Low flow anaesthesia

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Low Flow Anaesthesia

Definition and Classification

Low flow anaesthesia refers to techniques where fresh gas flow (FGF) is substantially less than minute ventilation, resulting in significant rebreathing of exhaled gases after CO₂ removal. The terminology is standardized as follows:

Term	Fresh Gas Flow	Rebreathing
High flow	>4-5 L/min	Minimal/none (absorber unnecessary)
Medium flow	1-4 L/min	Partial
Low flow	<1 L/min	>50% expired gas rebreathed
Minimal flow	≤0.5 L/min	Maximal
Closed circuit	= metabolic O₂ uptake (~250 mL/min)	Complete rebreathing

Low flow anesthesia "generally refers to a technique where fresh gas flow is far less than minute ventilation, and at least 50% of expired gas is rebreathed after carbon dioxide removal." - Miller's Anesthesia, 10e

Requirements (Prerequisites)

Low flow anaesthesia is only feasible with a circle breathing system containing:

Two unidirectional (one-way) valves - inspiratory and expiratory - to direct gas flow and prevent rebreathing of CO₂
CO₂ absorber (soda lime or calcium hydroxide lime) - essential; at flows >5 L/min, CO₂ absorption is largely unnecessary but becomes essential at low flows
Adjustable Pressure Limiting (APL) valve - to vent excess gas
Reservoir bag or ventilator bellows
A leak-free circuit - any leak is magnified at low flow because there is no excess gas to compensate

Physiological Basis

The anesthetic gas in the circuit is a mixture of fresh gas and exhaled gas (after CO₂ removal). The inspired concentration depends on the balance between FGF and minute ventilation:

High FGF → circuit gas approximates fresh gas composition → rapid changes in inspired concentration possible
Low FGF → circuit gas is diluted by exhaled gas → inspired concentration changes slowly

This creates two key kinetic consequences:

Changes at the vaporizer take longer to be reflected in inspired agent concentration
Oxygen consumption reduces inspired FiO₂ below the set value - FiO₂ must be monitored continuously

As Morgan & Mikhail state: "With low fresh gas flows, concentrations of oxygen and inhalation anesthetics can vary markedly between fresh gas and inspired gas." - Morgan & Mikhail, 7e

Advantages

1. Economy of Volatile Agent

The major benefit. Rebreathed gas retains volatile agent, so substantially less agent is consumed. Cost savings and reduced vaporizer refill frequency.

2. Environmental Impact Reduction

Volatile anaesthetics are greenhouse gases. Desflurane and nitrous oxide have particularly high global warming potential. Keeping FGF low, or eliminating desflurane, significantly reduces the environmental footprint of anaesthesia. - Miller's, 10e

3. Heat and Humidity Conservation

Medical gases supplied to the circuit are dry and at room temperature
Exhaled gas is saturated with water vapor at body temperature
At low flow, a greater proportion of inspired gas is warm, humidified exhaled gas
CO₂ absorber granules generate heat and moisture as a byproduct of the exothermic neutralization reaction
Result: reduced need for active humidification, better preservation of airway ciliary function and mucosal integrity

4. Reduced Theatre Pollution

Less waste gas escapes to the environment/operating room even with scavenging systems.

5. Reduces Cost

Disadvantages and Hazards

1. Accumulation of Unwanted Gases

Exhaled gas contains endogenous metabolic products (acetone, methane) and exogenous gases (CO, nitrogen). At low flow, these can accumulate to meaningful concentrations. Nitrogen washout is slow at induction.

