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| |
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| An in-depth review |
| by |
| M Ravishankar MD |
| Professor and Chair of Anesthesia |
| JIPMER |
| Pondicherry, India |
|
|
Requirements
of a Breathing System
Breathing
systems without CO2 absorption
Enclosed
afferent reservoir system
Since the introduction of
diethyl ether as an anaesthetic in 1846, the speciality of anaesthesia has come
a long way. In the initial phase, the attention was mainly diverted to
administering a single agent and apparatus were developed to suit the purpose.
The reintroduction of nitrous oxide in 1868, and facility to store it in
cylinders, created an interest in administering a combination of agents to
anaesthetise patients. Any resemblance to a breathing system was developed by
Barth (1907) using his valve with a nitrous oxide cylinder, a reservoir bag and
Clover’s inhaler. Different positions of the lever in the valve allowed a
range from complete rebreathing to completely breathing from the atmosphere. The
development of Boyle’s machine (1917) and mastering of endotracheal intubation
with a soft red rubber single lumen tube by Magill and Rowbothom were the
forerunners for development of a simple anaesthetic delivery system by Magill,
popularly known as the “Magill’s circuit”. Introduction of cyclopropane in
1929 and cuffed endotracheal tube in 1931; prompted Waters to develop the “to
and fro” canister and use it for closed system anaesthesia with cyclopropane.
In 1936, Brian Sword introduced the circle system. The Ayre’s T-piece was
introduced in 1937, EMO inhaler in 1941 and Minnitt’s ‘gas and air’
apparatus with demand valve in 1949.
With the introduction of
many breathing systems attempts were made to classify them in the 50’s and
60’s, but lack of a proper definition, lead to more confusion than clarity.
[Back]
A breathing system is
defined as an assembly of components which connects the patient’s airway to
the anaesthetic machine creating an artificial atmosphere, from and into which
the patient breathes.
It primarily consists of
a) A fresh gas entry
port/delivery tube through which the gases are delivered from the
machine to the systems;
b) A port to connect it to
the patient’s airway;
c) A reservoir for gas, in
the form of a bag or a corrugated tube to meet the peak inspiratory flow
requirements;
d) An expiratory port/valve
through which the expired gas is vented to the atmosphere;
e) A carbon dioxide
absorber if total rebreathing is to be allowed and
f) Corrugated tubes for
connecting these components.
Flow directing valves may or may not be used.
[Back]
The components when assembled should satisfy certain
requirements, some essential and others desirable.
Essential:
The breathing system must
a) deliver the gases from
the machine to the alveoli in the same concentration as set and in the shortest
possible time;
b) effectively eliminate
carbon-dioxide;
c) have minimal apparatus
dead space; and
d) have low resistance.
Desirable:
The desirable requirements are
a) economy of fresh gas;
b) conservation of heat;
c) adequate humidification
of inspired gas;
d) Light weight;
e) convenience during use;
f) efficiency during
spontaneous as well as controlled ventilation (Efficiency is determined in terms
of CO2 elimination and fresh gas utilization);
g) adaptability for adults, children and mechanical
ventilators;
h) provision to reduce theatre pollution.
[Back]
One will realize the reason
for the failure of the attempts at classification in the 50’s to 60’s, if
this definition and requirements are taken into account. There are numerous
classifications of breathing systems according to the whims and fancy of the
person classifying. Many of them are irrelevant as they do not define a
breathing system. Different authors classified the same system under different
headings, adding to confusion1.
McMohan in 1951 classified them as open, semiclosed and closed taking the
level of rebreathing into account. It as follows:
|
Open |
no rebreathing |
|
Semiclosed |
partial rebreathing |
|
Closed |
total rebreathing |
To overcome this problem
Conway2 suggested that a functional classification be used and
classified according to the method used for CO2 elimination as:
|
1. Breathing systems with CO2 absorber |
|
2. Breathing systems without CO2
absorber. |
Miller.D.M.3
in 1988 widened the scope of this classification so as to include the enclosed
afferent reservoir system.
