Low flow Anesthesia system

Low flow Anaesthesia system
Dr P K Maharana
KIMS, Bhubaneswar.

History of Low flow Anaesthesia
 As early as 1850 John Snow (1813-53) recognized that only a small
fraction that inhalation anaesthetics gets metabolized and most of it
exhaled unchanged in the expired air of the anaesthetized patients.
He also concluded that the narcotic effect of the volatile anaesthetics
could be markedly prolonged by re-inhaling these unused vapours.
 About 75 years later, in 1924, the rebreathing technique with carbon
dioxide absorption was introduced into routine clinical practice by Ralph
Waters (1883-1979), the to-and-fro absorption system.
 At the same time Carl Gauss (1875-1957) published his clinical experiences
of the successful use of a circle absorption system for use with narcylene,
i.e. acetylene, as an inhalation anaesthetic proved to be a milestone. Brian
C. Sword introduced the circle (CO2 absorption) system in the 1930s.
 1933 onwards ; with introduction and increasing clinical use of
cyclopropane it became nearly essential to close the overflow valve of the
breathing system to avoid serious danger of explosions; and it forced to
reuse the exhaled air completely

History of LFA( Conti-i)
 In 1954 , the world noticed a land mark event the introduction halothane to
clinical practice, a volatile anaesthetic characterized by high anaesthetic potency
but a narrow therapeutic index .
It’s use necessitated knowledge of the inhaled vapour concentration to ensure
patient safety.
It was easy to measure the concentration of halothane with high flow system with
no or minimal rebreathing.
Hence popularization of Halothane gave rise to use of high flow system.
In spite of the fact that most anaesthetic machines were equipped with circle
systems, but it became clinical obligation to use high fresh gas flows for patient’s
safety because rebreathing became negligible.
 The introduction of Isoflurane in the early 1980s gave way to a renewed interest in
LFA and use of closed-circuit anesthesia.
 This fact is further strengthen by the fact that anesthetic agents are atmospheric
pollutants especially N2O, the volatile agents like HAL, ENF, and to some extent
ISO, accused for ozone depletion & the green house effect.

History of LFA( Conti-ii)
 In 1990’s the low soluble volatile substances like sevoflurane and
desflurane became the most popular clinically. Factors like cost and theatre
pollution paved the way for the resurgence of low flow system and use of
these newer drugs. Xenon as an anesthetic gas was more relevant.
 Kleemann in 1990, had put forth the procedures of preserving the
functional and the anatomical integrity of the epithelial cells of the
respiratory tract using the LFA for the improvisation of heat and humidity
of the rebreathing anesthetic gases.
 Baum and Aitken Head in 1995, renovated the LFA by imposing on
important noted benefits from both environmental and economic aspects.

Definition
 Any technique that utilizes a fresh gas flow (FGF) that is less than the alveolar
ventilation can be classified as “Low flow anaesthesia.” However, there is no clear
definition as to what exactly the flow should be to level it as low flow?
o Baum et al : had defined it as a technique wherein at least 50% of the expired
gases had been returned to the lungs after carbon dioxide absorption in the next
inspiration.
o A technique for oxygen-nitrous oxide anaesthesia with a flow of 1liter per minute
was described by Foldes in 1952.
o In 1974, Virtue described technique using a fresh gas flow of 500ml per minute,
which is named as minimal flow anaesthesia.
Baker in his editorial described fresh gas flow (FGF) in anesthetic practice as
under;
Metabolic flow  250ml/minute
Minimal Flow  250-500ml/minute
Low Flow  500-1000ml/minute
Medium Flow  1000- 2000ml/minute
 For all practical purposes a FGF was less than about two litres per minute
may be considered as LFA

Principle of working of LFA system
• Low flow anaesthesia involves utilizing a fresh gas flow which is
higher than the metabolic flows but considerably lesser than the
conventional flows.
– The larger than metabolic flows provides considerably greater margin
of safety and helps in satisfactory maintenance of gas composition in
the inspired mixture.
• 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 sodalime.
- Hence in LFA, the conduct of anaesthesia is greatly simplified and at the
same time provides advantages for the economy of the fresh gas flows
minimising theatre pollution.

