Circle system low flow anesthesia

CIRCLE SYSTEM (LOW FLOW)
By Dr. Geeta
choudhary

CIRCLE SYSTEM CAN BE:
 closed (fresh gas inflow exactly equal to
patient uptake, complete rebreathing
after carbon dioxide absorbed, and pop-
off closed)
 semi-closed (some rebreathing
occurs, FGF and pop-off settings at
intermediate values), or
 semi-open (no rebreathing, high fresh
gas flow)

CIRCLE SYSTEM CONT.
 Circle systems
 Most commonly used
 Adult and child appropriate sizes
 Can be semiopen, semiclosed, or closed
dependent solely on fresh gas flow (FGF)
 Uses chemical neutralization of CO2
 Conservation of moisture and body heat
 Low FGF’s saves money

CONT.
 Circle system
Allows for mechanical ventilation of the
lungs using the attached ventilator
Allows for adjustment of ventilatory
pressure
Allows for semiopen, semiclosed, and
closed systems based solely on FGF
Is easily scavenged to avoid pollution of
OR environment

COMPONENTS
Absorber :
An absorber assembly consists of
 an absorber
 two ports for connection to breathing tubes
 a fresh gas inlet.
 Other components that may be mounted
 inspiratory and expiratory unidirectional
valves,
 an adjustable pressure limiting (APL) valve,
and a bag mount.

 Absorber with two canisters in series, a
dust/moisture trap at the bottom and a
drain at the side. The lever at the right is
used to tighten and loosen the canisters.
Note that the date the absorbent was last
changed is marked on the lower canister.

CANISTERS
 The absorbent is held in canisters.
 The side walls are transparent so that the
absorbent color can be monitored.
 A canister with tinted side walls may make
it difficult to detect color changes in the
absorbent.
 A screen at the bottom of each canister
holds the absorbent in place.

Absorber with a single canister. It is loosened and tightened by twisting.
Absorber with
a single
canister it is
loosen and
tightened by
twisting.
Absorber with single
disposable canister
A:with the canister in
place. B :with canister
removed. The two valve
at the top prevent loss
of gas when the canister
is removed

INDICATOR
 Color When Fresh WhenExhausted
 Phenolphthalein White Pink
 Ethyl violet White Purple
 Clayton yellow Red Yellow
 Ethyl orange Orange Yellow
 Mimosa Z Red White

REACTIONS BETWEEN ABSORBENTS
AND ANESTHETIC AGENTS
 Haloalkene Formation
 Halothane degradation most often occurs
during closed-circuit anesthesia and
produces the haloalkene 2-bromo-2-
chloro-1, 1-difluoroethene (BCDFE).
 Compound A Formation
 Sevoflurane decomposes in the presence
of some carbon dioxide absorbents to
compound A

SEVERAL FACTORS INFLUENCE THE
AMOUNT OF COMPOUND A IN THE
BREATHING SYSTEM.
 Fresh Gas Flow: more with low fresh gas flow
 Absorbent Composition: greatest with
absorbents containing potassium or sodium
hydroxide.
 Absorbent Temperature : low temp cause
decrease comp A formation
 Concentration of Sevoflurane
 Anesthetic Length
 Water Content :

CARBON MONOXIDE FORMATION
 Carbon monoxide is produced when
desflurane, enflurane, or isoflurane is
passed through dry absorbent containing a
strong alkali (potassium or sodium
hydroxide)
 When sevoflurane is degraded by
absorbent, carbon monoxide is formed if
the temperature exceeds 80°C

FACTORS ASSOCIATED WITH CARBON
MONOXIDE FORMATION
 Absorbent Composition
 Absorbent Desiccation
 Anesthetic Agent -
 The highest carbon monoxide levels have
been seen with desflurane
 followed by enflurane
 then isoflurane
 Temperature Inside the Absorber
 Fresh Gas Flow
 Carbon Dioxide Absorption