Table 1. Classification of breathing systems
|
BREATHING
SYSTEMS WITHOUT CO2
ABSORPTION. |
BREATHING
SYSTEMS WITH CO2
ABSORPTION. |
|
Unidirectional
flow: a) Non rebreathing systems. B) Circle systems.
|
Unidirectional
flow Circle system with absorber.
|
|
Bi-directional
flow: a) Afferent reservoir systems. Mapleson A Mapleson B Mapleson C Lack’s system. B) Enclosed afferent reservoir systems Miller’s (1988) c) Efferent reservoir systems Mapleson D Mapleson E Mapleson F Bain’s system d) Combined systems Humphrey ADE
|
Bi-directional flow
To and Fro system.
|
This classification also has a personal bias as the Humphrey ADE system is not included in the classification, even though he preferred to compare his system with that of Humphrey’s6. The classification suggested in table.1. is a partial modification of Miller’s3 classification.
[Back]
FIG 1. Nonrebreathing system. (Inset – Nonrebreathing Valve)
|
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a)
Nonrebreathing systems.
They use non rebreathing valves and there is no mixing of fresh gas and
the expired gas.
The fresh gas flow (FGF)
should be equal to the minute ventilation (MV)
of the patient. These systems satisfy all four essential requirements, but are
not very popular because of the following reasons:
1) Fresh gas flow has to be
constantly adjusted and is not economical.
2) There is no
humidification of inspired gas.
3) There is no conservation
of heat.
4) They are not convenient
as the bulk of the valve has to be positioned near the patient.
5) The valves can
malfunction due to condensation of moisture and lead to complications.
b)
Circle Systems:
These systems are designed with a CO2 absorber
as an essential component of the system. To use it without absorber is
uneconomical as it needs a FGF more than the alveolar ventilation.
The effect of arrangements
of various components in the effective elimination of CO2 and fresh
gas economy when used with high flows were analysed by Egar and Ethans7.
Detailed discussion on this is beyond the purview of this review.
However, some aspects of this is discussed under the section, Circle
system with absorber.
[Back]
Systems with bi-directional
flow are extensively used. These systems depend on the FGF for effective
elimination of CO2. Understanding these systems is most important as
their functioning can be manipulated by changing parameters like Fresh gas flow, alveolar ventilation,
apparatus dead space, etc. We will
analyze these in detail.
Fresh Gas Supply;
Fresh gas flow (FGF) forms one of the essential requirements of a
breathing system. If there is no
FGF into the system, the patient will get suffocated.
If the FGF is low, most systems do not eliminate carbon-dioxide
effectively, and if there is an excess flow there is wastage of gas.
So, it becomes imperative to specify optimum FGF for a breathing system
for efficient functioning.
If the system has to deliver a set concentration in the shortest
possible time to the alveoli, the FGF should
be delivered as near the patient’s airway as possible.
Elimination Of Carbon-Dioxide:
The following may be taken as an example for better understanding of CO2
elimination by the bi-directional flow systems.
Normal production of carbon-dioxide in a 70 kg adult is 200 ml per minute
and it is eliminated through the lungs. Normal
end-tidal concentration of carbon-dioxide is 5%.
Hence, for eliminating 200 ml of carbon-dioxide as a 5% gas mixture, the
alveolar ventilation has to be:
200 x 100
= 4,000 ml.
5
Apparatus Dead Space:
It is the volume of the breathing system from the patient-end to the
point up to which, to and fro movement of expired gas takes place.
FIG 2. Extent of dead space in
various systems
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|
In an afferent reservoir system with adequate FGF,
the apparatus dead space extends up to the expiratory valve positioned near the
patient (fig.2).