Requirements for LFA
1. A circle system with CO2 absorption.
2. A flow meter for the adjustments of FGF below 1 l/min.
3. Gas-tight breathing system, recommended test leakage
should be below 150 ml/min.
4. The breathing system must be having minimal internal
volume and a minimum number of components and
connections.
5. Continuous gas monitoring must be employed, essential in
controlling the patient's alveolar gas concentrations.
6. The vaporizers delivering the high concentrations must be
calibrated and must be accurate at low fresh gas flow.

Circle System
• The minimum requirement for conduct of low
flow anaesthesia is the effective absorption of
CO2 from the expired gas, so that the CO2 free
expired gas can be reutilized for alveolar
ventilation.
• In the past, two systems were in use; “To and
Fro” system introduced by Waters and the “circle
system” introduced by Brian Sword.
• But because of its bulkiness near the patient and other
disadvantages the “To and Fro” has gone out of favour
and the “circle system” using large sodalime canisters is
in use.

Describing a Circle System.
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.
The relative position of fresh gas
entry, pop off valve, and reservoir
bag are immaterial as long as they
are positioned between the
expiratory and the inspiratory
unidirectional valves that functions
properly and CO2 absorption is
efficient at all times.

THE PRACTICE OF LOW FLOW
ANAESTHESIA:
The practice of low flow anaesthesia can be
dealt with under the following three headings:
o 1. Initiation of Low flow anaesthesia
o 2. Maintenance of Low flow anaesthesia
o 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 1.3 MAC).
 Factors influencing to buildup of alveolar concentration should be
considered . These factors can broadly be classified into three
groups :
 1) Factors governing the inhaled tension of the anaesthetic agent,
 2) Factors responsible for rise in alveolar tension,
 3) Factors responsible for reducing the alveolar tension, uptake
from the lungs.

Factors governing the inhaled tension of
the anaesthetic.
Three most important factors responsible for
inhaled tension of anesthetics are :
 1.BREATHING CIRCUIT VOLUME
 2. RUBBER GAS SOLUBILITY
3. SET INSPIRED CONCENTRATION

Breathing Circuit Volume
 Circle system is often bulky and volume roughly equal to 6-7 liters,
FRC of the patient, which is roughly 3 litres, together constitutes a
reserve volume of 10 liters; to which the anaesthetic gases and
vapours have to be added.
• With the addition of FGF, the rate of change of composition of the reserve
volume is exponential, time required for the changes to occur is governed
by the time constant, which is equal to this reserve volume divided by the
fresh gas flow.
• Three time constants are needed for a 95% change in the gas
concentration to occur.
• Hence, if a FGF of 1L/min is used, then 30 minutes will be required
for the circuit concentration to reflect the gas concentration of the
FGF.
• If the FGF is still lower, then correspondingly longer time will be
required.

Rubber Gas Solubility of the agent
• The anaesthetic agent could be lost from the
breathing system due to solubility of the
agent in rubber, and permeability through
the corrugated tubes.
• The concentration will take a longer time if
solubility is more.
• Though the amount of loss will be minimal, it
should be considered at the start if the aimed
anaesthetic concentration is low.

SET INSPIRED CONCENTRATION
 The rate of rise of alveolar partial pressure of the
anaesthetic agent must bear a direct relationship
to the inspired concentration.
 Higher the inspired concentration, the more rapid
is the rise in alveolar concentration.
 The Inspired concentration of the agent depends
on several factors.
– Denitrogenation of the circuit volume.
– Alveolar ventilation.
– Concentration effect.

