2. ⦁ Closed circuit anaesthesia with carbon dioxide
absorption was used as early as 1850 by John Snow.
Because of the high costs and limited availability of
anaesthetic agents, early investigators frequently used
closed circuit techniques.
⦁ Brian Sword first described the circle breathing system
and delivery of closed circuit anaesthesia in 1930 . Lack
of efficient vaporizers along with poor understanding of
the pharmacokinetics of the newly developed halothane
led to the resurgence of high flow techniques.
DR.Aditi
3. Contd..
DR.Aditi
⦁ Low flow, partial rebreathing anaesthetic
techniques were first described by Foldes and
colleagues in 1952 which involved fresh gas
flows of 1 l/min.
⦁ Virtue, in 1974, reduced gas flows even further
in his minimal flow anaesthesia technique 500
ml/min.
4. Low flow anaesthesia has various definitions.
Any technique that utilizes a fresh gas flow (FGF)
that is less than the alveolar ventilation can be
classified as Low flow anaesthesia.
It can also be defined as a technique wherein at
least 50% of the expired gases had been returned
to the lungs after carbon dioxide absorption. This
would be satisfied when the FGF is less than about
two litres per minute.
DR.Aditi
5. According to Dorsch :
DR.Aditi
⦁ Low-flow anesthesia has been variously defined
as an inhalation technique in which a circle
system with absorbent is used with a fresh gas
inflow of
⦁ Less than the patient's alveolar minute
volume,
⦁ less than 4 L/minute
⦁ 3 L/minute or less
⦁ 0.5 to 2 L/minute
⦁ less than 1 or 1.5 L/minute or
⦁ 0.5 to 1 L/minute or 500 mL/minute
6. Baker classified the FGF used in anaesthetic practice
into :
1. Medium flow : 1 - 2 l/min.
2. Low flow : 500- 1000 ml/min.
3. Minimal flow : 250-500 ml/min.
4. Metabolic flow : about 250 ml /min
For most practical considerations, utilisation of a
fresh gas flow less than 2 litres/min may be
considered as low flow anaesthesia.
DR.Aditi
7. ⦁ Completely closed circuit anaesthesia is based upon the
reasoning that anaesthesia can be safely be 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 soda lime.
⦁ No gas escapes out of the circuit and would provide for
maximal efficiency for the utilisation of the fresh gas flow.
⦁ The very nature of this system requires that the exact
amount of anaesthetic agent taken up by the body be
known, since that exact amount has to be added into the
circuit. DR.Aditi
8. Contd…
DR.Aditi
⦁ Any error in this could lead to potentially
dangerous level of anaesthetic agent be present
in the inspired mixture with its attended
complications. So we have to be cautious while
using low flow anaesthesia.
9. 1. Circle rebreathing system with CO2 absorption
2.Accurate flow meters for adjustments of FGF below
1L/min
3.Gas tight breathing system. Recommended test
leakage should be below 150 mL/min at 30 cm H2 O
test pressure.
DR.Aditi
10. CO NTD ..
DR.Aditi
5. The breathing system should have minimal internal
volume and a minimum number of components and
connections.
6.Continuous gas monitoring MUST be employed. From
the clinical standpoint the measurement of expiratory
gas concentrations close to the Y-piece is of crucial
importance. That information is essential in controlling
the patient’s alveolar gas concentrations, whereas Fi
reflect the adequacy of gas concentrations into the
breathing circuit.
11. Contd..
DR.Aditi
⦁ 7.VAPOURIZERS :
Calibrated vaporizers - Vaporizers capable of
delivering high concentrations and that are
accurate at low fresh gas flows are required.
Liquid injection - Anesthetic liquid can be
injected directly into the expiratory limb
In circle vaporizer - In-system vaporizers
have been used successfully with both
spontaneous and controlled ventilation
13. Monitoring
Inspired O2 concentration should be monitored at
all times if N2O is used in more than 65%
concentration, as one of the adjuvant gas.
EtCO2 monitoring seems to be necessary to ensure
proper functioning of the absorber.
If monitoring of end tidal anaesthetic
concentration is available, the administration of
low flow anaesthesia becomes very easy.
