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Low Flow Anesthesia

Bambang WahjuprajitnoDept. of Anesthesiology & ReanimationFaculty of Medicine - Univ. of Airlangga

Rumah Sakit BedahSurabaya

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History

• 1850: John Snow recognized that a considerable amount of inhalation anaesthetics were exhaled  unchanged in the expired air of anaesthetized patients reinhaling?➞

• 1924: CO2 absorbers were introduced into anaesthetic practice by Ralph Waters (to and fro) and CJ Gauss-HD Wieland (circle system)

• 1933: highly combustible cyclopropane ➞ ↓FGF to reduce pollution

• 1954: halothane, high anesthetic potency yet narrow therapeutical width high FGF and low proportion of ➞rebreathing was kept patient safety➞

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Ideal technique, but low adoption

• Familiar with high FGF Constant inspired ➞anesthesia agent

• Loss of control over anesthetic concentration

• Complex process the need of knowledge, ➞skill, training and sophisticated apparatus

• No suitable anesthetic agent available

• No suitable anesthetic machine and monitor available

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Kyoto Protocol

• An international treaty that sets binding obligations on industrialised countries to reduce emissions of greenhouse gases

• carbon dioxide (CO2)

• methane (CH4)

• nitrous oxide (N2O)

• sulphur hexafluoride (SF6)

• two groups of gases: hydrofluorocarbons (HFCs) and perfluorocarbons (PFCs)

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Anesthesiologist contributions

• Use room air/oksigen, avoid N2O as carrier gas

• Gases with less impact: xenon

• Avoid unnecessarily high FGF, use low flow as routine LFA➞

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Characteristics of low freshgas flow techniques

• Increased rebreathing volume

• Less excess gas

• Difference of gas composition – Fresh gas versus gas in the circuit

• Long time constants

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Low Flow Anesthesia

• Low flow anesthesia occurs when the FGF rate is significantly less than minute ventilation

• A technique where significant re-breathing occurs

• Re-breathing fraction increases with the reduction of FGF with a reciprocal decrease in the volume of excess gas

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Definition

• Low flow anaesthesia: an inhalation anaesthetic technique via a rebreathing system in which the rebreathing fraction at least amounts to 50% 50% of the exhaled gas ➞volume is led back to the patient after carbon dioxide absorption in the next inspiration.

• Using modern anaesthetic machines this will be gained at a fresh gas flow rate between 2 to 1 L/min

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Classification of anesthesia circuits according to Baker and Simionescu

Circuit Fresh gas flow

Metabolic flow ~ 250 ml/min

Minimal flow 250-500 ml/min

Low flow 500-1000 ml/min

Medium flow 1-2 L/min

High flow 2-4 L/min

Open > 4 L/min

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Advantages

• Economy (desflurane & sevoflurane)

• Climatization of the inspired gases (inspired gas humidity & temperature)

• Ecology: Reduced atmospheric pollution (chlorine destruction of ozone layer)➞

• Anesthesiologist promotes greater ➞understanding of:

• breathing systems

• pharmacokinetics of inhalation anaesthesia

Cost savings by Low Flow versus Minimal Flow Anaesthesia

Nitrous oxide workplace concentration

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Inspired gas humidity & temperature

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Disadvantages

• Capital investment

• Increased consumption of absorbent

• Limitations of currently available vaporizers

• Accumulation of unwanted gases in the breathing system

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Concerns about safety in LFA

• Hypoxia

• Gas volume deficiency

• Misdosage of volatiles

• Reduced controllability

• Exhaustion of the absorbent

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Requirements

• Flow meters calibrated to flows down to 50 ml min-1

• A leak-free circle system

• A near-gas-tight breathing system

• CO2 absorber

• Anesthesia gas monitor

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Requirements

• Qualified personnel

• Suitable anesthetic machine

• Accurate fresh gas delivery(Accurate and consistent settings, accurate readability)

• Very low leakage of the system (must not exceed 100 mL/min)

• Monitoring for safe performance

• Standard : EKG, pulse oximetry, NIBP, EtCO2

• Continous measurement with alarm: Paw, MV, FiO2

• Optional : gas monitor

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Calibrated Flowmeters

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Leak-free circle system

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Vaporizers

• The vaporisers should feature pressure-, temperature-, and flow compensation

• Maximum output of the vaporisers at a concentration equalling 3-5 times the respective MAC impossible ➞for halothane & enflurane

• Future: Liquid injection vaporizers

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Trace gasesDue to decreased wash-out

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Caution!

• Sevoflurane interaction with carbon dioxide absorbents Compound A ➞

• fluoromethyl-2,2-difluoro-1-(trifluoromethyl)vinyl ether

• Factors

• low-flow or closed-circuit

• concentrations of sevoflurane

• higher absorbent temperatures

• fresh absorbent

• Baralyme dehydration ➞ ↑ compound A concentration

• Soda lime dehydration ➞ ↓ compound A concentration

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Caution!