Denitrogenation of FRC & Breathing
Circuit Volume
The functional residual capacity of the lung
and the body as a whole contain nitrogen
which will try to equilibrate with the circuit
volume and alter the gas concentration if
satisfactory denitrogenation is not achieved at
the start of anaesthesia.
 Hence, as a prelude to the initiation of closed or low flow
anaesthesia, thorough denitrogenation must be achieved with
either a non-rebreathing circuit or the closed circuit with a
large flow of oxygen and a tight fitting facemask.

Alveolar ventilation:
 The second factor governing the delivery of
anaesthetic agent to the lung is the level of
alveolar ventilation.
 The greater the alveolar ventilation, the
more rapid is the rise of alveolar
concentration towards the inspired
concentration.
• This effect is limited only by the lung volume, the larger
the functional residual capacity, the slower the wash. in
of the new anaesthetic gas.

Factors responsible for uptake from the lungs
thus reducing the alveolar tension
• 1. CARDIAC OUTPUT .
• 2. BLOOD GAS SOLUBILITY .
• 3. ALV – VENOUS GRADIENT.

Cardiac output
• Because blood carries anaesthetic away from the
lungs, the greater the cardiac output, the
greater the uptake, and consequently the slower
the rate of rise of alveolar tension.
• At low inspired concentration, the alveolar
concentration results from a balance between
the ventilatory input and circulatory uptake.
• If the later removes half the anaesthetic
introduced by ventilation, then the alveolar
concentration is half that inspired.

Blood gas solubility
• If other things are equal, the greater the blood gas
solubility coefficient, the greater the uptake of
anaesthetic, and slower the rate of rise of alveolar
concentration.
• “Solubility” is the term used to describe how a gas or vapour is
distributed between two media. At equilibrium, that is when the
partial pressure of the anaesthetic in the two phases is equal, the
concentration of the anaesthetic in the two phases might differ.
• This is calculated as a coefficient.
• When it is between blood and gas it is called blood gas solubility
coefficient.

Alveolar to venous partial pressure gradient:
• During induction the tissues remove all the
anaesthetic brought to them by the blood. Tissue
uptake lowers the venous anaesthetic partial
pressure far below that of the arterial blood.
• If tissue uptake is more then it results in a large alveolar
to venous anaesthetic partial pressure difference, which
promotes maximum anaesthetic uptake and hence
lowers the alveolar partial pressure.

Concentration effects
 The concentration effect modifies this influence of uptake.
o When appreciable volumes are taken up rapidly, the lungs do not
collapse; instead the sub atmospheric pressure created in the lung
by the anaesthetic uptake causes passive inspiration of an
additional volume of gas to replace that lost by uptake, thus
increasing the alveolar concentration and offsetting the
mathematical calculations.
o Similarly, if an insoluble gas (e.g., nitrogen) is present in the
inspired mixture, as the blood takes up the anaesthetic gas, the
concentration of the insoluble gas will go up in the alveoli, reducing
the concentration of the anaesthetic agent.
o Concentration effect: The concentration effect helps in raising the
alveolar tension towards the inspired tension, but hinders with it if
an insoluble gas is present in the mixture.

Methods to achieve desired gas and
agent concentration.
• High flow for short time
• Use of Prefilled Circuits with gases
• Use of large doses of anaesthetic agents.
• Injector technique.

Use of high flows for a short time
 This is by and far the commonest and the most effective technique of initiating closed circuit.
• By using high flows for a short time, the time constant is reduced thereby bringing the circuit
concentration to the desired concentration rapidly.
• Often, a fresh gas flow of 10L of the desired gas concentration and 2 MAC agent concentration is
used so that by the end of three minutes (three time constants) the circuit would be brought to the
desired concentration.
 Advantages:
--- Rapidity with which the desired concentration is achieved, the ability to prevent unexpected
raise in the agent concentration and the ability to plenum vaporizers to achieve the desired
concentration.
– This also has the added advantage of achieving better denitrogenation, so vital to the conduct of the low
flow anaesthesia.
 The chief disadvantage:
- Not economical & need scavenging systems to prevent theatre pollution
 Mapleson3 using a spreadsheet model of a circle breathing system has calculated that, by using a
FGF equal to minute ventilation and setting the anaesthetic agent partial pressure to 3 MAC, the
end expired partial pressure of halothane will reach 1 MAC in 4 minutes and that of isoflurane in
1.5 minutes.