DR.Aditi
15. HOW TO ADJUST FGF AT DIFFERENT
PHASES OF LFA
DR.Aditi
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
16. 1.Initial HIGH flow phase
⦁ 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.
⦁ The factors that can influence the build up of
alveolar concentration should all be considered
while trying to reach the desired alveolar
concentration. (MAC)
🞂
DR.Aditi
18. • 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 B G Coeff.)}
DR.Aditi
19. An alternative method for administering the large
amounts of the agents is by directly injecting the agent
into the circuit.
This is an old, time-tested method and is extremely
reliable. Each ml of the liquid halothane, on vaporisation
yields 226 ml of vapour and each ml of liquid isoflurane
yields 196 ml of vapour at 20oC.
Hence, the requirement of about 2ml of the agent is
injected in small increments into the circuit. The high
volatility coupled with the high temperature in the circle
results in instantaneous vaporisation of the agent.
DR.Aditi
21. The intermittent injections are often made in 0.2-
DR.Aditi
0.5 ml aliquots manually.
Doses should never exceed 1ml at a time. Doses
exceeding 2 ml bolus invite disaster.
Intermittent injections can often be easily
substituted with a continuous infusion with the
added advantage of doing away with the peaks and
troughs associated with intermittent injections.
23. This priming dose is the dose required to bring the
circuit volume + FRC to the desired concentration
and is injected over the first few minutes of the
closed circuit anaesthesia.
Besides this, an amount of agent necessary to
compensate for the uptake of the body must also
be added and this is calculated depending on the
uptake model being used.
DR.Aditi
24. 2. Low flow phase
DR.Aditi
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.
25. ⦁ If the flow provided is too small for the patient’s
needs the bellow will gradually go down, down
down...
DR.Aditi
26. THE MAINTENANCE OF LOW FLOW
ANAESTHESIA
DR.Aditi
⦁ 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.
27. ⦁ 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.
DR.Aditi
Management of the oxygen and nitrous
oxide flow during the maintenance
phase
28. Constant circuit volume :
DR.Aditi
Constant reservoir bag size : if the bag decreases in size.
The fresh gas flow rate is increased. If the bag increases in
size, the flow is decreased.
Ventilator with Ascending bellows: constant volume can be
achieved by adjusting the fresh gas flow so that bellows is
below the top of its housing at the end of exhalation.
Ventilator with Descending bellows: the fresh gas flow so
that bellows just reaches the bottom of its housing at the
end of exhalation.
29. Important Equations :
DR.Aditi
⦁ VO2 = 10 × BW KG ^ ¾ BRODY EQUATION
⦁ V N20 = 1000 × t ^ ½
EQUATION
SEVERINGHAUS
⦁ V AN = desired concentration × λB/G × Q × t − 1/2
LOWE EQUATION
30. ⦁ Time constant :
⦁ The time constant is a measure for the time it takes,
that alterations of the fresh gas composition will lead to
corresponding alterations of the gas composition within
the breathing system. According to a formula given by
Conway the time constant (T) can be calculated by the
division of the system's volume (VS) by the difference
between the amount of anaesthetic agent delivered into
the system with the fresh gas (VD) and the individual
gas uptake (VU):
⦁ T = VS / (VD - VU)
DR.Aditi
31. 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%.
DR.Aditi
32. Management of the potent anaesthetic
agents during maintenance phase
DR.Aditi
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.
33. 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.
DR.Aditi
34. Category
DR.Aditi
High flow
5L/Min
Inter
Mediate 2.5
l/min
Low flow 1
l/min
Minimal
Flow 0.5
l/min
Closed
circuit 0.3 –
0.5 l/min
Percentage 0 %
rebreathing
52% 86% 97% 100%
Circuit
concentratio
n with
vaporizer set
to 1% (at 1
h)
1 % 0.94% 0.68% 0.54% N2O 0.52%
NO N2O
0.39 %
Vaporizer
setting for in
inspired
concentratio
n of 1% ( at
1 h)
1% 1.1% 1.4% 2.0% N2O 2.4%
NO N20
3.2%
35. Low flow with Dräger machine :
DR.Aditi
⦁ With the connection of the patient to the anaesthesia
machine, an initial phase of high fresh gas flow (4-6
L/min) follows. During this initial phase, adequate
denitrogenation and distribution of anaesthetic gases in
the desired concentration throughout the system is
achieved and the desired level of anaesthesia is attained.
