Low flow anaesthesia

12 min read Last updated April 8, 2026

How to run a low-flow case (practical sequence)

  • Pre-use checks and setup
    • Full anaesthetic machine check: leaks (LP and breathing system), ventilator function, O2 failure protection, vapouriser seating/filling, scavenging patency
    • Circle system: correct assembly, unidirectional valves competent, APL/ventilator relief valve function, reservoir bag integrity, HMEF/filters as appropriate
    • CO2 absorbent: fresh, not desiccated; correct canister seating; check colour change is not sole indicator
    • Monitoring available: inspired O2 (FiO2), end-tidal CO2, agent monitoring (FiAA/EtAA), airway pressures/volumes, capnography waveform; consider CO monitoring if available
  • Denitrogenation / wash-in phase
    • Use higher fresh gas flow (FGF) initially to rapidly achieve target FiO2 and agent concentration and wash out nitrogen from lungs and circuit
    • Typical approach: FGF 4–6 L/min for several minutes (or until EtAA near target and FiO2 stable), then reduce
  • Transition to low flow / minimal flow
    • Reduce FGF to low flow (commonly 0.5–1.0 L/min total) once circuit equilibrated and patient stable
    • Increase vapouriser setting transiently after reducing FGF to maintain EtAA (lower FGF slows changes in circuit concentration)
    • Maintain adequate FiO2 by choosing appropriate O2:air mix and monitoring inspired O2 continuously
  • Maintenance and ongoing adjustments
    • Aim for stable EtAA and EtCO2; adjust ventilation to control CO2; adjust FGF/O2 fraction to maintain FiO2
    • Watch for absorbent exhaustion: rising inspired CO2, increasing EtCO2 despite ventilation, heat in canister, channeling
    • If rapid change needed (depth, FiO2, washout), temporarily increase FGF (e.g., 4–10 L/min) then return to low flow
  • Emergence / washout
    • Increase FGF (e.g., 6–10 L/min) with 100% O2 (or O2/air as appropriate) to wash out agent and nitrogen
    • Consider circuit volume and agent solubility: desflurane/sevoflurane wash out faster than isoflurane; low FGF prolongs washout

Definition and principles

  • Low flow anaesthesia: use of a circle system with rebreathing and CO2 absorption, with FGF sufficiently low that a substantial proportion of inspired gas is rebreathed.
  • Common working definitions (vary by source):
    • Low flow: total FGF ~1 L/min (or ≤1 L/min).
    • Minimal flow: total FGF ~0.5 L/min.
    • Metabolic flow: FGF approximates patient O2 consumption (~3–4 mL/kg/min; ~200–300 mL/min adult).
  • Key concept: in a circle system, CO2 is removed by absorbent; O2 and volatile agent must be supplied to match uptake and losses; nitrogen and trace gases accumulate unless washed out.

Why use low flow? (advantages)

  • Economy: reduced volatile agent and carrier gas consumption.
  • Environmental: lower greenhouse gas emissions (particularly relevant for desflurane and N2O).
  • Humidity and heat conservation: rebreathing warmed, humidified gas reduces airway drying and heat loss.
  • Reduced operating theatre pollution when system is tight and scavenging works (though leaks become more clinically significant).

Disadvantages and hazards

  • Hypoxic mixture risk if FiO2 not monitored and O2 fraction/FGF not adjusted for O2 uptake.
    • Inspired O2 can drift down over time due to O2 consumption exceeding supplied O2 fraction at low FGF.
  • Slower response to changes in depth or FiO2: low FGF reduces the ability to rapidly change circuit concentrations.
  • Accumulation of unwanted gases: nitrogen (especially early), methane/acetone, carbon monoxide (rare), compound A (sevoflurane) depending on conditions.
  • CO2 absorbent issues: exhaustion → rebreathing CO2; desiccated absorbent → CO production (esp desflurane/isoflurane) and heat; channeling can cause breakthrough.
  • Greater reliance on monitoring and machine integrity: small leaks can cause significant under-delivery of O2/agent and increased pollution.