If the FG enters the system
near the patient-end as in an efferent reservoir system, the dead space extends
upto the point of FG entry. In
systems where inspiratory and
expiratory limbs are separate, it extends upto the point of bifurcation.
The dynamic dead space will depend on the FGF and the alveolar
ventilation. The dead space is
minimal with optimal FGF. If the
FGF is reduced below the optimal level, the dead space increases and the whole
system will act as dead space if there is no FGF.
Increasing the FGF above the optimum level will only lead to wastage of
FG.
[Back]
Mapleson8 did a theoretical analysis of the fresh gas requirements of the semiclosed systems available at that time. It is only proper to refer to it as Mapleson systems as he gave a nomenclature as A, B, C, D and E for easy identification as per their construction. For better understanding of the functional analyses, they have been classified as:
|
1 |
Afferent reservoir system (ARS). |
|
2 |
Enclosed afferent reservoir systems (EARS). |
|
3 |
Efferent reservoir systems (ERS). |
|
4 |
Combined systems. |
The efferent limb is that part of the
breathing system which carries expired gas from the patient and vents it to the
atmosphere through the expiratory valve/port.
If the reservoir is placed in this limb as in Mapleson D, E, F and Bain
systems, they are called efferent reservoir systems (ERS).
Enclosed afferent reservoir system has been described by Miller and Miller.
[Back]
The Mapleson A, B and C systems have the reservoir in
the afferent limb, and do not have an efferent limb (Fig.3).
Lack system has an afferent limb reservoir and an efferent limb through which
the expired gas traverses before being vented into the atmosphere (Fig.6).
This limb is coaxially placed inside the afferent limb.
FIG 3. Afferent reservoir
systems (Mapleson’s A, B and C systems)
|
|
Mapleson8 has analysed these bi-directional flow systems using mathematical calculations. He made a few basic assumptions while analyzing breathing systems. These are
(1) Gases move enblock. They maintain their identity as fresh gas, dead space gas and alveolar gas. There is no mixing of these gases.
(2) The reservoir bag continues to fill up, without offering any resistance till it is full.
(3) The expiratory valve opens as soon as the reservoir bag is full and the pressure inside the system goes above atmospheric pressure.
(4)
The valve remains open throughout the expiratory phase without offering any
resistance to gas flow and closes at the start of the next inspiration.
[Back]
Functional analysis:
Spontaneous breathing: The system is filled with fresh
gas before connecting to the patient. When
the patient inspires, the fresh gas from the machine and the reservoir bag flows
to the patient, and as a result the reservoir bag collapses (Fig.4a).
During expiration, the FG continues to flow into the system and fill the
reservoir bag. The expired gas,
initial part of which is the dead space gas, pushes the FG from the corrugated
tube into the reservoir bag and collects inside the corrugated tube (Fig.4b).
FIG 4.Functional analysis of Mapleson A (Magill system), Spontaneous breathing.
FGF 
Dead space gas 
Alveolar gas 
|
|
Controlled ventilation: To facilitate IPPV the
expiratory valve has to be partly closed. During
inspiration, the patient gets ventilated with FG and part of the FG is vented
through the valve (Fig.5a) after sufficient pressure has
developed to open the valve. During
expiration, the FG from the machine flows into the reservoir bag and all
the expired gas (i.e., dead space gas and alveolar gas) flows back into the
corrugated tube till the system is full (Fig.5b).
During the next inspiration the alveolar gas is pushed back into the
alveoli followed by the FG. When
sufficient pressure is developed, part of the expired gas and part of the FG
escape through the valve (Fig.5c).
This leads to considerable rebreathing, as well as excessive waste of
fresh gas. Hence these systems are
inefficient for controlled ventilation.
FIG 5. Mapleson A (Magill system), Controlled ventilation.
FGF
Dead space gas
Alveolar gas
|
|
This system functions like a Mapleson A system both
during spontaneous and controlled ventilation.
