Fresh gas flow and rebreathing

10 min read Last updated April 8, 2026

Core concepts (definitions you must be able to state)

  • Fresh gas flow (FGF) is the flow of gas from the anaesthetic machine into the breathing system (L/min). It determines wash-in of gases/volatile and the extent of rebreathing in non-absorber systems.
  • Rebreathing is inspiration of previously exhaled gas. It may include CO2 unless CO2 is removed (e.g. soda lime in a circle system).
    • Rebreathing is not inherently harmful if CO2 is removed and oxygen/volatile concentrations are appropriate.
  • Alveolar ventilation (VA) = (VT − VD) × f. Minute ventilation (VE) = VT × f. FGF requirements are often expressed relative to VE.
  • In non-absorber systems, preventing CO2 rebreathing requires sufficient FGF to wash out exhaled CO2 from the system before the next inspiration.

What determines rebreathing in practice

  • FGF relative to VE: higher FGF reduces rebreathing in non-absorber systems; in circle systems CO2 is removed so rebreathing is acceptable and expected at low flows.
  • System design: location of APL valve, reservoir bag, and fresh gas inlet determines how effectively exhaled gas is vented vs rebreathed (Mapleson classification).
  • Ventilation mode: controlled ventilation changes flow patterns and alters Mapleson efficiency (e.g. Mapleson D is efficient for controlled; Mapleson A is efficient for spontaneous).
  • Patient factors: high VE (e.g. sepsis, pregnancy, paediatrics) increases required FGF to avoid CO2 rebreathing in non-absorber systems.
  • Equipment factors: leaks, incompetent valves, exhausted CO2 absorbent, incorrect assembly can cause CO2 rebreathing even with apparently adequate FGF.

Clinical approach to choosing FGF

  • Induction/wash-in: use higher FGF initially to rapidly achieve target inspired O2 and volatile concentration, then reduce once stable.
  • Maintenance: choose FGF based on system type (circle vs Mapleson), ventilation mode, and monitoring (FiO2, agent, capnography).
  • Low-flow/closed-circuit: only appropriate with a circle system with functional unidirectional valves and CO2 absorber; requires vigilant monitoring and understanding of uptake.

Rebreathing and FGF in Mapleson systems (key exam numbers)

  • General rule: to avoid CO2 rebreathing, FGF must exceed the patient’s CO2-containing exhaled gas remaining in the system at the start of inspiration. Practical exam rules use multiples of minute ventilation (VE).
  • Mapleson A (Magill): most efficient for spontaneous ventilation.
    • Spontaneous: FGF ≈ VE (often quoted ~1 × VE) to minimise rebreathing.
    • Controlled: inefficient; may require very high FGF (often quoted ~2–3 × VE) and is generally avoided for controlled ventilation.
  • Mapleson D (including Bain): most efficient for controlled ventilation.
    • Controlled: FGF ≈ 1–1.5 × VE (commonly taught ~1 × VE).
    • Spontaneous: less efficient; FGF ≈ 2–3 × VE.
  • Mapleson C: compact version of B; similar performance to B (generally inefficient).
    • Often requires high FGF to prevent rebreathing; commonly quoted ~2–3 × VE for both modes (varies by source).
  • Mapleson B: generally inefficient for both spontaneous and controlled ventilation; high FGF required.
    • Commonly quoted ~2–3 × VE to minimise rebreathing.
  • Mapleson E (Ayre’s T-piece) and F (Jackson-Rees): used in paediatrics; no valves; rely on high FGF to prevent rebreathing.
    • Spontaneous: FGF often ~2–3 × VE (or ~2–3 L/min in small infants, scaled to size).
    • Controlled: FGF often ~2–3 × VE (some teaching uses ~1–2 × VE for controlled with Jackson-Rees depending on technique), but exam-safe answer: high flows required.
  • Remember: these are pragmatic exam rules. In real practice, confirm adequacy with capnography (inspired CO2) and clinical context.