Physiology and gas kinetics (what changes at low flow)

  • O2 balance: O2 supplied (FGF × FiO2) must exceed O2 consumption (VO2) plus circuit losses; otherwise inspired O2 falls.
    • Typical adult VO2 ~200–250 mL/min (higher in fever, sepsis, pregnancy, paediatrics).
  • Volatile agent: uptake is greatest early (high alveolar-to-venous gradient), so low flow is easier after equilibration; agent monitoring (inspired and end-tidal) is important.
  • Nitrogen: initially washed out from FRC and tissues; with very low FGF, nitrogen can accumulate in the circuit unless adequately denitrogenated first.
  • Time constants: circuit + lungs behave as a mixing volume; decreasing FGF increases time constant for changes in FiO2 and FiAA.

Equipment requirements and monitoring

  • Breathing system: circle system with CO2 absorber and unidirectional valves; low compliance/leak; ventilator suitable for circle use.
  • Essential monitoring for safe low flow:
    • Inspired O2 (FiO2) continuously (not just SpO2).
    • Capnography including inspired CO2 (FiCO2) if available; rising inspired CO2 suggests absorbent exhaustion or valve fault.
    • Agent monitoring (inspired and end-tidal) to guide vapouriser adjustments and detect under-delivery.
    • Airway pressure, tidal volume, minute ventilation; alarms set appropriately.
  • Scavenging: must be functional; low flow reduces total waste gas but leaks become proportionally more important.

CO2 absorbent and low flow-specific chemistry

  • Absorbent function: removes CO2 allowing rebreathing of other gases; reaction is exothermic and produces water.
  • Absorbent exhaustion indicators: rising inspired CO2, rising EtCO2, increased canister temperature early then cool when exhausted; colour change may revert on standing.
  • Carbon monoxide production: increased with desiccated absorbent (classically Baralyme > soda lime; modern absorbents reduce risk). Risk increased with high agent concentrations and dry gas flows overnight through machine.
    • Clinical clue: unexplained high carboxyhaemoglobin, low SpO2 with normal PaO2, cherry-red blood (late/unreliable).
  • Sevoflurane and compound A: degradation product formed in CO2 absorbents; risk increases with low flows, high sevo concentration, warm/dry absorbent; clinical nephrotoxicity in humans is not clearly demonstrated at usual clinical practice but manufacturer guidance historically advised avoiding very low flows for prolonged periods.

Practical flow/oxygen strategies (numbers that help in viva)

  • If total FGF = 1 L/min, ensure O2 flow comfortably exceeds VO2 (e.g., O2 0.5–0.6 L/min with air 0.4–0.5 L/min) and titrate to FiO2 target using inspired O2 monitor.
  • If total FGF = 0.5 L/min, O2 fraction must be higher; typical approach O2 0.3–0.5 L/min with minimal/zero air depending on FiO2 target and patient factors.
  • Avoid N2O in true minimal-flow if you need precise FiO2 control; N2O uptake and diffusion can complicate concentrations and hypoxia risk if O2 delivery marginal.
  • When changing depth quickly: increase FGF and/or use bolus IV agent; when reducing depth for wake-up: high FGF washout is usually most effective.

Troubleshooting patterns

  • Falling FiO2 at constant settings
    • Check O2 analyser calibration/sampling line; check pipeline/cylinder supply and rotameters; increase O2 flow; consider increased VO2 (fever, sepsis, shivering).
    • Look for leaks (cuff leak, circuit disconnection, absorber/canister seal, APL/ventilator relief valve, sampling line leak).
  • Rising inspired CO2 / rising EtCO2 despite ventilation
    • Absorbent exhausted or channeling; canister incorrectly seated; unidirectional valve incompetence; APL/ventilator valve fault.
    • Immediate actions: increase FGF, change absorbent, check valves and circuit assembly.
  • Low agent concentration despite high vapouriser setting
    • High uptake early; leak; vapouriser empty/malposition; incorrect agent; sampling line issues; too low FGF for desired rapid change.
    • Actions: increase FGF temporarily, confirm vapouriser fill and seating, check agent monitor and sampling line, consider IV supplementation.
Define low flow and minimal flow anaesthesia. What system is required?

