃, then neutralisation to carbonates. Reaction is exothermic and generates water.
- Clinical effects: warms and humidifies inspired gases (useful during low-flow).
Signs of exhaustion: rising inspired CO₂ (FiCO₂), capnography baseline elevation, increased ETCO₂ despite adequate ventilation, canister colour change (unreliable), canister coolness.
Important hazards: channeling (poor packing), desiccation (dry gas flows), dust, caustic injury if granules enter airway, and compound A (sevoflurane) and CO (desiccated absorbent with some agents).
- Risk reduction: avoid prolonged high FGF through absorber when not in use, change absorbent if suspected dry, use modern absorbents with reduced strong bases, monitor FiCO₂ and agent.

Unidirectional valves: function and failure patterns

Normal: inspiratory valve opens during inspiration and closes during expiration, expiratory valve does the opposite.
Inspiratory valve stuck open: expired gas can flow back into inspiratory limb → rebreathing, capnograph may show inspired CO₂ and altered waveform.
Expiratory valve stuck open: inspired gas may pass into expiratory limb, can cause rebreathing and ineffective ventilation, may see abnormal capnography and bag/ventilator behaviour.
Valve stuck closed: causes obstruction (inspiration if inspiratory valve closed, expiration if expiratory valve closed) with rising airway pressure or failure to exhale.

Fresh gas flow (FGF) and circle system behaviour

With adequate CO₂ absorption, the circle can be run with low flows (e.g., ~0.5–1 L/min) after initial wash-in, reducing cost and pollution.
At very low flows, inspired O₂ and agent concentrations depend on patient uptake, circuit volume, leaks, and absorber function, requires continuous gas monitoring (FiO₂ and agent).
If FGF is high relative to minute ventilation, the circle behaves more like a non-rebreathing system (less rebreathing, more waste).

Advantages and disadvantages (equipment viva style)

Advantages: low-flow capability, economical volatile use, reduced theatre pollution, conservation of heat and humidity, versatile for spontaneous/controlled ventilation, stable inspired concentrations once equilibrated.
Disadvantages: more complex, higher resistance than some systems (valves + absorber), risk of rebreathing if faults, slower changes in agent concentration at low flows, potential absorbent-related toxicities, bulk and multiple connections (leaks/disconnections).

Checks and fault-finding (high yield)

Pre-use: integrity of circuit, correct assembly and orientation of unidirectional valves, absorber seated and sealed, adequate absorbent, reservoir bag present, APL functioning, scavenging connected, leak test performed.
Intra-op indicators of problems: rising FiCO₂, rising ETCO₂ with unchanged ventilation, unexpected airway pressure changes, abnormal bag/bellows movement, agent/FiO₂ drift, audible leaks.
Immediate actions if rebreathing suspected: increase FGF temporarily, switch to a non-rebreathing mode/system if available, check valves and absorber, check for exhausted absorbent/channeling, confirm capnograph sampling and calibration.

Test yourself…

Describe the components of a circle breathing system.

A structured equipment list with functions scores well.

FGF inlet, inspiratory limb, expiratory limb, inspiratory and expiratory unidirectional valves, CO₂ absorber, reservoir bag and/or ventilator, APL valve (in manual/spontaneous mode), patient connection (Y-piece, catheter mount, HMEF), pressure relief/monitoring, scavenging connection.

Explain how the circle system prevents rebreathing of CO₂.

CO₂ removal is chemical, rebreathing of other gases is allowed.

Expired gas is directed by the expiratory unidirectional valve through the CO₂ absorber, where CO₂ is removed before gas returns to the reservoir and is re-inspired.
Unidirectional valves prevent reverse flow and mixing that would otherwise increase inspired CO₂.

What are the functions of the APL valve in a circle system?

Limits maximum circuit pressure during manual/spontaneous ventilation by venting excess gas to scavenging.
Allows control of circuit pressure during manual ventilation, incorrect setting (closed) risks barotrauma, too open may cause inadequate ventilation if large leak/insufficient reservoir.

A previous FRCA-style scenario: Capnography shows a rising baseline (inspired CO₂) during controlled ventilation on a circle. Give causes and how you would manage it.

Think: absorber failure, valve failure, inadequate flows, and measurement issues.

Causes: exhausted CO₂ absorbent, channeling due to poor packing, unidirectional valve incompetence (stuck open/misassembled), inadequate FGF relative to system leak/ventilation (less common if absorber working), faulty capnograph sampling/zeroing.
Immediate management: increase FGF (washout), increase minute ventilation as needed, check patient and circuit for obstruction/leak, inspect valve discs movement, check absorber colour/temperature and seating, replace absorbent/canister if in doubt, consider switching to alternative circuit.

What will happen if the inspiratory unidirectional valve is stuck closed? How would it present?

Inspiratory obstruction: inability to deliver tidal volume, rising airway pressure (if ventilator) or inability to squeeze bag, minimal chest movement, possible low exhaled volumes/alarms.
Management: disconnect and ventilate via self-inflating bag, inspect/replace valve or circuit, ensure correct assembly.

What will happen if the expiratory unidirectional valve is stuck closed? How would it present?

Expiratory obstruction: difficulty exhaling, breath stacking, rising airway pressures, prolonged expiration, possible hypotension/barotrauma, bag may become tight and not refill normally.
Management: immediate disconnection to allow exhalation, then troubleshoot/replace expiratory valve/circuit.

A previous FRCA-style question: Describe the CO₂ absorber and the factors affecting its efficiency.

Absorbent: soda lime (mainly Ca(OH)₂ with small amounts of strong bases and water). Efficiency depends on: granule size/surface area, water content (too dry reduces absorption and increases toxic by-products), temperature, gas flow pattern, canister design, packing and channeling, and duration of use.
Clinical detection: inspired CO₂ (FiCO₂) rise is most reliable, colour change can be misleading (regeneration on standing).

Why does the circle system conserve heat and humidity?

Rebreathing returns warmed, humidified exhaled gas to the patient after CO₂ removal, the absorber reaction is exothermic and produces water, further warming/humidifying inspired gas (especially at low flows).

How does fresh gas flow influence inspired agent concentration in a circle system?

High FGF: faster wash-in/wash-out, inspired concentration approaches vaporiser setting, less rebreathing.
Low FGF: slower changes, inspired concentration influenced by patient uptake, circuit volume, leaks, and rebreathing, requires agent monitoring.

A previous FRCA-style question: List hazards specific to circle systems and how they are detected.

CO₂ rebreathing (valve fault, exhausted absorbent, channeling) detected by FiCO₂ and capnography baseline rise.
Obstruction (valve stuck closed, kinked limb, water) detected by airway pressure changes, reduced delivered/exhaled volumes, clinical signs.
Leaks/disconnections detected by low pressure/volume alarms, inability to pressurise circuit, agent/FiO₂ drift.
Absorbent-related toxicities (CO, compound A) suspected with desiccated absorbent/high flows/agent choice, mitigated by correct absorbent management and monitoring.