• Desiccated soda lime and Baralyme

• carbon monoxide

• after disuse of an absorber for at least 2 days, especially over a weekend

• Several factors appear to increase the production of CO and carboxyhemoglobin:

• Anesthetic agents (desflurane ≥ enflurane > isoflurane ≥ halothane = sevoflurane)

• The absorbent dryness (completely dry absorbent produces more carbon monoxide than hydrated absorbent)

• The type of absorbent (at a given water content, Baralyme produces more carbon monoxide than does soda lime)

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Caution!

• Several factors appear to increase the production of CO and carboxyhemoglobin:

• The temperature (a higher temperature increases carbon monoxide production)

• The anesthetic concentration (more carbon monoxide is produced from higher anesthetic concentrations)

• Low fresh gas flow rates (LFA)

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Prevention

• Educating anesthesia personnel regarding the cause of carbon monoxide production

• Turning off the anesthesia machine at the conclusion of the last case of the day to eliminate FGF, which dries the absorbent

• Changing carbon dioxide absorbent if fresh gas was found flowing during the morning machine check

• Rehydrating desiccated absorbent by adding water to the absorbent

• Changing the chemical composition of soda lime (e.g., Dragersorb 800 plus, Sofnolime, Spherasorb) to reduce or eliminate potassium hydroxide

• Using absorbent materials such as calcium hydroxide lime that are free of sodium and potassium hydroxides

How we do it?

FGF

Breathingcircuit

FI

Arterialblood

VenousbloodLungs

Anesthesia machine• FGF (fresh gas flow) is determined by:

• the vaporizer and flowmeter settings

• FI (inspired gas concentration) is determined by:

1.FGF rate

2.Breathing circuit volume and

3.Circuit absorption.• FA (alveolar gas concentration) is determined by:

1.Uptake: Uptake = λ b/g x C(A-V) x Q

2.Ventilation and

3.The concentration effect and second gas effecta)Concentrating effectb)Augmented inflow effect

• Fa (arterial gas concentration) is affected by ventilation/perfusion mismatching

Uptake of volatile anesthetic agents

Minutes of Anesthesia

Up

take a

t M

AC

(m

L/m

in)

Pharmacokinetic and pharmacodynamic properties of different inhalation anesthetics

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3 phases of LFA

1. Initial high flow

2.Low flow

3.Recovery

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Initial High Flow Phase

• Sufficient denitrogenation

• Rapid wash in of the desired gas composition into the breathing system

• Establishing of the desired anaesthetic concentration

• Avoiding gas volume deficiency

Initial High Flow Phasehigh FGF lasting 10 to 20minutes

Denitrogenation Wash-in

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Oxygen concentration during low fresh gas flow

• During Low Flow Anesthesia (1.0 L/min)

• Oxygen concentration shall be increased to 50 vol.%

• At least to 40 vol %

• Minimal Flow Anaesthesia (0.5 L/min)

• Oxygen concentration shall be increased to 60 vol.%

• O2 300 mL/min + N2O 200 mL/min

Insp. O2 Concentration during Low Flow Phase

Insp. oxygen concentration during the course of Low Flow Anaesthesia, different oxygen concentrations of the fresh

gas

Expiratory isoflurane concentration (nominal value, 0.9 vol% ≈ 0.8 MAC, patient 75 kg) and vaporiser settings required for the different

FGFs

LFA

The lower the fresh gas flow, the higher the concentration to be dialled at the vaporiser to maintain a desired expiratory sevoflurane concentration of about 1.7 vol% during the remaining course of anaesthesia

Expiratory sevoflurane concentration (nominal value,1.7 vol% ≈ 0.8 MAC, patient 75 kg) and vaporiser settings required for the different

FGFs

Expiratory sevoflurane

concentration

The lower the fresh gas flow, the higher the concentration to be dialled at the vaporiser to maintain a desired expiratory sevoflurane concentration of about 1.7 vol% during the remaining course of anaesthesia

Expiratory desflurane concentration (nominal value, 3.5 vol% ≈ 0.8 MAC, patient 75 kg) and vaporiser settings required for the different

FGFs

When desflurane is used in Low Flow Anaesthesia the flow can be reduced from 4.5 to 1.0 L/min already 10 minutes after induction without any alteration of the vaporiser setting.

How can anaesthetic depth be changed rapidly during low flow inhalation

anaesthesia?

Changing from long to short time constant by varying not only vaporiser setting but also fresh gas flow during performance of Minimal Flow Anaesthesia to quickly alter the depth of anaesthesia.

Decreasedepth of anesthesia

Increasedepth of anesthesia

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Absolute contraindications

• Continuous wash out of potentially dangerous gases is required Smoke or gas intoxication➞

• High Individual gas uptake Malignant ➞hyperthermia

• The equipment does not meet essential requirements

• Soda lime exhaustion

• Failure of the oxygen monitor (unless pure oxygen is used as carrier gas)

• Failure of the anaesthetic agent monitor (if it is part of the dosing system itself)

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Relative contraindications

• Inhalation anaesthesia lasting less than 10-15min risk of:➞

• Insufficient denitrogenation

• Inadequate depth of anaesthesia

• Gas volume deficiency

• Leakage in the anesthesia system

• Risk of accumulation of potentially dangerous trace gases:

• Decompensated diabetes mellitus

• The state of long-term starvation

• Chronic alcoholics or acute alcohol intoxication

• Heavy smookers

References

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