Low Flow Anaes

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