2. Compound A (Sevoflurane-specific)

Sevoflurane undergoes base-catalyzed degradation by CO₂ absorbents to form Compound A (fluoromethyl-2,2-difluoro-1-(trifluoromethyl) vinyl ether), a nephrotoxic vinyl ether
Production is enhanced by: low flow/closed circuit, warm or desiccated absorbents, high sevoflurane concentrations, long duration
At 1 L/min FGF with soda lime: ~20 ppm; with Baralyme: ~30 ppm
In rats: nephrotoxic above ~150 ppm-hours cumulative exposure
In humans: despite exposures >200-320 ppm-hours, no clinically significant renal injury has been demonstrated in multiple randomized studies - attributed to far lower renal β-lyase activity in humans vs rats
Most countries (including UK) impose no minimum FGF restriction for sevoflurane based on this evidence
The FDA (USA) recommends FGF ≥2 L/min for sevoflurane based on earlier conservatism - now considered outdated by many authorities

3. Carbon Monoxide Formation

When CO₂ absorbents containing strong bases (NaOH, KOH - present in soda lime and Baralyme) become desiccated (<5% water content), they degrade desflurane, isoflurane, and to a lesser extent sevoflurane to carbon monoxide
Risk highest when a machine is left on with high FGF overnight (desiccating the absorbent)
Modern absorbents (Amsorb Plus, Drägersorb Free) contain no strong base, dramatically reducing this risk and enabling safer low flow with sevoflurane
"Carbon dioxide absorbents without strong bases such as potassium hydroxide or sodium hydroxide decrease this risk, allowing for low-flow anesthesia." - Miller's, 10e

4. Difficulty Controlling Inspired Concentration

Slow response to vaporizer changes
Requires experience and understanding of uptake/distribution kinetics
Can make rapid deepening or lightening difficult in an emergency

5. Oxygen Monitoring is Mandatory

O₂ is consumed metabolically (~250 mL/min in adults) but is not replenished by rebreathed gas
FiO₂ can fall below set value - inline O₂ analyzer is essential

Practical Conduct

Induction Phase

High FGF (4-6 L/min) is used initially to:

Flush nitrogen from the circuit
Rapidly achieve target inspired concentrations
Ensure adequate oxygenation before switching to low flow

This high-flow phase typically lasts 10-15 minutes until equilibration.

Maintenance Phase

FGF is then reduced to ≤1 L/min. The vaporizer dial setting must be higher than at high flow (since inspired concentration lags behind delivered concentration). End-tidal agent concentration monitoring (agent analyzer) is the best guide to adequacy.

Recovery

Increasing FGF again washes out volatile agent and speeds recovery.

Monitoring Requirements

Mandatory for safe low flow anaesthesia:

Monitor	Purpose
Inline O₂ analyzer	Detect falling FiO₂
CO₂ capnograph	Detect CO₂ absorber exhaustion
Inspired/expired agent analyzer	Guide vaporizer setting
Airway pressure	Detect circuit leaks
Respiratory volume	Detect disconnection
Pulse oximetry	Continuous SpO₂

CO₂ Absorbers

Absorbent	Strong Base?	Compound A risk	CO risk	Notes
Soda lime	Yes (NaOH + KOH)	Moderate	Yes (if dry)	Traditional
Baralyme	Yes (Ba(OH)₂)	Higher	Higher	Removed from US market
Amsorb Plus / Drägersorb Free	No	Minimal	Very low	Preferred for low flow

Key Points Summary

Low flow = FGF <1 L/min with ≥50% rebreathing; minimal flow = ≤0.5 L/min; closed circuit = FGF equals metabolic O₂ consumption
Requires circle system with functioning CO₂ absorber and leak-free circuit
Key benefits: cost savings (volatile agent economy), humidity preservation, environmental benefit
Key risks: Compound A accumulation (sevoflurane), CO from desiccated absorbent, O₂ depletion, nitrogen accumulation
Compound A is nephrotoxic in animals but no clinically significant renal toxicity in humans has been demonstrated
O₂ analyzer is mandatory; agent monitoring is essential
Start at high flow for 10-15 min to denitrogenate, then reduce FGF

Sources: Miller's Anesthesia 10e; Morgan & Mikhail's Clinical Anesthesiology 7e; Barash Clinical Anesthesia 9e

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