THE APSF HAS PROVIDED A NUMBER OF
RECOMMENDATIONS
 that an anesthesia department should take
to prevent absorbent desiccation if the
department continues to use strong alkali
absorbents with volatile anesthetic
agents. These, including
 All gas flows should be turned OFF after
each case
 Vaporizers should be turned OFF when not
in use

CONT.
 The absorbent should be changed routinely, at
least once a week, preferably on a Monday
morning, and whenever fresh gas has been flowing
for an extensive or indeterminate period of time.
 The canister should be labeled with the filling
date.
 Checking this date should be part of the daily
machine checklist.
 If a double-chamber absorber is used, the
absorbent in both canisters should be changed at
the same time.

CONT.
 Canisters on an anesthesia machine that is
commonly not used for a long period of time
should not be filled with absorbent that
contains strong alkali or should be filled with
fresh absorbent before each use.
 The integrity of the absorbent packaging
should be verified prior to use.
 The practice of supplying oxygen for
administration to a patient who is not
receiving general anesthesia through the
circle system should be strongly discouraged

CONT.
 Using fresh gas to dry breathing system
components should be discouraged
 The temperature in the canister should be
monitored and the absorbent changed if
excessive heat is detected.
 Consideration should be given to removing
absorbent from canisters in induction
rooms and to using high fresh gas flows to
eliminate rebreathing.

WHEN AND HOW TO CHANGE THE
ABSORBENT
 Inspired Carbon Dioxide-Most reliable
method
 Indicator Color Change
 Heat in the Canister
 If excessive dust is present

UNIDIRECTIONAL VALVES
 Two unidirectional (flutter, one-
way, check, directional, dome, flap, nonret
urn, inspiratory, and expiratory)
 valves are used in each circle system to
ensure that gases flow toward the patient
in one breathing tube and away in the
other.
 They are usually part of the absorber
assembly

Unidirectional valve. Left: Reversing the gas flow causes the disc to
contact its seat, stopping further retrograde flow. Right: Gas flowing
into the valve raises the disc from its seat and then passes through the
valve. The guide (cage) prevents lateral or vertical displacement of the
disc. The transparent dome allows observation of disc movement.

Horizontal unidirectional valves.
Note the cages that prevent the
discs from being displaced.

INSPIRATORY AND EXPIRATORY PORTS
 The inspiratory port- has a 22-mm male
connector downstream of the inspiratory
unidirectional valve through which gases pass
toward the patient during inspiration.
 The expiratory port- has a 22-mm male
connector upstream of the unidirectional
valve through which gases pass during
exhalation.
 These ports are usually mounted on the
absorber

Y-PIECE
 is a three-way tubular connector
 with two 22-mm male ports for connection to
the breathing tubes
 a 15-mm female patient connector for a
tracheal tube or supraglottic airway device.
 The patient connection port usually has a
coaxial 22-mm male fitting to allow direct
connection between the Y-piece and a face
mask.
 A septum may be placed in the Y-piece to
decrease the dead space.

The breathing tubes attach to the
inspiratory and expiratory ports

FRESH GAS INLET
 The fresh gas inlet may be connected to
the common gas outlet on the anesthesia
machine by flexible tubing.
 the fresh gas inlet port, or nipple, has an
inside diameter of at least 4.0 mm and
that the fresh gas delivery tube has an
inside diameter of at least 6.4 mm

ADJUSTABLE PRESSURE-LIMITING
VALVE
 During spontaneous breathing, the valve is
left fully open and gas flows through the
valve during exhalation.
 When manually assisted or controlled
ventilation is used, the APL valve should be
closed enough that the desired inspiratory
pressure can be achieved.
 When this pressure is reached, the valve
opens and excess gas is vented to the
scavenging system during inspiration.

PRESSURE GAUGE
 Many circle systems have an analog pressure
gauge (manometer) attached to the
exhalation pathway.
 The gauge is usually the diaphragm type.
 Changes in pressure in the breathing system
are transmitted to the space between two
diaphragms, causing them to move inward or
outward. Movements of one diaphragm are
transmitted to the pointer, which moves over
a calibrated scale.

