anesthesia breathing circuits

8/2/2019 Anesthesia Breathing Circuits

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An in-depth review

byM Ravishankar MDProfessor and Chair of Anesthesia

JIPMERPondicherry, India

Table of contents

Introduction

Definition

Requirements of a Breathing System

Classification (Table 1)

Breathing systems without CO2 absorption

Unidirectional Flow

Bidirectional Flow

Subclassification

Afferent reservoir system

Magill system

Enclosed afferent reservoir system

Efferent reservoir system

Bain system

Combined system

Breathing systems with CO2 absorption

Circle system

Totally closed system

References

INTRODUCTION

hesia breathing circuits http://www.capnography.com/Circuits/breathingcirc

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Since the introduction of diethyl ether as an anaesthetic in 1846, the speciality of anaesthesiahas come a long way. In the initial phase, the attention was mainly diverted to administering a singleagent 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 toanaesthetise patients. Any resemblance to a breathing system was developed by Barth (1907) usinghis valve with a nitrous oxide cylinder, a reservoir bag and Clovers inhaler. Different positions of thelever in the valve allowed a range from complete rebreathing to completely breathing from theatmosphere. The development of Boyles machine (1917) and mastering of endotracheal intubationwith a soft red rubber single lumen tube by Magill and Rowbothom were the forerunners fordevelopment of a simple anaesthetic delivery system by Magill, popularly known as the Magillscircuit. Introduction of cyclopropane in 1929 and cuffed endotracheal tube in 1931; prompted Watersto develop the to and fro canister and use it for closed system anaesthesia with cyclopropane. In1936, Brian Sword introduced the circle system. The Ayres T-piece was introduced in 1937, EMOinhaler in 1941 and Minnitts gas and air apparatus with demand valve in 1949.

With the introduction of many breathing systems attempts were made to classify them in the50s and 60s, but lack of a proper definition, lead to more confusion than clarity.

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DEFINITION:

A breathing system is defined as an assembly of components which connects the patientsairway to the anaesthetic machine creating an artificial atmosphere, from and into which the patientbreathes.

It primarily consists of

a) A fresh gas entry port/delivery tube through which the gases are delivered from the machine tothe systems;

b) A port to connect it to the patients airway;

c) A reservoir for gas, in the form of a bag or a corrugated tube to meet the peak inspiratory flowrequirements;

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.

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REQUIREMENTS OF A BREATHING SYSTEM:

The components when assembled should satisfy certain requirements, some essential andothers desirable.

Essential:


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The breathing system must

a) deliver the gases from the machine to the alveoli in the same concentration as set and in theshortest 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 ofCO2 elimination and fresh gas utilization);

g) adaptability for adults, children and mechanical ventilators;

h) provision to reduce theatre pollution.

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CLASSIFICATION OF BREATHING SYSTEMS:

One will realize the reason for the failure of the attempts at classification in the 50s to 60s, ifthis definition and requirements are taken into account. There are numerous classifications ofbreathing systems according to the whims and fancy of the person classifying. Many of them areirrelevant 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

Dripps et al have classified them as Insufflation, Open, Semiopen, Semiclosed and Closed taking into

account the presence or absence of Reservoir, Rebreathing, CO2 absorption and Directional valves1

The ambiguity of the terminology used as open, semi open, semi closed and closed allowed inclusionof apparatus that are not breathing systems at all into the classification.


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To overcome this problem Conway2 suggested that a functional classification be used andclassified 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 afferentreservoir system.

A new breathing system called The Maxima4 has been designed by Miller in 1995 and to include it in

the classification5, the enclosed afferent reservoir systems have been grouped under displacementafferent reservoir systems.

Table 1. Classification of breathing systems

BREATHING SYSTEMS WITHOUT CO2ABSORPTION.

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

Lacks system.

B) Enclosed afferent reservoir systems

Millers (1988)

c) Efferent reservoir systems

Mapleson D

Mapleson E

Mapleson F

Bains system

Bi-directional flow

To and Fro system.


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d) Combined systems

Humphrey ADE

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 Humphreys6. The

classification suggested in table.1. is a partial modification of Millers3 classification.