Definitions vary; examiners want principles and safe practice requirements.

  • Low flow: typically total FGF around 1 L/min (or ≤1 L/min) with significant rebreathing.
  • Minimal flow: typically total FGF around 0.5 L/min.
  • Requires a circle breathing system with unidirectional valves and a CO2 absorber to permit rebreathing without CO2 accumulation.
  • Requires continuous monitoring of inspired oxygen and (ideally) inspired/end-tidal agent concentrations.
Explain why inspired oxygen can fall during low flow anaesthesia even if SpO2 is normal.

This is a common FRCA viva theme: supply vs consumption and lag in pulse oximetry.

  • At low FGF, the O2 delivered per minute (FGF × FiO2) may approach or be less than patient VO2, so the circuit O2 fraction drifts down.
  • SpO2 may remain normal for a period due to the shape of the oxyhaemoglobin dissociation curve and oxygen reserve in FRC; FiO2 monitoring detects the problem earlier.
  • Increased VO2 (fever, sepsis, shivering, pregnancy, paediatrics) accelerates FiO2 decline.
Describe a safe technique to start low flow anaesthesia from induction to maintenance.

Examiners look for wash-in then reduce flow, with monitoring and contingency plans.

  • After induction and securing airway: start with high FGF (e.g., 4–6 L/min) to denitrogenate and rapidly achieve target FiO2 and agent concentration.
  • Once EtAA approaches target and FiO2 stable: reduce to low flow (e.g., 1 L/min) or minimal flow (0.5 L/min) depending on case and monitoring.
  • Increase vapouriser setting transiently when reducing FGF to maintain EtAA (slower kinetics at low flow).
  • Continuously monitor FiO2, EtCO2 (and inspired CO2 if available), FiAA/EtAA; set appropriate alarms; be ready to increase FGF if instability occurs.
What are the advantages of low flow anaesthesia?
  • Reduced volatile agent and carrier gas consumption (cost saving).
  • Reduced environmental impact (lower total anaesthetic gas emissions).
  • Improved heat and humidity conservation via rebreathing.
  • Potentially reduced theatre pollution if system is tight and scavenging effective.
What are the main hazards/complications specific to low flow anaesthesia and how do you mitigate them?

A common structured viva: list hazards then link each to a mitigation.

  • Hypoxic mixture: mitigate with continuous FiO2 monitoring, adequate O2 flow, appropriate alarms, and readiness to increase FGF.
  • CO2 rebreathing from absorbent exhaustion or valve faults: mitigate with capnography (including inspired CO2), routine absorbent checks/changes, correct assembly, and increasing FGF if suspected.
  • Slow response to changes in depth/FiO2: mitigate by temporary high FGF for wash-in/washout, agent monitoring, and IV supplementation if needed.
  • Accumulation of unwanted gases (nitrogen, trace gases): mitigate with adequate initial denitrogenation and periodic higher flows if required.
  • Absorbent-related toxicities (CO with desiccated absorbent; compound A with sevo): mitigate by avoiding desiccated absorbent, regular replacement, avoiding prolonged very low flows with hot/dry absorbent, and using modern absorbents.
You notice inspired CO2 appearing on the capnogram during low flow anaesthesia. Give causes and immediate management.

This is a classic FRCA troubleshooting viva based on capnography.

  • Causes: exhausted absorbent; channeling in canister; canister not seated; unidirectional valve incompetence; incorrect circuit assembly; ventilator/APL valve malfunction.
  • Immediate management: increase FGF to reduce rebreathing; switch to manual ventilation if needed; check circuit and valves; change absorbent; confirm capnograph sampling line not contaminated/blocked.
  • Assess patient: check EtCO2, ventilation, haemodynamics; treat hypercapnia if present.
How does low fresh gas flow affect control of volatile anaesthetic concentration? Include what you would do if you need to deepen anaesthesia quickly.
  • Lower FGF increases the time constant for changing circuit/alveolar concentrations, so changes in vapouriser setting translate more slowly to EtAA changes.
  • To deepen quickly: temporarily increase FGF (e.g., 6–10 L/min) and increase vapouriser; consider IV agent/analgesia as a bridge; ensure agent monitoring guides effect.
Describe the relationship between oxygen consumption and the minimum oxygen flow you would set during minimal-flow anaesthesia.