The only difference is that the expired gas instead of getting vented
through the valve near the patient, is carried by an efferent tube placed
coaxially and vented through the valve placed near the machine end (Fig.6).
This facilitates easy scavenging of expired gas.
|
|
In order to reduce the
rebreathing of alveolar gas and to improve the utilization of FG during
controlled ventilation, the FG entry
was shifted near the patient(Fig.3).
This allows a complete mixing of FG and expired gas.
The end result is that these systems are neither efficient during
spontaneous nor during controlled ventilation.
[Back]
This has been described by
Miller & Miller10. The
system consisted of a Mapleson A system enclosed within a non distensible
structure (Fig.7a). It
may also be constructed by enclosing the reservoir bag alone in a bottle and
connecting the expiratory port to the bottle with a corrugated tube and a one
way valve (Fig.7b). To the bottle is also attached a reservoir bag and a variable
orifice for providing positive pressure ventilation.
FIG 7. Enclosed afferent reservoir
system.
|
|
Functional analysis:
During spontaneous ventilation, the gas is vented from the system in a manner
which is identical to the Mapleson A system.
In this mode the variable orifice
is kept widely open to allow free communication to the atmosphere. In controlled
ventilation the reservoir bag ‘B’
is squeezed intermittently and the variable
orifice is partly closed to allow building up of pressure in the bottle.
The pressure thus developed (1) closes the expiratory valve, (2) squeezes
the enclosed afferent reservoir and the patient gets ventilated. The expiration
takes place in a manner similar to that described during spontaneous ventilation
when the pressure is released in reservoir ‘B’,.
Hence this system should function efficiently during spontaneous and
controlled ventilation with a FGF equivalent to alveolar ventilation. The fresh
gas requirement and the utilization of this system has been investigated by a
group of investigators from Manchester11-13 and a group from Wales14
under the guidance of Mapleson. They have reported varying figures for
utilization as 82%, 93% and 74% respectively11,12,14.
The reasons for this lesser percentage of utilization have been quoted as
faulty methodology for calculation15, resistance offered by the
reservoir bag and tubing and early opening of the unidirectional valve during
expiration14 etc. Though the fresh gas requirement is higher than the
alveolar ventilation in this system as shown by the above studies, it is still
more efficient than the Bain system for controlled ventilation.
[Back]
FIG 8. Efferent reservoir systems (Mapleson’s D, E and F)
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The Mapleson D, E, F and Bain systems have a 6 mm tube as the afferent limb that supplies the FG from the machine. The efferent limb is a wide-bore corrugated tube to which the reservoir bag is attached and the expiratory valve is positioned near the bag. In Mapleson E system, the corrugated tube itself acts as the reservoir (Fig.8). In Bain system, the afferent and efferent limbs are coaxially placed (Fig.9).
All
these ER systems are modifications of Ayre’s T-piece. This consists of a light metal tube 1 cm in diameter, 5 cm in
length with a side arm (Fig.10).
Used as such, it functions as a non-rebreathing system.
Fresh gas enters the system through the side arm and the expired gas is
vented into the atmosphere and there is no rebreathing.
The dead space is minimal as it is only up to the point of FG entry and
elimination of CO2 is achieved by breathing into the atmosphere.
FGF equal to peak inspiratory flow rate of the patient has to be used to
prevent air dilution.
|
|
In
an attempt to reduce FGF requirements, ER systems are constructed with
reservoirs in the efferent limb. The
functioning of all these systems are similar.
These systems work efficiently and economically
for controlled ventilation as long as the FG entry and the expiratory valve are
separated by a volume equivalent to at least one tidal volume of the patient.
They are not economical during spontaneous breathing.
[Back]
Spontaneous respiration:
The breathing system should be filled with FG before connecting to the
patient. When the patient takes an inspiration, the FG from the machine, the
reservoir bag and the corrugated tube flow to the patient (Fig.11a).