Circle system: FGF, rebreathing, and low-flow anaesthesia

  • In a circle system with functioning unidirectional valves and CO2 absorbent, CO2 is removed so rebreathing of CO2-free gas is expected and acceptable.
  • FGF in circle system primarily affects: speed of changes in FiO2 and volatile concentration, degree of reliance on absorbent, humidity/heat conservation, and cost/pollution.
  • Definitions (commonly used):
    • High-flow: FGF > patient VE (minimal rebreathing; faster control; more waste).
    • Low-flow: FGF less than VE (significant rebreathing; economical; requires monitoring).
    • Minimal-flow: very low FGF (often ~0.5 L/min).
    • Closed-circuit: FGF approximates patient uptake (O2 + agent), with minimal/no gas vented via APL during steady state.
  • Oxygen consumption (adult): typically ~200–250 mL/min at rest; higher with fever, pregnancy, sepsis, children.
  • Implication: if FGF is very low, delivered O2 fraction must be sufficient to avoid hypoxic mixture as O2 is preferentially taken up.
  • Low-flow advantages: reduced theatre pollution, reduced volatile use/cost, improved heat and humidity conservation, quieter/less drying.
  • Low-flow disadvantages/risks: slower changes in FiO2 and agent concentration, risk of hypoxia if O2 not set appropriately, accumulation of trace gases (e.g. CO, compound A with sevo under specific conditions), dependence on absorbent and valve integrity, harder to detect leaks.
  • Monitoring required for low-flow: inspired O2 (FiO2), end-tidal O2 if available, inspired/expired agent concentration, capnography, airway pressures/volumes, and ideally inspired CO2 (should be ~0).

How to recognise and troubleshoot rebreathing (practical FRCA)

  • Capnography: inspired CO2 > 0 suggests rebreathing (or sampling/valve issues). Check baseline returns to zero during inspiration in a normal circle system.
  • Circle system causes of CO2 rebreathing: exhausted absorbent, stuck/incompetent unidirectional valves, missing/incorrectly seated valves, channeling in absorbent, bypass of absorber, excessive dead space, incorrect assembly.
  • Non-absorber systems: inadequate FGF for the mode of ventilation, excessive apparatus dead space, inappropriate use (e.g. Mapleson A for controlled ventilation).
  • Immediate actions if rebreathing suspected: increase FGF, switch to a known-good circuit (often circle with high flows), check absorbent and valves, check for misconnections, confirm with capnography.

Worked exam-style calculations (typical viva prompts)

  • If VE = 6 L/min: Mapleson A spontaneous needs FGF ~6 L/min; Mapleson D controlled needs ~6–9 L/min; Mapleson D spontaneous may need ~12–18 L/min.
  • If using low-flow circle at FGF 1 L/min with O2 consumption ~250 mL/min, ensure delivered O2 fraction is high enough that FiO2 does not drift down (monitor inspired O2 continuously).
Define fresh gas flow and rebreathing. Is rebreathing always undesirable?

A common Primary FRCA viva theme is to separate the concept of rebreathing from CO2 rebreathing, and to link this to circle vs Mapleson systems.

  • FGF: flow from the anaesthetic machine into the breathing system (L/min).
  • Rebreathing: inspiration of previously exhaled gas; may include CO2 unless removed.
  • Rebreathing is acceptable in a circle system because CO2 is absorbed; it improves heat/humidity and reduces gas use.
  • CO2 rebreathing is undesirable and indicates inadequate CO2 removal (circle fault/absorbent exhausted) or inadequate FGF in non-absorber systems.
How does FGF determine rebreathing in Mapleson systems? Give FGF requirements for Mapleson A and D in spontaneous vs controlled ventilation.

This is a classic Primary FRCA question; examiners expect the direction of efficiency and approximate multiples of minute ventilation.

  • Mapleson A: most efficient for spontaneous ventilation; FGF ≈ 1 × VE to minimise rebreathing.
  • Mapleson A: inefficient for controlled ventilation; may require ~2–3 × VE (so generally avoided).
  • Mapleson D (Bain): most efficient for controlled ventilation; FGF ≈ 1–1.5 × VE (commonly taught ~1×VE).
  • Mapleson D: less efficient for spontaneous ventilation; FGF ≈ 2–3 × VE.
A patient is breathing spontaneously through a Mapleson A circuit. Minute ventilation is 7 L/min. What FGF would you choose and how would you confirm it is adequate?

Often asked as a short calculation plus monitoring/safety answer.

  • Choose FGF approximately equal to VE: start around 7 L/min (and adjust to clinical context).
  • Confirm adequacy with capnography: inspired CO2 should be ~0; look for absence of inspired CO2 and stable ETCO2.
  • Also assess work of breathing, reservoir bag movement, and ensure no excessive resistance/obstruction.
Describe what happens to inspired oxygen concentration when you reduce FGF in a circle system. Why can hypoxia occur with low flows?