Diaphragm-activated pressure gauge.
Two thin metal diaphragms are sealed
together, with a space between them.
This space is connected to the
breathing system. Variations in
pressure in the breathing system are
transmitted to the diaphragms, which
bulge outward or inward. A series of
levers is activated, moving the
pointer, which records the pressure.

RESERVOIR BAG
 The bag is usually attached to a 22-mm
male bag port (bag mount or extension).
 It may also be placed at the end of a
length of corrugated tubing or
 a metal tube leading from the bag mount
providing some freedom of movement for
the anesthesia provider.

VENTILATOR
 Ventilator is also part of circle system.

BAG/VENTILATOR SELECTOR SWITCH
 A bag/ventilator selector switch
 provides a convenient method to shift
rapidly between manual or spontaneous
respiration and automatic ventilation
without removing the bag or the ventilator
hose from its mount.
 the selector switch is essentially a three-
way stopcock.

 One port connects to the breathing
system.
 The second is attached to the bag mount.
 The third attaches to the ventilator hose.
 The handle or knob that is used to select
the position indicates the position in which
the switch is set.

Bag/ventilator selector
switch. In the Bag position,
the reservoir bag and APL
valve are connected to the
breathing system. In the
Ventilator position, the APL
valve and bag are excluded
from the breathing system.

 Respiratory Gas Monitor Sensor or
Connector
 Both mainstream and sidestream devices
can be used with the circle system.
 Airway Pressure Monitor Sensor
 The sensor can be inserted into the circle
system by using an adaptor, or it may be
incorporated into the absorber assembly.

ADVANTAGES OF CIRCLE SYSTEM
 Cost reduction (use less agent and O2)
 Increased tracheal warmth and humidity
 Decreased exposure of OR personnel to waste
gases
 Decreased pollution of the environment
 REMEMBER that the degree of
rebreathing in an anesthesia circuit is
increased as the fresh gas flow (FGF)
supplied to the circuit is decreased

DISADVANTAGES OF CIRCLE SYSTEM
 Greater size, less portability
 Increased complexity
 Higher risk of disconnection or malfunction
 Increased resistance (of valves during
spontaneous ventilation)
 Dissuading use in Pediatrics (unless a circle pedi
system used)
 Difficult prediction of inspired gas
concentration during low fresh gas flow

LOW FLOW ANESTHESIA
 Any technique that utilises a fresh gas
flow (FGF) that is less than the alveolar
ventilation can be classified as „Low flow
anaesthesia‟.
 a technique wherein at least 50% of the
expired gases had been returned to the
lungs after carbon dioxide absorption.

CONT.
 Baker had classified the FGF used in
anaesthetic practice into the following
categories:
 Metabolic flow : about 250 ml /min
 Minimal flow : 250-500 ml/min.
 Low flow : 500- 1000 ml/min.
 Medium flow : 1 - 2 l/min.
 For most practical considerations, utilisation
of a fresh gas flow less than 2 litres/min may
be considered as low flow anaesthesia.

CONT.
 there exists a need for a system that
provided the advantages of the completely
closed circuit and at the same time,
reduced the dangers associated with it.
 Low flow anaesthesia fulfilled these
requirements.

CONT.
 Low flow anaesthesia involves utilising a
fresh gas flow which is higher than the
metabolic
flows but which is considerably lesser
than the conventional flows.
The larger than metabolic flows provides
for considerably greater margin of safety
and satisfactory maintenance of gas
composition in the inspired mixture.

THE NEED FOR LOW FLOW
ANAESTHESIA
 Completely closed circuit anaesthesia is
based upon the reasoning that anaesthesia
can be safely maintained if the gases
which are taken up by the body alone are
replaced into the circuit taking care to
remove the expired carbon dioxide with
sodalime. No gas escapes out of the circuit
and would provide for maximal efficiency
for the utilisation of the fresh gas flows.