During expiration, there is a continuous FGF into the system at the
patient end. The expired gas gets
continuously mixed with the FG as it flows back into the corrugated tube and the
reservoir bag (Fig.11b). Once the system is full the excess gas is vented to the
atmosphere through the valve situated at the end of the corrugated tube near the
reservoir bag. During the
expiratory pause the FG continues
to flow and fill the proximal portion of the corrugated tube while the mixed gas
is vented through the valve (Fig.11c).
During the next inspiration, the patient breaths FG as well as the mixed
gas from the corrugated tube (Fig.11d).
Many factors influence the composition of the inspired mixture.
They are FGF, respiratory rate, expiratory pause, tidal volume and CO2
production in the body. Factors
other than FGF cannot be manipulated in a spontaneously breathing patient.
It has been mathematically calculated and clinically proved8,16
that the FGF should be atleast 1.5 to 2 times the patient’s minute ventilation
in order to minimise rebreathing to acceptable levels.
FIG 11Functional analysis, Bain’s system (Mapleson D), Spontaneous breathing.
FGF
Dead space gas
Alveolar gas
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|
Controlled
ventilation:
To facilitate intermittent positive pressure ventilation, the expiratory
valve has to be partly closed so that it opens only after sufficient pressure
has developed in the system. When
the system is filled with fresh gas, the patient gets ventilated with the FGF
from the machine, the corrugated tube and the reservoir bag (Fig.12a).
During expiration, the expired gas continuously gets mixed with the fresh
gas that is flowing into the system at the patient end.
During the expiratory pause the FG continues to enter the system and
pushes the mixed gas towards the reservoir (Fig.12B).
When the next inspiration is initiated, the patient gets ventilated with
the gas in the corrugated tube i.e., a mixture of FG, alveolar gas and dead
space gas (Fig.12c).
As the pressure in the system increases, the expiratory valve opens and
the contents of the reservoir bag are discharged into the atmosphere.
FIG 12. Bain’s system, Controlled ventilation.
FGF
Dead space gas
Alveolar gas
|
|
Factors
that influence the composition of gas mixture in the corrugated tube with which
the patient gets ventilated are the same as for spontaneous respiration namely
FGF, respiratory rate, tidal volume and pattern of ventilation.
The only difference is that these parameters can be totally controlled by
the anaesthesiologist and do not depend on the patient. Using a low respiratory
rate with a long expiratory pause and a high tidal volume, most of the FG could
be utilized for alveolar ventilation without wastage.
FIG 13. Relation between alveolar ventilation and FGF
Analyzing
the performance of these systems during controlled ventilation, two
relationships have become evident. 1)
When FGF is very high the PaCO2 becomes ventilation dependent (as
during spontaneous respiration). 2)
When the minute volume exceeds the FGF substantially, the PaCO2 is
dependent on the FGF17. Combining
these influences a graph can be constructed as shown in Fig.13.
An infinite number of combinations of FGF and minute ventilation can be
chosen to achieve a desired PaCO2. One can use a high FGF and a
normal minute volume of 70 ml/kg to achieve a normal PaCO2 of 40 mm
Hg. This is uneconomical and leads
to low humidity and heat loss. Alternately,
a FGF equivalent to the predicted minute volume i.e., 70 ml/kg can be chosen and
the patient ventilated with at least twice the predicted minute volume i.e. 140
ml/kg. Here a deliberate controlled
rebreathing is allowed in order to maintain normal PaCO2 along with
high humidity, less heat loss and greater economy of fresh gas.
Combinations between these two extremes can also be used.
It is important to remember that using a low FGF with normal minute
ventilation, can lead to hypercarbia; a moderate FGF and hyperventilation, can
lead to hypocarbia.
[Back]
To over come the
difficulties of changing the breathing systems for different modes of
ventilation, Humphrey designed a system called Humphrey ADE18, with
two reservoirs, one in the afferent limb and the other in the efferent limb.