This tests understanding of uptake and the need for inspired O2 monitoring during low-flow anaesthesia.

  • At low FGF, a larger proportion of inspired gas is rebreathed; the final inspired mixture is strongly influenced by patient uptake.
  • O2 is preferentially taken up (e.g. ~200–250 mL/min adult), so the O2 fraction in the circuit can fall if not replenished adequately.
  • Hypoxia risk increases if O2 flow is too low, if there is a leak, or if inspired O2 is not monitored continuously.
Capnography shows inspired CO2 of 0.5 kPa on a circle system. Give causes and immediate management.

A common equipment viva: list causes in a structured way (absorbent, valves, assembly) and give safe immediate steps.

  • Immediate: increase FGF, switch to a known-good circuit if needed, ensure adequate ventilation/oxygenation while troubleshooting.
  • Absorbent: exhausted soda lime, channeling, desiccated absorbent (also raises risk of CO with some agents).
  • Valves: stuck/incompetent inspiratory or expiratory unidirectional valve; missing valve disc; incorrect seating.
  • Assembly/bypass: incorrect connections allowing gas to bypass absorber; excessive dead space; faulty APL/scavenging arrangement causing abnormal flow patterns.
  • Confirm resolution: inspired CO2 returns to ~0 and ETCO2 normalises.
Explain why Mapleson A is efficient for spontaneous ventilation but inefficient for controlled ventilation.

Examiners want a flow-path explanation using the position of the APL valve and fresh gas inlet.

  • In Mapleson A, the APL valve is near the patient end; during spontaneous expiration, alveolar gas reaches the APL and is vented, while fresh gas preferentially fills the reservoir bag.
  • During the next inspiration, the patient draws mainly from the reservoir bag containing fresh gas → minimal rebreathing at FGF ~VE.
  • During controlled ventilation, positive pressure tends to push fresh gas out via the APL near the patient, while exhaled gas may remain in the limb/bag → more rebreathing unless FGF is very high.
You are using a Bain circuit for controlled ventilation. How do you choose FGF and what specific safety check is associated with the Bain?

Bain is Mapleson D coaxial; questions often combine FGF requirements with the inner tube integrity check.

  • Controlled ventilation: choose FGF about 1–1.5 × VE (commonly start ~1×VE and adjust using capnography).
  • Safety: check integrity of the inner fresh gas tube (risk of disconnection causing severe rebreathing and hypercapnia).
    • Perform a Pethick test (or equivalent institutional check) and visually inspect connections.
  • Monitor inspired CO2 and ETCO2 continuously to detect rebreathing early.
What are the advantages and disadvantages of low-flow anaesthesia in a circle system?

This is frequently asked; structure into patient, environmental, and equipment/monitoring issues.

  • Advantages: reduced volatile consumption and cost; reduced theatre pollution; better heat and humidity conservation; quieter system.
  • Disadvantages/risks: slower changes in FiO2 and agent; risk of hypoxic mixture if O2 not set/monitored; reliance on absorber and valve function; accumulation of trace gases; harder leak detection.
  • Requirements: continuous inspired O2 monitoring, agent monitoring, capnography; regular absorbent checks; appropriate machine/circuit integrity.
How would you detect rebreathing on a capnogram? Give at least two patterns and their likely causes.

FRCA often tests capnography interpretation linked to equipment faults.

  • Inspired CO2 not returning to zero (baseline elevated) suggests rebreathing (e.g. exhausted absorbent, incompetent valves, inadequate FGF in Mapleson).
  • Progressive rise in ETCO2 with elevated inspired CO2 may indicate worsening rebreathing (e.g. absorbent exhaustion).
  • If baseline elevated but system otherwise normal, consider sampling line issues or contamination; correlate clinically and check equipment.
Describe the relationship between FGF, uptake, and achieving a target end-tidal volatile concentration in a circle system.

This is a conceptual viva: faster control with higher FGF; slower with low-flow due to circuit volume and uptake.

  • Higher FGF increases delivered agent to the circuit and reduces the proportion of rebreathed gas → faster rise in inspired and end-tidal agent concentration.
  • At low flows, agent concentration changes slowly because the circuit acts as a reservoir and patient uptake becomes a larger fraction of delivered agent.
  • Practical approach: high flows for wash-in, then reduce to low-flow once stable while monitoring inspired/expired agent.

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