EQUIPMENT
 The minimum requirement for conduct of low flow
anaesthesia is effective absorption of
 CO2 from the expired gas, so that the CO2 free
gas can be reutilised for alveolar ventilation.
 The circle system should have the basic
configuration with
 two unidirectional valves on either side of the
sodalime canister, fresh gas entry, reservoir bag,
 pop off valve, and corrugated tubes and „Y‟ piece
to connect to the patient.

CIRCLE SYSTEM
CAN: Canister of CO2 absorber
RB: Reservoir bag

SPECIFIC SAFETY FEATURES OF
ANESTHETIC TECHNIQUES WITH
REDUCED FGF
1. Improved equipment maintenance
2. The long time constant:
3. Improved knowledge of the theory and
practice of inhalational anesthesia.

Circle system low flow anesthesia

HOW TO ADJUST FGF AT
DIFFERENT PHASES OF LFA
Premedication, pre-oxygenation and
induction of sleep are performed according
to the usual practice. Concerning adjustment
of FGF anesthesia can be divided into 3
phases:
1. Initial HIGH flow
2. Low flow
3. Recovery

1.INITIAL HIGH FLOW PHASE
At the beginning of anesthesia high FGF of
5-6 LPM is necessary to wash out nitrogen
(N2) from the patients body tissues.
High initial flow facilitates the filling of the
breathing system with the desired gas
composition which in turn influences patient
uptake and distribution of the anesthetic
agents.

Reproduced with permission from “Low Flow Anesthesia with Draeger machines”
by Prof.J.A.Baum

Reproduced with permission from “Low Flow Anesthesia with Draeger
machines” by Prof.J.A.Baum

2. LOW FLOW PHASE
After the high flow phase of 5-15 min, or when
the target gas concentrations has been
achieved FGF can be reduced at the desired low
flow level. The lower the FGF the greater the
difference between the vaporizer setting and
inspired concentration of the anesthetic agent in
the breathing circuit will be.
With low FGF, time to reach the desired
concentration in the inspiratory gas will be
prolonged.
Hence, monitoring of oxygen and anesthetic agent
concentration is essential and necessary in LFA.

IF THE FLOW PROVIDED IS TOO SMALL
FOR THE PATIENT’S NEEDS THE
BELLOW WILL GRADUALLY GO DOWN,
DOWN DOWN...

THE PRACTICE OF LOW FLOW
ANAESTHESIA
 under the following three categories:
 1. Initiation of Low flow anaesthesia
 2. Maintenance of Low flow anaesthesia
 3. Termination of Low flow anaesthesia.

INITIATION OF LOW FLOW
ANAESTHESIA.
 Primary aim at the start of low flow
anaesthesia is to achieve an alveolar
concentration of the anaesthetic agent
that is adequate for producing surgical
anaesthesia (approximately 1.3MAC).
 The factors that can influence the build
up of alveolar concentration should all be
considered while trying to reach the
desired alveolar concentration.

CONT.
 These factors can broadly be classified
into three groups
1) Factors governing the inhaled tension of
the anaesthetic,
2)Factors responsible for rise in alveolar
tension,
3) Factors responsible for uptake from
the lungs
thus reducing the alveolar tension.

FACTORS AFFECTING THE BUILD UP OF
ALVEOLAR TENSION
 Factors governing the inhaled tension of
the anaesthetic,
1 BREATHING CIRCUIT VOLUME
2. RUBBER GAS SOLUBILITY
3. SET INSPIRED CONCENTRATION

CONT.
 FACTORS AFFECTING THE RISE IN
ALVEOLAR TENSION
1. CONCENTRATION EFFECT
2. ALVEOLAR VENTILATION
 UPTAKE BY THE BLOOD
1. CARDIAC OUTPUT
2. BLOOD GAS SOLUBILITY
3. ALV – VENOUS GRADIENT

METHODS TO ACHIEVE DESIRED GAS
AND AGENT CONCENTRATION
 Use of high flows for a short time
 Prefilled circuit
 Use of large doses of anaesthetic agents.
 Injection techniques :The exact dose to be
used is calculated thus:
 Priming dose (ml vapour) = Desired
concentration x {( FRC + Circuit volume)
+( Cardiac output x BG Coeff.)}

THE MAINTENANCE OF LOW FLOW
ANAESTHESIA
 This phase is characterised by
1. Need for a steady state anaesthesia often
meaning a steady alveolar concentration of
respiratory gases.
2. Minimal uptake of the anaesthetic agents
by the body.
3. Need to prevent hypoxic gas mixtures.