Examiners want you to quote VO2 and show you will set O2 flow above it with a safety margin.

  • Adult VO2 is typically ~200–250 mL/min (higher in some states).
  • O2 flow should exceed VO2 to prevent FiO2 drift down; in practice you set a margin (e.g., ≥300–500 mL/min depending on patient and target FiO2) and titrate using inspired O2 monitoring.
  • If using air as balance gas, ensure total FGF and O2 fraction still provide sufficient absolute O2 delivery.
What is compound A and why is it discussed in low flow anaesthesia with sevoflurane?
  • Compound A is a degradation product of sevoflurane formed in CO2 absorbents; formation is promoted by low flows, higher sevo concentrations, and warm/dry absorbent.
  • It caused renal injury in animal studies; clinically significant nephrotoxicity in humans at usual practice is not clearly established, but prudent practice is to avoid prolonged very low flows with sevo in conditions that increase formation and to use modern absorbents.
Explain how carbon monoxide can be produced in the breathing system and how you would prevent it.
  • CO can be produced when volatile agents (especially desflurane/isoflurane) react with desiccated CO2 absorbent; risk increased if absorbent dried by prolonged high gas flows through the machine when not in use.
  • Prevention: ensure absorbent is not desiccated (change regularly; avoid leaving high flows running); use modern absorbents with reduced strong alkali; consider CO monitoring if available; be vigilant for unexplained COHb.
A previous FRCA-style scenario: During low flow anaesthesia, the agent monitor shows falling end-tidal sevoflurane despite an unchanged vapouriser setting. Give differential diagnosis and management.

This mirrors common exam themes: uptake, delivery failure, leaks, and monitoring artefact.

  • Differential: increased uptake (lighter plane, increased CO, early phase); leak in circuit/ETT cuff; vapouriser empty or not seated; wrong vapouriser/agent; sampling line leak/obstruction; too low FGF to maintain concentration after a change in uptake.
  • Management: assess patient depth and haemodynamics; increase FGF temporarily and increase vapouriser; check vapouriser fill and seating; check circuit integrity and cuff pressure; check sampling line; consider IV agent/analgesia while resolving.
How do you wash out nitrogen and why is this important before going to very low flows?
  • Use high FGF with high FiO2 during initial phase to replace nitrogen in the lungs and circuit with O2/air mixture; this reduces nitrogen accumulation in the circuit once flows are reduced.
  • If not adequately denitrogenated, nitrogen can accumulate and dilute oxygen/agent, making control less predictable at minimal flows.
You are asked to run low flow anaesthesia at 0.5 L/min for a 70 kg adult. What monitoring do you insist on and what initial settings might you choose?
  • Insist on: continuous inspired O2, capnography (preferably with inspired CO2), agent monitoring (inspired and end-tidal), airway pressure/volume monitoring, and appropriately set alarms.
  • Technique: start with wash-in at higher FGF (e.g., 4–6 L/min) until FiO2 and EtAA stable, then reduce to 0.5 L/min.
  • At 0.5 L/min, set O2 flow with margin above VO2 (e.g., 0.3–0.5 L/min O2 depending on target FiO2 and patient factors) and titrate to inspired O2 reading; adjust vapouriser to maintain EtAA.
In low flow anaesthesia, what alarms would you set and why?
  • Low inspired O2 alarm (and high O2 if desired): early detection of hypoxic mixture.
  • High inspired CO2 / rebreathing alarm if available; EtCO2 high alarm: detect absorbent exhaustion/valve faults/hypoventilation.
  • Agent low alarm (end-tidal): detect under-delivery/leaks/empty vapouriser.
  • Pressure/volume alarms: disconnection, obstruction, leak, compliance changes.

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