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BREATHING SYSTEMS WITHOUT CO2 ABSORPTION

Unidirectional Flow

FIG 1. Nonrebreathing system. (Inset Nonrebreathing Valve)


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a) NONREBREATHING SYSTEMS. They use non rebreathing valves and there is no mixing offresh gas and the expired gas.

Functional analysis: When the patient takes a breath, or if the reservoir bag is squeezed, theinspiratory unidirectional valve opens and the gases flow into the patients lungs (Fig.1). Theexpiratory unidirectional valve closes the expiratory port during spontaneous breathing. Theinspiratory unidirectional valve itself closes the expiratory port during controlled ventilation. At thestart of expiration, the inspiratory unidirectional valve returns back to position and expiration takes

place through the expiratory port, opening the expiratory valve.

The fresh gas flow (FGF) should be equal to the minute ventilation (MV) of the patient. Thesesystems satisfy all four essential requirements, but are not very popular because of the followingreasons:

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 componentof the system. To use it without absorber is uneconomical as it needs a FGF more than the alveolarventilation.

The effect of arrangements of various components in the effective elimination of CO 2 and

fresh gas economy when used with high flows were analysed by Egar and Ethans7. Detaileddiscussion on this is beyond the purview of this review. However, some aspects of this is discussed

under the section, Circle system with absorber.

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Bi-Directional Flow:

Systems with bi-directional flow are extensively used. These systems depend on the FGF foreffective 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 deadspace, etc. We will analyze these in detail.

Fresh Gas Supply; Fresh gas flow (FGF) forms one of the essential requirements of abreathing system. If there is no FGF into the system, the patient will get suffocated. If the FGF islow, most systems do not eliminate carbon-dioxide effectively, and if there is an excess flow there iswastage of gas. So, it becomes imperative to specify optimum FGF for a breathing system forefficient functioning.

If the system has to deliver a set concentration in the shortest possible time to the alveoli, theFGF should be delivered as near the patients airway as possible.

Elimination Of Carbon-Dioxide: The following may be taken as an example for betterunderstanding of CO2 elimination by the bi-directional flow systems. Normal production of carbon-


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dioxide in a 70 kg adult is 200 ml per minute and it is eliminated through the lungs. Normal end-tidaconcentration of carbon-dioxide is 5%. Hence, for eliminating 200 ml of carbon-dioxide as a 5% gasmixture, the alveolar ventilation has to be:

200 x 100 = 4,000 ml.

5

This 4000 ml or 4 litres is the normal alveolar ventilation. Any breathing system connected toan adults airway should provide a minimum of 4 litres per minute of carbon-dioxide free gas to thealveoli for eliminating carbon-dioxide. If the alveolar ventilation becomes less than 4 litres per minute,it would lead to hypercarbia. If the alveoli are ventilated with 5 litres/minute of a gas containing 1%carbon-dioxide, or 8 litres/minute of a gas containing 2.5% carbon-dioxide, it could still eliminate 200ml of carbon-dioxide per minute from the alveoli. It may be construed as 4 litres of CO2 free gas and

1 litre of gas with 5% CO2 in the first instant and as 4 litres of CO2 free gas and 4 litres of gas with

5% CO2 in the second instant. In effect, 4 litres of alveolar ventilation with CO2 free gas is provided

in both cases.

Apparatus Dead Space: It is the volume of the breathing system from the patient-end to the pointup to which, to and fro movement of expired gas takes place.

FIG 2. Extent of dead space in various systems

In an afferent reservoir system with adequate FGF, the apparatus dead space extends up to theexpiratory valve positioned near the patient (fig.2).

If the FG enters the system near the patient-end as in an efferent reservoir system, the deadspace extends upto the point of FG entry. In systems where inspiratory and expiratory limbs areseparate, 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 reducedbelow the optimal level, the dead space increases and the whole system will act as dead space ifthere is no FGF. Increasing the FGF above the optimum level will only lead to wastage of FG.

