Fresh gas flow and rebreathing

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).