Low Flow Anaesthesia (LFA)

Definition

Classification by Fresh Gas Flow

The lower threshold of 250 mL/min is the absolute minimum oxygen requirement for resting metabolic processes in a normothermic adult. This must be delivered as 100% O2.

Prerequisites / Equipment Requirements

1. Circle breathing system - essential; Mapleson circuits are unsuitable (no CO2 absorber support, wasteful)
2. Functional CO2 absorber (soda lime / Amsorb) - prevents CO2 rebreathing
3. Accurate vaporizer - must be calibrated and leak-free
4. Monitoring (mandatory for safe LFA):
   • Inspired oxygen analyzer (lower alarm at 28-30%)
   • Capnography (end-tidal CO2) - most sensitive indicator of absorber exhaustion
   • Volatile agent analyzer (inspired + expired)
   • Oxygen saturation (SpO2)
   • Airway pressure / minute volume monitoring
   • Disconnection alarms

Advantages

1. Economic

• Volatile agent consumption reduced by up to 75% versus high-flow
• Greatest savings occur when FGF drops from 4 L/min to 2 L/min; returns diminish below 1 L/min

2. Environmental / Ecological

• Reduction of greenhouse gas emissions by up to 90%
• Volatile agents (sevoflurane, desflurane, isoflurane) and N2O are greenhouse gases and ozone-depleting substances
• Reduced operating-room pollution and staff exposure, especially to nitrous oxide

3. Physiological / Pulmonary

• Inspired gases are warmed and humidified by rebreathing - aids mucociliary clearance, body temperature maintenance, reduces airway water loss
• Improved flow dynamics of inhaled gases
• Reduction in respiratory heat loss - particularly important in paediatrics and long cases

Disadvantages and Hazards

1. Slow changes in anaesthetic depth

• When circuit FGF is low, altering the vaporizer dial changes inspired concentration slowly (the "time constant" of the circuit is long)
• If the volatile agent is switched off at low flow, emergence can take 10-20 minutes as concentration slowly decreases
• Remedy: temporarily increase FGF to flush/wash in the desired change, then return to low flow

2. Accumulation of unwanted gases

• Nitrogen (released from body tissues early in anaesthesia) - can dilute the inspired O2 and cause hypoxia
• Carbon monoxide (CO) - produced when desiccated CO2 absorbents (especially those containing strong bases like KOH/NaOH) interact with desflurane, isoflurane, or sevoflurane. Risk is highest with absorbents dried out by unused machine left running over a weekend (Monday morning risk). FGF ≥ 5 L/min through the absorber without a patient connected is enough to desiccate it critically.
• Acetone, methane - metabolic by-products exhaled by patients
• Potentially interfere with gas monitoring if trifluoromethane accumulates

3. Compound A (Sevoflurane specific)

• Sevoflurane + CO2 absorbent (base-catalyzed) → Compound A (fluoromethyl-2,2-difluoro-1-[trifluoromethyl] vinyl ether), a nephrotoxic vinyl ether
• Production is enhanced at: low/closed-circuit FGF, warm or dry absorbent, high sevoflurane concentration, long duration, use of Baralyme (now withdrawn from market)
• Concentrations of 8-32 ppm are produced at 1 L/min with soda lime
• Clinical significance: extensive human studies show no nephrotoxicity or renal injury at low-flow concentrations. No adverse renal effects detected in randomized, multicenter trials including patients with pre-existing renal disease
• Most countries have no FGF restriction for sevoflurane; some clinicians recommend ≥ 2 L/min for procedures lasting more than a few hours (cautious practice only)
• Rats are susceptible to compound A nephrotoxicity; humans appear not to be, due to absence of the relevant renal enzymatic pathway

4. Hypoxia risk

• Nitrogen washout from tissues early in anaesthesia dilutes O2 in a closed/near-closed circuit
• Inspired O2 analyzer is the only reliable protection in the low-pressure section of the machine
• Inspired O2 alarm must be set at 28-30%

5. Absorber monitoring and CO2 buildup

• Exhausted absorber causes CO2 rebreathing - capnography (inspiratory CO2 > 0) is the most sensitive indicator
• Ethyl violet colour change in soda lime is not always reliable (can be photo-inactivated by fluorescent light exposure)

Practical Technique

1. Induction: Use higher FGF (4-6 L/min) initially to denitrogenate the circuit and the patient, achieve target concentration quickly, and fill the circuit volume
2. Transition to low flow: After ~10-15 minutes (nitrogen has been washed out, equilibration achieved), reduce FGF to target low-flow rate
3. Maintenance: Titrate vaporizer dial upward to compensate for agent uptake (vaporizer dial reading will be higher than measured inspired concentration)
4. Depth changes: Temporarily increase FGF to alter inspired concentration rapidly, then reduce again
5. Emergence: Increase FGF to flush volatile out of circuit; do NOT turn off the vaporizer at minimal flow and expect rapid emergence
6. Check absorber: Change soda lime when colour indicates exhaustion, or if inspiratory CO2 rises on capnograph

Monitoring for Safe LFA - Summary

Closed Circuit Anaesthesia

Recent Evidence (PubMed 2024-2026)

Key Points Summary

• LFA = FGF < minute ventilation with ≥ 50% rebreathing; requires circle system + CO2 absorber
• Benefits: cost savings (up to 75%), environmental (up to 90% fewer emissions), improved humidity/temperature
• Main hazards: slow depth changes, N2 accumulation/hypoxia, CO production from desiccated absorber, compound A (sevoflurane - not clinically toxic in humans)
• Monitoring is mandatory: O2 analyzer, capnography, agent analyzer, SpO2, pressure alarms
• Start at high flow for induction/denitrogenation; transition to low flow after ~10-15 min
• Closed circuit anaesthesia is the extreme form - maximizes benefits but impractical for routine use