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Sub-Classification Of Bi-Directional Flow Systems:

Mapleson8 did a theoretical analysis of the fresh gas requirements of the semiclosed systemsavailable at that time. It is only proper to refer to it as Mapleson systemsas he gave a nomenclature


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as A, B, C, D and E for easy identification as per their construction. For better understanding of thefunctional 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 afferent limb is that part of the breathing system which delivers the fresh gas from the machineto the patient. If the reservoir is placed in this limb as in Mapleson A, B, C and Lacks systems, theyare called afferent reservoir systems (ARS).

The efferent limb is that part of the breathing system which carries expired gas from the patient andvents it to the atmosphere through the expiratory valve/port. If the reservoir is placed in this limb asin 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.

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AFFERENT RESERVOIR (AR) SYSTEMS

The Mapleson A, B and C systems have the reservoir in the afferent limb, and do not have anefferent limb (Fig.3). Lack system has an afferent limb reservoir and an efferent limb through whichthe expired gas traverses before being vented into the atmosphere (Fig.6). This limb is coaxiallyplaced inside the afferent limb.

FIG 3. Afferent reservoir systems (Maplesons A, B and C systems)

These AR systems work efficiently during spontaneous breathing provided the expiratory valve isseparated from the reservoir bag and FGF by at least one tidal volume of the patient and apparatusdead space is minimal. They do not function efficiently during controlled ventilation. If the FGF isclose to the expiratory valve as in Mapleson B & C, the system is inefficient both during spontaneousand controlled ventilation. The efficiency is determined in terms of carbon-dioxide elimination andFGF utilization.

Mapleson8 has analysed these bi-directional flow systems using mathematical calculations. He made a


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few basic assumptions while analyzing breathing systems. These are

(1) Gases move enblock. They maintain their identity as fresh gas, dead space gas andalveolar 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 togas flow and closes at the start of the next inspiration.

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Mapleson A/Magills system:

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 intothe system and fill the reservoir bag. The expired gas, initial part of which is the dead space gaspushes 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

As soon as the reservoir bag is full, the expiratory valve opens and the alveolar gas is vented into the


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atmosphere (Fig.4c). During the expiratory pause, alveolar gas that had come into the corrugatedtube is also pushed out through the valve, depending on the FGF. The system is filled with only freshgas and dead space gas at the start of the next inspiration when FGF is equal to the alveolarventilation (Fig.4d). The entire alveolar gas and dead space gas is vented through the valve andsome FG also escapes, if the FGF is higher than the minute ventilation. Some amount of alveolar gaswill remain in the system and lead to rebreathing with a FGF less than the alveolar ventilation. This

has been confirmed theoretically and experimentally by many investigators8,9. The system functions

at maximum efficiency, when the FGF equals the alveolar ventilation and the dead space gas (whichhas not taken part in gas exchange) is allowed to be rebreathed and utilized for alveolar ventilation.

Controlled ventilation: To facilitate IPPV the expiratory valve has to be partly closed. Duringinspiration, 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 themachine 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 thealveolar gas is pushed back into the alveoli followed by the FG. When sufficient pressure isdeveloped, part of the expired gas and part of the FG escape through the valve (Fig.5c). This leadsto 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.


Lacks system:

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


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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.

FIG 6. Lacks system.

Mapleson B & C systems:

In order to reduce the rebreathing of alveolar gas and to improve the utilization of FG duringcontrolled ventilation, the FG entry was shifted near the patient(Fig.3). This allows a completemixing of FG and expired gas. The end result is that these systems are neither efficient duringspontaneous nor during controlled ventilation.

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ENCLOSED AFFERENT RESERVOIR (EAR) SYSTEMS

This has been described by Miller & Miller10. The system consisted of a Mapleson A systemenclosed within a non distensible structure (Fig.7a). It may also be constructed by enclosing thereservoir bag alone in a bottle and connecting the expiratory port to the bottle with a corrugated tubeand a one way valve (Fig.7b). To the bottle is also attached a reservoir bag and a variable orificeforproviding positive pressure ventilation.

FIG 7. Enclosed afferent reservoir system.


