Low flow anaesthesia (LFA) techniques aim to minimize fresh gas flow (FGF) and maximize gas rebreathing during general anaesthesia. This reduces waste of anaesthetic gases and has environmental and economic benefits. The document outlines the history and concepts of LFA, requirements for its safe use including accurate gas monitoring, advantages like reduced waste, and disadvantages like potentially slower induction and emergence times. Conducting LFA requires vigilance to prevent hypoxic mixtures and ensure steady anaesthetic concentrations. With modern equipment and guidelines, LFA can be smoothly and practically applied in clinical practice while maintaining patient safety.

1. LOW FLOW ANAESTHESIA
Dr Ravi Wadke
Dr Shreyas Kate

2. Introduction
• Reusing anaesthetic agents in exhaled gases.
• Environmental pollution.
• Breakthroughs:-
– Modern anaesthetic machines
– Gas analyser monitors
– Precision vapourisers
– Potent volatile agents

3. History
• Snow- exhaled gases re-inspired-prolong
narcotic effect.
• Waters-1924- To and fro absorption system-
avoid CO2 rebreathing.
• Brian Sword- 1930- Circle breathing system
with sodalime absorber-closed circuit
anaesthesia.
• Cyclopropane-1933- minimise excessive
overflow-risk of explosion.

4. Circle breathing system

5. History
• Halothane-1954- highly potent volatile
anesthetic agents with narrow therapeutic
indices.
• High fresh gas flows-negligible rebreathing-
semi closed use of rebreathing systems-
common practice.
• Virtue-1974- Minimal flow anaesthesia
techniques.

6. History
• Aldrete et al., Lowe , Ernst – 1980- Revive low
flow and closed system anaesthesia.
• Logan, Farmer 1989- Environmental hazards-
anaesthetic agents.
• Modern volatile anaesthetic - partially
substituted halogenated hydrocarbons &
nitrous oxide-
– Depletion of stratospheric ozone layer
– Greenhouse effect

7. History
• Kleeman 1990- improving heat and humidity
of rebreathed anaesthetic gases- preserving
functional and anatomical integrity of
epithelial cells in respiratory tract.
• Baum and Aitkenhead-1995- Economic and
environmental advantage.

8. Concept
• Administer FG mixture + composition of
anaesthetic gases.
• Uptake of agents in body-physical
characteristics of agent.
• Exhaled gas mixture- different composition-
uptake of agents & addition CO2.
• Low flow Anesthesia- replenish consumed
gases with as minimum FG as possible &
remove CO2 before recirculating.

9. Concept
• Challenge- control dynamic equilibrium of FG
composition
– Uptake & metabolism time sensitive
– Many factors influence consumption & production of
gaseous components
• Frequent adjustments gas flow controls
• Complex equations –estimate uptake
• Monitoring inspired and end tidal concentrations
using gas analyser –more accurate and
convenient.

10. Definition
• Any technique that employs FG flow less than the
alveolar ventilation –Low flow anaesthesia.
• Inhalation anesthetic technique –rebreathing
fraction at least amounts to 50% where at least
50% of exhaled gas mixture is returned to patient
after CO2 removal in next inspiration.
• Modern anaesthetic machine FGF reduced to
2L/min or less.

12. Advantages

13. Requirements for use of Low Flow
technique
• Flow meters calibrated to flows down to
50ml/min.
• Leak proof circle breathing system and airway
devices like cuffed ETT (Well fitting supraglottic
airway device can be used).
• Gas monitoring system –inspired and end tidal
concentrations of agents.
• Measurement of expiratory gas concentrations
closer to Y piece crucial (reflects pts alveolar gas
concentration).

14. Requirements for use of Low Flow
technique
• Vapourisers capable of delivering high
concentrations and calibrated accurate at low
FGF.
• Breathing system with minimal internal
volume to minimise reserve volume.

15. LFA not suitable
• Short term anaesthesia with FM.
• Procedures with imperfectly gas –tight airways
(i.e. bronchoscopies with rigid bronchoscope).
• Technically unsatisfactory equipment with a
high gas leakage, unsuitable for leak free
closed breathing systems.
• Inadequate monitoring (malfunction of gas
analyser).

16. LFA not suitable
• Situations –clinical issues like hemodynamic
instability.
• Anaesthesiologist not familiar with LFA.

17. Concerns
• Dilution of anaesthetic agents-
– Low FGF added to significant large reserve volume
– Rate of change of composition of gas in reserve
volume exponential
– time constant (RV/FGF)
– 3 time constants- to effect 95% change in gas
composition
– Steady state- LFA most economical use of
anaesthetic agents

18. Concerns
• Differential uptake of agents modifying
composition of gas mixture
– N2O carrier gas
– Uptake high initially followed by gross reduction in
uptake
– Change in trend in differential uptake –hypoxic
mixtures delivered

19. Concerns
• Ensuring enough oxygen for metabolism
– Wide range of variations in the gas composition is
possible while reducing the FG flow
– Pulse oximeter less sensitive surrogate monitor of
tissue oxygenation
– Oxygen analyser
– Lower limit FiO2 0.30

20. Concerns
• Delay in recovery from anaesthesia
– Long time constant –slow reduction in
concentration of volatile anaesthetic agents –
recovery phase
– Change over to high FGF – to reduce time constant
– Switch off vapourisers early
– Some machines- special charcoal filter