While in use, only one reservoir will be in operation and the system can
be changed from ARS to ERS by changing the position of a lever.
It can be used for adults as well as children.
The functional analysis is the same as Mapleson A in ARS mode and as Bain
in ERS mode. It is not yet widely
used.
Systems so far described have relied on FGF for
effective elimination of CO2. Any
desire to economize on FGF by allowing a total rebreathing, should be
accompanied by removal of the expired CO2 by chemical
absorption using sodalime or baralyme.
The systems designed for these purpose are again classified as:
Unidirectional flow.
-Circle system.
Bi-directional flow.
-To and fro system.
The
essential components of the circle system are, (1) a sodalime canister, (2) Two
unidirectional valves, (3) Fresh gas entry, (4) Y-piece to connect to the
patient, (5) Reservoir bag (6) a relief valve and (7) low resistance
interconnecting tubing. The
arrangement of the components is shown in fig.14.
FIG 14. Components of
circle system
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|
For
efficient
functioning of the system the following criteria should fulfilled.
(1) There should be two unidirectional valves on either side of the
reservoir bag, (2) Relief valve should be positioned in the expiratory limb
only, (3) The FGF should enter the system proximal to the inspiratory
unidirectional valve.
Functional analysis:
During inspiration the FG along with the CO2 free gas in the
reservoir bag flow through the inspiratory limb and inspiratory unidirectional
valve to the patient. No flow takes
place in the expiratory limb as the expiratory unidirectional valve is closed by
back pressure transmitted to the valve. During expiration the inspiratory unidirectional valve closes
and the expired gas flows through the expiratory unidirectional valve in the
expiratory limb to the sodalime canister and to the reservoir bag.
The CO2 is absorbed in the canister.
The FGF from the machine continues to fill the reservoir bag.
When the reservoir is full the relief valve opens and the excess gas is
vented to atmosphere. By selecting
a suitable position for the relief valve, the expired gas can be selectively
vented when the FGF is more than the alveolar ventilation. To facilitate
controlled ventilation the relief valve has to be partly closed and the excess
gas is vented during inspiration. The gas flow pattern is similar to that
described above.
The advantages and
disadvantages of the various arrangements of the components were analyzed by
Eger and Ethans7. The
relative positions of the components of the circle system are of particular
importance to the functioning of the system only when the FGF is high, the gas
components of the system unmixed and CO2 absorber not used.
When the FGF is reduced below the alveolar ventilation, the CO2
absorber is a must as the gas in the system become more uniformly mixed, and the
relative position of the system’s components become less important
[Back]
The systems with CO2
absorption can be used in a completely closed mode. After a period of approximately 10-20 minutes breathing with
high inflow of fresh gas for denitrogenation, the expiratory valve is closed.
The FGF is then adjusted to meet only the patients basal oxygen
requirements together with anaesthetic. A
number of advantages have been demonstrated for totally closed systems.
A) Economy:
The FGF could be reduced to as low as 250 - 500 ml of oxygen.
The consumption of Halothane/Isoflurane has been found to be around 3.5
ml/hour19.
b) Humidification:
In the completely closed system, once the equilibrium has been
established, the inspired gas will be fully saturated with water vapour20.
C) Reduction of heat loss:
In addition to conserving water the totally closed system will also
conserve heat. The CO2 absorption is an exothermic reaction and the
system may actively assist in maintaining body temperature.
D) Reduction in atmospheric
pollution: Once the expiratory
valve has been closed, no anaesthetic escapes, except for the small percutaneous
loss from the patient.
E) Control of anaesthesia:
It is possible to compute the time course of uptake of anaesthetic in a
patient of known size and add the appropriate quantity of the anaesthetic to the
circuit at a rate decreasing in a manner calculated to maintain a constant
alveolar concentration21. In
practice an alveolar concentration of about 1.3 x MAC is found to be suitable.