MANAGEMENT OF THE OXYGEN AND
NITROUS OXIDE FLOW DURING THE
MAINTENANCE PHASE
 A high flow of 10 lit/min at the start, for a
period of 3 minutes,
 is followed by a flow of 400 ml of O2 and 600
ml of N2O for the initial 20 minutes and
 a flow of 500 ml of O2 and 500 ml of N2O
thereafter.
 This has been shown to maintain the oxygen
concentration between 33 and 40 % at all
times.

 The Gothenburg Technique :
 Initially high flows, oxygen at 1.5 l/min and nitrous
oxide at 3.5 l/min had to be used for a period of
six minutes after the induction of anaesthesia and
this constitutes the loading phase.
 This is followed by the maintenance phase in which
the oxygen flow is reduced to about 4ml/kg
 and nitrous oxide flow adjusted to maintain a
constant oxygen concentration in the circuit.
 The usual desired oxygen concentration is about
40%.

MANAGEMENT OF THE POTENT
ANAESTHETIC AGENTS DURING
MAINTENANCE PHASE
 Weir and Kennedy recommend infusion of
halothane (in liquid ml/hr) at the following rates
 for a 50 kg adult at different time intervals.
 0-5 min 27 ml/hr
 5-30 min 5.71 ml/hr
 30-60 min 3.33ml/hr
 60-120 min 2.36 ml/hr
 These infusion rates had been derived from the
Lowe's theory of the uptake of anaesthetic
 agent.

 They had approximated isoflurane infusion (in liquid
ml/hr) based on the Lowe's formula as follows:
 0 - 5 min. 14 + 0.4X wt. ml/hr.
 5 - 30 min. 0.2 X initial rate.
 30-60 min. 0.12Xinitial rate.
 60-120min. 0.08X initial rate.
 For halothane infusion, they had suggested that the
above said rates be multiplied by 0.8
 and for enflurane, multiplied by 1.6. These rates had
been suggested to produce 1.3 MAC without the use of
nitrous oxide. The infusion rates had to be halved if
nitrous oxide is used.

TERMINATION OF LOW FLOW
ANAESTHESIA
 There are only two recognised methods of
termination of the closed circuit. They are as
follows:
1 )Towards the end of the anaesthesia, the circuit is
opened and a high flow of gas is used to flush out
the anaesthetic agents which accelerates the
washout of the anaesthetic agents.
 This has the obvious advantage of simplicity but
would result in wastage of gases.
2 )The second method is the use of activated
charcoal Activated charcoal when heated to 220oC
adsorbs the potent vapours almost completely.

ADVANTAGES OF THE LFA
1. QUALITY OF PATIENT CARE
2. ECONOMIC BENEFITS
3. ENVIRONMENTAL BENEFITS
4. REDUCE OPERATING ROOM POLUTION
5. ESTIMATION OF ANESTHETIC AGENT
UPTAKE AND OXYGEN CONSUMPTION
6. BUFFERED CHANGES IN INSPIRED CONC.
7. HEAT AND HUMIDITY CONSERVATION
8. LESS DANGER OF BAROTRAUMA

DISADVANTAGES OF LFA:
More Attention Required
Inability to Quickly Alter Inspired
Concentrations
Danger of Hypercarbia
Accumulation of Undesirable Gases in the
System
Uncertainty about Inspired Concentrations
Faster Absorbent Exhaustion

Circle system low flow anesthesia

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