Use of Prefilled circuit
• The second method is utilizing a different
circuit like Magill’s for pre-oxygenation.
• Simultaneously, the circle is fitted with a test
lung and the entire circuit is filled with the gas
mixture of the desired concentration.
• Following intubation, the patient is connected to the circuit
thereby ensuring rapid achievement of the desired
concentration in the circuit.

Use of large doses of anaesthetic
agents.
 The third method consists of adding large amounts of anaesthetic agent
into the circuit so that the circuit volume + FRC rapidly achieves the
desired concentration as well as compensates for the initial large
anaesthetic gas uptake.
 To execute this, Pre-oxygenation after intubation, fresh gas flow is started
with metabolic flows of oxygen and a large amount of nitrous oxide often
in the range of 3-5 litres per minute.
 Oxygen concentration in the circuit gradually falls, is continuously
monitor Oxygen and the nitrous oxide concentration, N2O flow is reduced
once the desired oxygen concentration (33 - 40%) is achieved .
 Disadvantage of this method : is hypoxia if the oxygen monitor were to
malfunction.
o Hence this method is seldom used for N2O.
o In the contemporary anaesthesia machines it is not possible to administer
lesser than 25% of oxygen and the described technique cannot be
executed.

Injection techniques.
 An alternative method for administering the large amounts of the
agents is by directly injecting the agent into the circuit. The high
volatility coupled with the high temperature in the circle results in
instantaneous vaporization of the agent.
 The injection is made through a self sealing rubber diaphragm
covering one limb of a metal t piece or a sampling port, inserted into
either the inspiratory or the expiratory limb.
o This is an old, time-tested method and is extremely reliable.
o Each ml of the liquid halothane, on vaporization 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 MAINTENANCE OF LOW FLOW
ANAESTHESIA
 This is the most important phase as this is
stretched over a long period of time and financial
savings result directly from this; this phase is
characterized by :
1. Need a steady alveolar concentration of respiratory gases.
2. Minimal uptake of the anaesthetic agents by the body.
3. Need to prevent delivery of hypoxic gas mixtures.
 Since the uptake of the anaesthetic agent is small in this phase,
adding small amounts of the anaesthetic gases to match the uptake
and providing oxygen for the basal metabolism should suffice.

Management of the oxygen and nitrous oxide flow
during the maintenance phase
 The need to discuss the flow rates of N2O and O2 arises specifically
because of the possible danger of administration of a hypoxic mixture.
 Let us analyze the following example. 33% oxygen is set using a flow of 500 ml of O2 and
1000 ml of N2O.
o Oxygen is taken up from the lungs at a constant rate of about 4 ml/kg/min.
o N2O is a relatively insoluble gas and after the initial equilibration with the FRC and vessel
rich group of tissues, the up take is considerably reduced.
o In this situation, there is a constant removal of O2 at a rate of 200 - 250 ml/min, where as
the insoluble gas N2O uptake is minimal. Hence the gas that is partly vented and partly
returning to the circuit will have more N2O and less of O2.
o Over a period of time, due to the mixing of fresh gas that has 66% N2O and the expired CO2
free gas that has N2O much higher than that, the percentage of N2O will go up and that of
O2 will fall, sometimes dangerously to produce hypoxic mixtures.
 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.