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Functional analysis:During spontaneous ventilation, the gas is vented from the system in amanner which is identical to the Mapleson A system. In this mode the variable orifice is kept widelyopen to allow free communication to the atmosphere. In controlled ventilation the reservoir bag B issqueezed intermittently and the variable orifice is partly closed to allow building up of pressure in thebottle. The pressure thus developed (1) closes the expiratory valve, (2) squeezes the enclosedafferent reservoir and the patient gets ventilated. The expiration takes place in a manner similar tothat described during spontaneous ventilation when the pressure is released in reservoir B,. Hencethis system should function efficiently during spontaneous and controlled ventilation with a FGFequivalent 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 thanthe alveolar ventilation in this system as shown by the above studies, it is still more efficient than theBain system for controlled ventilation.

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EFFERENT RESERVOIR (ER) SYSTEMS:

FIG 8. Efferent reservoir systems (Maplesons D, E and F)


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FIG 9. Bain system

The Mapleson D, E, F and Bain systems have a 6 mm tube as the afferent limb that supplies the FGfrom the machine. The efferent limb is a wide-bore corrugated tube to which the reservoir bag isattached and the expiratory valve is positioned near the bag. In Mapleson E system, the corrugatedtube itself acts as the reservoir (Fig.8). In Bain system, the afferent and efferent limbs are coaxiallyplaced (Fig.9).

All these ER systems are modifications of Ayres T-piece. This consists of a light metal tube 1 cm indiameter, 5 cm in length with a side arm (Fig.10). Used as such, it functions as a non-rebreathingsystem. Fresh gas enters the system through the side arm and the expired gas is vented into theatmosphere and there is no rebreathing. The dead space is minimal as it is only up to the point of FGentry 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.

FIG 10. Ayres T piece.


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In an attempt to reduce FGF requirements, ER systems are constructed with reservoirs in theefferent limb. The functioning of all these systems are similar. These systems work efficiently andeconomically for controlled ventilation as long as the FG entry and the expiratory valve are separatedby a volume equivalent to at least one tidal volume of the patient. They are not economical duringspontaneous breathing.

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Bain system (Mapleson D)

Spontaneous respiration: The breathing system should be filled with FG before connecting to thepatient. When the patient takes an inspiration, the FG from the machine, the reservoir bag and thecorrugated tube flow to the patient (Fig.11a). During expiration, there is a continuous FGF into thesystem at the patient end. The expired gas gets continuously mixed with the FG as it flows back intothe 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 reservoirbag. During the expiratory pause the FG continues to flow and fill the proximal portion of thecorrugated tube while the mixed gas is vented through the valve (Fig.11c). During the nextinspiration, 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 patients minute ventilation in order tominimise rebreathing to acceptable levels.

FIG 11Functional analysis, Bains system (Mapleson D), Spontaneous breathing.



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Controlled ventilation: To facilitate intermittent positive pressure ventilation, the expiratory valve hasto be partly closed so that it opens only after sufficient pressure has developed in the system. Whenthe system is filled with fresh gas, the patient gets ventilated with the FGF from the machine, thecorrugated tube and the reservoir bag (Fig.12a). During expiration, the expired gas continuously getsmixed with the fresh gas that is flowing into the system at the patient end. During the expiratorypause 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 thecorrugated tube i.e., a mixture of FG, alveolar gas and dead space gas (Fig.12c). As the pressure inthe system increases, the expiratory valve opens and the contents of the reservoir bag aredischarged into the atmosphere.

FIG 12. Bains system, Controlled ventilation.



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Factors that influence the composition of gas mixture in the corrugated tube with which the patientgets ventilated are the same as for spontaneous respiration namely FGF, respiratory rate, tidalvolume and pattern of ventilation. The only difference is that these parameters can be totallycontrolled by the anaesthesiologist and do not depend on the patient. Using a low respiratory ratewith a long expiratory pause and a high tidal volume, most of the FG could be utilized for alveolarventilation without wastage.

FIG 13. Relation between alveolar ventilation and FGF

Analyzing the performance of these systems during controlled ventilation, two relationships havebecome 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 inFig.13. An infinite number of combinations of FGF and minute ventilation can be chosen to achieve adesired 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 ventilatedwith at least twice the predicted minute volume i.e. 140 ml/kg. Here a deliberate controlled


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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 isimportant to remember that using a low FGF with normal minute ventilation, can lead to hypercarbia;a moderate FGF and hyperventilation, can lead to hypocarbia.