The technique has several potential disadvantages.
i) A greater knowledge of
uptake and distribution is required to master closed circuit anaesthesia.
ii) Inability to alter any
concentration quickly.
iii) Real danger of
hypercapnia may result from, a) an inactive absorber, B) incompetent
unidirectional valves and c) incorrect use of absorber bypass.
Bi-directional flow systems
The Waters to and fro system is valveless and
conveniently portable. It has been widely used in the past and now is only of
historical importance. The reader
may refer to any standard text book for furtherdetails.
[Back]
1. Dorsch, J.A., Dorsch,S.E.
Breathing systems II. In
understanding anaesthesia equipment. 2nd
ed. 1984. Williams & Wilkins.
2. Conway, C.M.:
Anaesthetic breathing systems. British Journal of Anaesthesia. 1985, 57,
649-57.
3. Miller, D.M.: Breathing
systems for use in anaesthesia. Evaluation
using a physical lung model and classification.
British Journal of Anaesthesia. 1988, 60, 555-64.
4. Miller DM. An enclosed
efferent afferent reservoir system: The Maxima. Anaesthesia and intensive care
1995; 23:284-91.
5. Miller DM. Breathing
systems reclassified. Anaesthesia and intensive care 1995; 23: 281-83.
6. Miller DM, Palm A.
Comparison in spontaneous ventilation of the Maxima with the Humphrey ADE
breathing system and between four methods for detecting rebreathing. Anaesthesia
and intensive care 1995; 23: 296-01.
7. Eager EI., Ethans, C.T.:
The effects of inflow, overflow and valve placement
on economy of circle system. Anesthesiology,
1968, 29, 93-100.
8. Mapleson WW. The
elimination of rebreathing in various semiclosed anaesthetic systems. British
journal of Anaesthesia; 1954;26: 323-32.
9. White, DC, Calkins, J.
Anaesthetic machine and breasting apparatus.
In: Nunn, Utting and Brown eds. General anaesthesia.
5th ed.. Butterworths, London 1989:428-56
10.
Miller, D.M., Miller, J.C.: Enclosed
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11. Droppert PM, Meakin G,
Beatty PCW, Mortimer AJ, Healy TEJ. Efficiency of a new afferent reservoir
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1991; 66: 638-42.
12. Meakin G, Jennings AD,
Beatty PCW, Healy TEJ. Fresh gas requirements of an enclosed afferent reservoir
breathing system during controlled
ventilation in children. British Journal of Anaesthesia 1992; 68: 43-47.
13. Beatty PCW, Meakin G,
Healy TEJ. Fractional delivery of fresh gas: a new index of the efficiency of
semiclosed breathing systems. British Journal of Anaesthesia 1992; 68: 474-77.
14. Tham EJ, Davies R,
Slade JM, Mapleson WW. Efficiency of breathing systems A and D in the Carden
Ventmasta ventilator. British Journal of Anaesthesia 1993; 71: 741-46.
15. Ravishankar M,
Chatterjee S. Fractional utilisation of fresh gas by breathing systems without
carbon dioxide absorption. British Journal of Anaesthesia 1993; 71: 706-07
16. Ward CS. In:
Anaesthetic equipment. Physical
principles and maintenance; W.B.Saunders, London; 2nd ed. 1985.
17. Rose DK, Froese AB. The
regulation of PaCO2 during controlled ventilation of children with a
T-piece. Canadian Anaesthetists Society Journal 1979; 26: 104-13.
18. Humphrey D. A new
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Anaesthesia 1983; 38: 361-72
19. Baum JA, Aitkenhead AR.
Low flow anaesthesia. Anaesthesia 1995; 50(supplement):37-44.
20. Kleemann, P.P. Humidity
of anaesthetic gases with respect to low flow anaesthesia.
Anaesthesia and Intensive Care, 1994, 22 (4), 396-408.
21. Lowe HJ, Ernst EA. The
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