Management of the potent anaesthetic agents during
maintenance phase
 This is easily accomplished by dialling in the calculated
concentration on the plenum vaporizer for the flow being used.
 For example, suppose the anaesthetic uptake for a desired
concentration of 0.5% halothane is 7.5ml/min.
o If a FGF of 500ml/min is being used, then the dial setting should be
1.5% for at this setting and for the used flow, the total vapour
output would be 7.5ml/min.
o If a flow of 1000ml/min is being used, then the dial setting should
be 0.8%.
 In practice the actual dial setting often over estimates the actual
output since the plenum vaporizer under delivers the agent at low
flows.
Hence, the dial setting is fine-tuned depending on the endpoints
being achieved.

Lowe's formula
T approximated 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, the above 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.

Weir and Kennedy recommend
infusion of halothane
 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.

Salient points to be considered during the
maintenance phase
The other salient points to be considered
during the maintenance phase are the
following:
o a) Leaks must be meticulously sought for and prevented, flows must
be adjusted to compensate for the gas lost in the leaks.
o b) Most of the gas monitors sample gases at the rate of 200 ml/min,
which may be sometimes as high as half the FGF. Hence, care must be
taken to return the sample back to the circuit to maximize the economy of
FGF utilisation.

Sevoflurane Controversies
Compound A is produced by degradation of
sevoflurane in the presence of soda lime or
Baralyme, is proven to be a nephrotoxic in rats.
o As such, it is not a metabolite produced by biotransformation of
sevoflurane in the body, but is rather a degradation product
generated in the anaesthesia circuit.
o Changing the composition of the absorbent by eliminating the
potassium hydroxide has reduced the formation of compound A to a
large extent.
o Eliminating the NaOH also has made it safer.
• Amsorb® plus is now available in India.

TERMINATION OF LOW FLOW
ANAESTHESIA.
 Unlike the initiation or the maintenance of the closed circuit,
termination is less controversial. There are only two recognized
methods of termination of the closed circuit.
 A. Use of high flow of gases: the circuit is opened and a high flow of gas is
used to flush out the anaesthetic agents .
 B . Use of activated charcoal.
o Activated charcoal when heated to 220oC adsorbs the potent vapours
almost completely. Hence, a charcoal-containing canister with a bypass is
placed in the circuit, towards the end of the anaesthesia, the gas is
directed through the activated charcoal canister.
o This results in the activated charcoal adsorbing the anaesthetic agent
resulting in rapid recovery and at the same time, reducing theatre
pollution. Nitrous oxide, due to its low solubility is washed off towards the
end by using 100% oxygen.

Emergence Phase
• In general, 100% O2 must be required in
facilitating the washout of the anesthetic
agent in the patient and later removing the
agent to the scavenging the system by
following high FGF.
• A charcoal filter can also be placed in the
expiratory limb, which causes a rapid decrease
in the concentration of the volatile agent.

Adjustments of FGF at different phases
of LFA
Premedication, pre-oxygenation, and induction of
sleep are conducted as per the usual practice.
Concerning the adjustment, it can be classified as
Initial High Flow: Parameters that can influence
the building up of alveolar concentration.
Sufficient denitrogenation
Rapid wash in the desired gas composition into
the breathing system
Constituting the required anesthetic
concentration

Maintenance
The stage of maintenance is marked by
The need for steady-state anesthesia often meant
as a steady alveolar concentration of respiratory
gases.
The basic uptake of anesthetic agents through the
body.
Minimal uptake of anesthetic agents by the body.
The requirement of preventing the different
mixture of gases.
Need to prevent hypoxic gas mixtures.

Flow Pattern during LFA
 During the initial stage, the flow must be a
high flow of 10 L/min for 3 min
• Pursuing the next flow of 400 ml of O2 and
600 ml of N2O in the first 20min
• and thereafter flow of O2 500ml and N2O
500ml,this state of maintenance states the 33-
44% of O2 concentrations each time.