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COMBINED SYSTEMS

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 theafferent limb and the other in the efferent limb. While in use, only one reservoir will be in operationand the system can be changed from ARS to ERS by changing the position of a lever. It can be usedfor adults as well as children. The functional analysis is the same as Mapleson A in ARS mode and asBain in ERS mode. It is not yet widely used.

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BREATHING SYSTEMS WITH CO2 ABSORPTION

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 expiredCO2 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.

Circle System

The essential components of the circle system are, (1) a sodalime canister, (2) Two unidirectionavalves, (3) Fresh gas entry, (4) Y-piece to connect to the patient, (5) Reservoir bag (6) a relief valveand (7) low resistance interconnecting tubing. The arrangement of the components is shown infig.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 twounidirectional valves on either side of the reservoir bag, (2) Relief valve should be positioned in theexpiratory limb only, (3) The FGF should enter the system proximal to the inspiratory unidirectionavalve.

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 inthe expiratory limb as the expiratory unidirectional valve is closed by back pressure transmitted to thevalve. During expiration the inspiratory unidirectional valve closes and the expired gas flows throughthe expiratory unidirectional valve in the expiratory limb to the sodalime canister and to the reservoirbag. 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 whenthe FGF is more than the alveolar ventilation. To facilitate controlled ventilation the relief valve has tobe 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 ofparticular importance to the functioning of the system only when the FGF is high, the gas componentsof 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 systems components become less important

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Totally closed system:

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, theexpiratory valve is closed. The FGF is then adjusted to meet only the patients basal oxygenrequirements together with anaesthetic. A number of advantages have been demonstrated for totallyclosed 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.


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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 conserveheat. 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 anaestheticescapes, 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 apatient 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 analveolar 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 unidirectionavalves 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 thepast and now is only of historical importance. The reader may refer to any standard text book forfurtherdetails.


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References

1. Dorsch, J.A., Dorsch,S.E. Breathing systems II. In understanding anaesthesia equipment. 2nd ed1984. 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 andclassification. British Journal of Anaesthesia. 1988, 60, 555-64.

4. Miller DM. An enclosed efferent afferent reservoir system: The Maxima. Anaesthesia and intensivecare 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 ADEbreathing system and between four methods for detecting rebreathing. Anaesthesia and intensivecare 1995; 23: 296-01.

7. Eager EI., Ethans, C.T.: The effects of inflow, overflow and valve placement on economy of circlesystem. Anesthesiology, 1968, 29, 93-100.

8. Mapleson WW. The elimination of rebreathing in various semiclosed anaesthetic systems. Britishjournal of Anaesthesia; 1954;26: 323-32.

9. White, DC, Calkins, J. Anaesthetic machine and breasting apparatus. In: Nunn, Utting and Browneds. General anaesthesia. 5th ed.. Butterworths, London 1989:428-56

10. Miller, D.M., Miller, J.C.: Enclosed afferent reservoir breathing systems. Description and clinicaevaluation. British Journal of Anaesthesia. 1988, 60, 469-75.

11. Droppert PM, Meakin G, Beatty PCW, Mortimer AJ, Healy TEJ. Efficiency of a new af ferentreservoir breathing system during controlled ventilation. British Journal of Anaesthesia 1991; 66:638-42.

12. Meakin G, Jennings AD, Beatty PCW, Healy TEJ. Fresh gas requirements of an enclosed afferentreservoir breathing system during controlled ventilation in children. British Journal of Anaesthesia1992; 68: 43-47.

13. Beatty PCW, Meakin G, Healy TEJ. Fractional delivery of fresh gas: a new index of the efficiencyof 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 theCarden Ventmasta ventilator. British Journal of Anaesthesia 1993; 71: 741-46.

15. Ravishankar M, Chatterjee S. Fractional utilisation of fresh gas by breathing systems withoutcarbon dioxide absorption. British Journal of Anaesthesia 1993; 71: 706-07

16. Ward CS. In: Anaesthetic equipment. Physical principles and maintenance; W.B.Saunders


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London; 2nd ed. 1985.

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T-piece. Canadian Anaesthetists Society Journal 1979; 26: 104-13.

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