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.