21. Disadvantages
• Low flow rate & long time constant- slower
induction and emergence
• Quick alteration of inspired concentrations not
possible
• Continuous vigilance and frequent flow
adjustments
– avoid hypoxic mixtures
– Under/over dosage anaesthetic agents

22. Disadvantages
• Higher consumption of CO2 absorbents
– frequent exhaustion of absorbers
– CO2 rebreathing
– Risk of hypercarbia
• Possible accumulation of undesirable trace
gases in system
– CO, Acetone, Methane, Hydrogen, Ethanol,
Compound A- haloalkene
– Flush high FGF once an hour

23. Disadvantages
• US FDA recommendations limit Sevoflurane
exposure to 2MAC hrs at flow rates 1 to <2
l/min of FGF rates
• FGF rates <1 l/min not recommended

24. Calculation of gas update
• Total gas uptake =sum of uptakes of O2+N2O
+Anaesthetic agents.

25. Use of N2O in LFA

26. LFA in paediatric population
• Concerns
– Leaks –use of uncuffed ETT
– Additional monitoring & breathing valves in circuit
adding to dead spaces & resistance to breathing
circuit
• Airway sealing uncuffed ETT or LMA sufficient to
perform LFA.
• Frink et al.-low concentrations compound A
measured in children during sevoflurane
anaesthesia -2l/min FGF.
• Younger the child, lower the concentrations of
Compound A.
• Practical & safe.

27. Conducting LFA
• Premedication, preoxygenation and induction
–as usual practice.
• Initiation-
– Objective-To achieve alveolar concentration of
anaesthetic agent adequate for producing surgical
anaesthesia.

28. Conducting LFA- Initiation
• Use of high flows during initial phase
– Time constant reduced, bringing the circuit
concentration to the desired concentration rapidly
– FGF 10L of desired gas concentration and 2 MAC
agent concentration
– 3min-3Time constant- circuit brought to desired
concentration
– Better denitrogenation
– Rapid achievement of desired concentration
– Counterbalancing large uptake encountered at the
start of anaesthesia

29. Conducting LFA- Initiation
• Use of prefilled circuit
– Use different circuit like magill’s for
preoxygenation
– Simultaneously circle system fitted with test lung
& circuit filled with gas mixture of desired
concentrations
– After tracheal intubation patient connected to
circle system
– Rapid achievement of the desired concentration in
circuit

30. Conducting LFA- Initiation
• Injection of volatile agent into the breathing
circuit
– Usual requirement of anaesthetic agent -400-500ml of
vapours in first 10 min( 40-50 ml/min)
– At 20degree C 1ml Isoflurane yields 196ml
– About 2 ml liquid agent injected in small increments
into expiratory limb of circuit
– Intermittent injections 0.2-0.5 ml aliquots manually
– Alternatively, continuous infusion –added advantage
doing away with peaks and troughs of intermittent
injections

31. Conducting LFA- Initiation
• Accurate dose requirement formulae
• Priming dose (ml vapour)=
Desired concentration*
([FRC+Circuit volume]+
[Cardiac output*blood gas coefficient])

32. Conducting LFA-Maintenance
• Maintain steady state concentration of
anaesthetic agents.
• Oxygen uptake constant- 200-250 ml/min
• Uptake of anaesthesia agents minimal
• Oxygen analyser- maintain O2 concentration at
least 30% at all times.
• Return sampling gas(200ml/min) back to circuit
to boost economy of FGF utilisation.
• Plenum vapourisers under delivers agent at low
flows-achievement of desired end tidal agent
concentration accurate- agent analyser or
haemodynamic stability.

33. Conducting LFA-Maintenance

34. Termination LFA
• Long time constants-recovery delayed
– Switch over to HFGF-wash out agent
– Use of activated charcoal to remove potent
vapours by absorption-rapid recovery
– N2O washed off- change over to 100% O2

35. Automated LFA
• Uses proprietary software algorithm to
determine agent and carrier gas administration to
attain targets with lowest surplus.
• ALFA machines-
– ZEUS-Drager Lubeck Germany- Closed circuit
– AISYS- GE Madison Wisconsin- minimal flow
(500ml/min FGF)
– FLOW-i –Maquet Solna Sweden
• Anaesthesiologist selects
– Target alveolar concentration(FAt) of inhaled
anesthetic
– Target O2%
• Either inspired –FiO2 ZEUS
• Or end expired –FaO2 AISYS

36. LFA for sustainable Anaesthesia
• Environmental stewardship as a prime priority.
• We decide FGF-directly responsible for
environmental impact of anaesthetic vapours
and gases.
• Average working day each anaesthesiologist
administering N2O or Desflurane contribute to
CO2 equivalent of more than 1000 km car
driving.

37. LFA for sustainable Anaesthesia
• Research underway to collect and resuse
anaesthetic gases.
• Zeolite filters –Deltasorb –Canada –
commercial use- scavenging system of
anaesthesia machine to adsorb anaesthetic.
• Later retrieving and purifying agents for reuse.

38. Summary
• Smooth and practical conduct of LFA
– Safety features of anaesthetic machines
– Accurate gas monitoring- gas analysers – FiO2, ETCO2,
agent monitoring
• Clinical application of LFA is simplified by
availability of reliable guidelines for safe
performance in routine clinical practice.
• Anaesthesiologist should take up LFA as
professional obligation to environment and
present and future generations.

39. Reference

40. •Thank You