Advantages of LFA system.
o 1.Enormous financial savings due to use of low fresh gas
flows as well as the agent.
o 2. High humidity in the system leads to fewer post
anaesthetic complications.
o 3.Maintenance of body temperature during prolonged
procedures due to conservation of heat.
o 4. Reduction in the theatre pollution.
o 5. Environmental Pollution ( Ozone depletion/green house
effect)

Disadvantages of LFA
• Inability to quickly alter the inspired concentrations.
• More attention is required
• Danger of hypercarbia.
• Uncertainty about inspired concentrations
• Faster absorbent exhaustion
• Accumulation of undesirable trace gases in the system like CO,
acetone, methane, hydrogen, ethanol, argon, nitrogen,
compound A, etc.

Undesired Gas effects
o A matter of concern remains the accumulation of trace gases resulting from the diminution
of the wash out effects, may decrease the concentration of nitrous oxide and oxygen in the
delivered mixture. That may, for instance, be the case if nitrogen accumulates because of
insufficient denitrogenation, or the argon concentration may rise as a result of the use of an
oxygen concentrator.
o Methane, exhaled physiologically by the patient, in high concentrations may compromise
anaesthetic gas monitoring, e.g. measurement of halothane concentration.
o Accumulation of acetone may prolong the emergence from anaesthesia and provoke nausea
or vomiting. However, only in the very rare cases of severely ketoacidotic patients does this
become clinically relevant.
o Recently it was revealed, that the volatile anaesthetic agents desflurane, enflurane,
isoflurane, and presumably also halothane, are liable to react with absolutely dry carbon
dioxide absorbents to generate carbon monoxide.
Subsequent recommendations have been made not to use fresh gas flows lower than 5 L
min−1 to safely avoid accidental carbon monoxide intoxication resulting from trace gas
accumulation.
o Halothane and sevoflurane, in their part, may react with carbon dioxide absorbents by
generating haloalkenes, e.g. 1, bromo-1,chloro-2,2,difluoro-ethylene, (BCDFE), or
fluoromethyl-2,2,difluoro-1,trifluoromethyl-vinyl-ether, (Compound A). Compound A is
nephrotoxic.

Monitoring for the safe performance
of LFA
• Inspiratory oxygen concentration.
• Airway pressure and minute volume.
• Anesthetic agent concentration in the circuit.
• Expiratory CO2 concentration.

Concerns about Safety in LFA
 Points of concern in providing safe
anaesthesia are:
o Hypoxia.
o Misdosage of the volatiles.
o Exhaustion of the absorbent.
o Gas volume deficiency.
o Reduced controllability.

Anesthesia Machines & low flow delivery
• The lower is the flow, the greater the difference between the fresh gas
and the gas composition within the breathing system.
• If the fresh gas flow is lower than 1 L min−1 even experienced anaesthetists
may fail to estimate precisely the gas composition within the breathing
system from the settings of the fresh gas controls.
• Furthermore, in a flow range lower than 1 L min−1 the performance of the
fine needle valves and the flow meter tubes approaches the limits of
accuracy. Older machines are not suitable for this.
• Newer Plenum vaporizers and newer machines can solve this problem
• The implementation of advanced computer technology, electronically
controlling the fresh gas supply by closed loop feedback according to
preset values, will overcome these technical problems.
• The Dräger company already offers commercially such a machine,
featuring electronic control of the anaesthetic gas composition and
volume circulating within the rebreathing system.

Summary
• Many new anesthesia machines have been designed and
developed in Europe in the past few years like the Physioflex
and Drager Zeus machines.
• The modern equipment’s has an implanted and innate algorithm for the
calculations of the uptake and later adjustments of the desired volumes
and concentrations in accordance with playing the most crucial role.
• Today there are ample varieties of gas analyzers for the monitoring
processes of FiO2, ETCO2 and agent monitoring in modern anesthesia
workstations which results in effortless and pragmatic conduct of LFA.
• Anaesthesiologists must take up LFA as their professional
commitment for the present and future generations.