⦁ The duration of the initial phase is determined by the
amount of flow reduction and the individual gas uptake
(4-5 L/min, 6-8 min).
36. ⦁ After this time, oxygen and nitrous oxide levels reach 30% O2 and
65% N2 O, respectively . If isoflurane is used, the vaporizer is set to
1.5%; enflurane and sevoflurane require 2.5% setting and desflurane
requires a 4-6% setting.
⦁ After 6-8 min, an expiratory concentration corresponding to 0.8
MAC of the respective anaesthetic agent is attained. At an additive
nitrous oxide concentration of 50-60%, this value approximately
corresponds to AD95, or agent concentration at which 95% of
patients tolerate a skin incision without noticeable reaction.
⦁ After 10 min, the total gas uptake of an adult patient is about 600
ml/min, so that at this time, the flow can be reduced to 1.0 L/min
and low flow anaesthesia can be initiated.
DR.Aditi
37. ⦁ An inspiratory oxygen concentration level of 30% can only be
maintained if the fresh gas oxygen concentration is increased to
50% once the flow has been reduced.
⦁ Flow reduction also decreases the amount of anaesthetic agent,
which is introduced into the fresh gas flow by the vaporizer.
⦁ If the initially selected anaesthetic agent concentration of 0.8 ×
MAC is to be maintained, the vaporizer must be adjusted for 3.0%.
⦁ For desflurane, no changes are necessary.
⦁ Before reducing fresh gas flow rates to even lower levels, i.e., 0.5
L/min, to perform minimal flow anaesthesia, the initial phase
should be extended to 15 min and for very large patients to 20
min. DR.Aditi
39. ⦁ Because the rebreathing fraction is increased
compared with low flow anaesthesia, oxygen
concentration should be increased to 60%; 50%
represents an absolute minimum value.
⦁ An expiratory anaesthetic agent concentration of 0.8 ×
MAC should be maintained with appropriate dial
setting.
⦁ In low flow anaesthesia, the composition of anaesthetic
gases changes continuously during the course of
anaesthesia, making intermittent adjustments to fresh
gas composition necessary.
DR.Aditi
40. Precautions :
DR.Aditi
⦁ If a rapid change in any component of inspired
mixture is desired, FGF should be increased.
⦁ If integrity of circle is broken high flows should be
used for several minutes before returning to low flow.
⦁ If closed anaesthesia system is used its
recommended that high flow be used for 1-2 minutes
every hour to eliminate gases such as nitrogen and
carbon monoxide.
41. 3. Emergence phase
DR.Aditi
At the end of anesthesia high FGF, usually 100%
O2, is necessary, to facilitate the washout of the
anesthetic agent from the patient and to remove the
agent to the scavenging system.
A charcoal filter can be placed in expiratory limb. It
will cause a rapid decrease in volatile agent
concentration.
42. 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.
DR.Aditi
43. Advantage :
DR.Aditi
Economic Benefits
Reduce Operating Room Pollution
Environmental Benefits
Estimation Of Anesthetic Agent Uptake And Oxygen
Consumption
Buffered Changes In Inspired Conc.
Heat And Humidity Conservation
Less Danger Of Barotrauma
Quality Of Patient Care
44. Disadvantage :
DR.Aditi
More Attention Required
Inability to Quickly Alter Inspired Concentrations
Danger of Hypercarbia
Accumulation of Undesirable Gases in the
System(CO, Methane, Acetone, Hydrogen, Ethanol,
Compound A, Argon, Nitrogen)
Uncertainty about Inspired Concentrations
Faster Absorbent Exhaustion
46. ⦁ Many new anesthesia machines have been
developed in Europe within the past few years.
One of these is the Physioflex machine .
Another is Drager Zeus.
⦁ These machines have an inbuilt algorithm to
calculate the uptake and adjust concentration
and volume accordingly.
DR.Aditi