References
• 1. Baum JA, Aithkenhead: Low flow Anaesthesia. Anaesthesia. 50 (suppl).: 37-44, 1995
• 2. Baker AB: Editorial. Low flow and Closed Circuits. Anaesthesia and Intensive Care. 22:
• 341-342, 1994
• 3. Mapleson W: The theoretical ideal fresh gas flow sequence at the start of low flow
• anaesthesia. Anaesthesia 53(3): 264-72, 1998
• 4. Weir HM, Kennedy RR: Infusing liquid anaesthetic agents into the closed circle
• anaesthesia. Anaesthesia and Intensive Care. 22: 376-379, 1994
• 5. Wolfson B: Closed Circuit Anaesthesia by Intermittent Injections of Halothane. British
• Journal of Anaesthesia. 34: 733 - 737., 1962
• 6. Thorpe CM, Kennedy RR: Vaporisation of Isoflurane by Liquid Infusion. Anaesthesia and
• Intensive Care. 22: 380-82, 1994
• 7. Hampton JL, Flickinger H: Closed Circuit Anesthesia utilising known increments of
• Halothane. Anesthesiology 22: 413-418, 1961
• 8. Philip JH: 'Closed Circuit Anaesthesia' in 'Anesthesia Equipment: Principles and
• Applications'. Edited by Ehrenwerth J, Eisenkraft JB, Mosby Year Book Inc., 1993, Chap 30.
• 9. Dale O, Stenqvist O: Low flow Anaesthesia : Available today - A routine tomorrow. Survey
• of Anesthesiology. 36: 334-336, 1992
• 10. Cullen SC: Who is watching the patient? Anesthesiology 37: 361-362, 1972
• 11. Baker AB: Back to Basics - A Simplified Non - Mathematical Approach to Low Flow
• Techniques in Anaesthesia. Anaesthesia and Intensive Care. 22: 394-395., 1994
• 12. Lin CY, Benson JW, Mostert DW: Closed Circle Systems - A new direction in the practice
• of Anaesthesia. Acta Anaesthesiologica Scandinavica. 24: 354-361., 1980
• 13. Lowe HJ: 'The Anesthetic Continuum' in the book, 'Low flow and closed circuit anesthesia'.
• Edited by Aldrete JA, Lowe HJ, Virtue RW, Grune & Stratton, 1979, pp 11-38
• 14. Lowe H: 'Closed- circuit anesthesia', in the book 'Clinical Anesthesiology' Edited by Morgan
• GE, Mikhail MS, Appleton and Lange, 1992, pp 112 - 115.
• 15. Da Silva CJM, Mapleson WW, Vickers MD: Quantitative study of Lowe's square root of
• time method of closed system anaesthesia. British Journal of Anaesthesia. 79: 103-112., 1997
• 16. El - Attar AM: Guided Isoflurane injection in a totally closed circuit. Anaesthesia. 46.: 1059-
• 1063., 1991.
• 17. Eger II E: “Uptake and Distribution”, in the book “Anesthesia”, Edited by Miller RD, Ed 4,
• Churchill Livingstone,1994, p118.
• 18. Bengtson J, Bengtsson J, Bengtsson A, Stenqvist O: Sampled gas need not be returned
• during low-flow anaesthesia. Journal of Clinical Monitoring 9(5): 330-4, 1993
• 19. Lin CY: Uptake of Anaesthetic Gases and Vapours. Anaesthesia and Intensive Care. 22:
• 363-373, 1994
• 20. Morita S, Latta W, Hambro K, Snider MT: Accumilation of methane, acetone and nitrogen
• in the inspired gas during closed circuit anesthesia. Anesthesia and analgesia. 64: 343-347,
• 1985
• 21. Baumgarten R: Much ado about nothing: Trace gaseous metabolites in closed circuit.
• Anesthesia and Analgesia. 64: 1029-1030, 1985

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Low flow Anesthesia system

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