Breathings Systems

7/30/2019 Breathings Systems

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PENDAHULUAN

The function of breathing is to maintain a supply of oxygen to the lungs for the blood to transport

to the tissues and to remove carbon dioxide from the body. The delivery systems which conduct

anaesthetic gases from an anaesthetic machine to the patient are known as the breathing systems

or circuits. A breathing circuit must enable a patient to breathe satisfactorily without significantly

increasing the work of breathing or the physiological deadspace. It must also conduct

inhalational anaesthetic agents to the patient. The volume of gas inspired and expired with each

breath is the tidal volume (normally 6-10mls/kg), the total volume breathed in a minute is the

minute volume and the volume of gas in the lungs at the end of normal expiration is the

Functional Residual Capacity (FRC).

The concentration of carbon dioxide in an exhaled breath varies with time; the first portion

contains no carbon dioxide and comes from the upper respiratory tract where no gas exchange

takes place (the anatomical dead space - 2mls/kg). The concentration of carbon dioxide then rises

rapidly to a plateau of about 5% as alveolar gas is breathed out. The volume of alveolar gas

expired per minute is called the alveolar minute ventilation. The anatomical dead space is 25-

35% of each tidal volume. Any areas of lung that are ventilated with gas but are not perfused by

blood cannot take part in gas exchange and represent the alveolar dead space. The total dead

space in the patient is the physiological dead space.

The term rebreathing implies that expired alveolar gas containing 5% carbon dioxide (and less

oxygen than normal) is inspired as part of the next tidal volume. Anaesthetic circuits are

designed to minimise this occuring as it may lead to serious elevations in blood CO2 levels. The

amount of rebreathing that occurs with any particular anaesthetic breathing system depends on

four factors; the design of the individual breathing circuit, the mode of ventilation (spontaneous

or controlled), the fresh gas flow rate and the patient's respiratory pattern. Circuits may eliminate

rebreathing either by ensuring an adequate flow of fresh gas which flushes the circuit clear ofalveolar gas, or, in the case of a circle system by the use of sodalime which absorbs the CO2 so

that low fresh gas flows may be used. For each of the circuits described below, fresh gas flow

rates that will ensure minimal rebreathing will be suggested.


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They are designed to allow either spontaneous respiration or intermittent positive pressure

ventilation (IPPV) and consist of a reservoir bag, anaesthetic tubing, and a pressure relief valve.

A number of mechanical ventilators include a specific breathing system eg the Manley series.

Other ventilators have been designed to operate with existing breathing systems e.g. the Penlon

Nuffield 200.

DEFINITION:

A breathing system is defined as an assembly of components which connects the patients

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 patients 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.

Properties of the ideal

breathing system

1. Simple and safe to use.

2. Delivers the intended inspired gas


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mixture.

3. Permits spontaneous, manual and

controlled ventilation in all age

groups.

4. Efficient, requiring low fresh gas

flow rates.

5. Protects the patient from

barotrauma.

6. Sturdy, compact and lightweight

in design.

7. Permits the easy removal of waste

exhaled gases.

8. Easy to maintain with minimal

running costs.

REQUIREMENTS OF A BREATHING SYSTEM

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:


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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.

Components of the

breathing systemsADJUSTABLE PRESSURE LIMITING

(APL) VALVE

A valve which allows the exhaled

gases and excess fresh gas flow to

leave the breathing system (Fig. 4.2).

It does not allow room air to enter the

breathing system. Synonymous terms

for the APL valve are expiratory

valve, spill valve and relief valve.


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Fig. 4.1 Mapleson classification of anaesthetic breathing systems. The arrow indicates

entry of fresh gas to the system (reproduced with permission from Aitkenhead R and

Smith G; Textbook of Anaesthesia 3rd edition, 1996, Churchill Livingstone).


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Fig. 4.2 Diagram of an adjustable

pressure-limiting (APL) valve (reproducedwith permission from Aitkenhead R and

Smith G; Textbook of Anaesthesia 3rd

edition, 1996, Churchill Livingstone).

Components


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1. Three ports: the inlet, the patient

and the exhaust ports. The latter

can be open to the atmosphere or

connected to the scavenging

system using a shroud.

2. A lightweight disc rests on a knifeedge

seating. The disc is held onto

its seating by a spring. The tension

in the spring, and therefore the

valves opening pressure, are

controlled by the valve dial.

Mechanism of action

1. This is a one-way, adjustable,

springloaded valve. The spring is

used to adjust the pressure

required to open the valve. The

disc rests on a knife-edge seating

in order to minimize its area of

contact.

2. The valve allows gases to escape

when the pressure in the breathing

system exceeds the valves opening

pressure.

3. During spontaneous ventilation,

the patient generates a positive

pressure in the system during

expiration, causing the valve to

open. A pressure of less than

1 cmH2O (0.1 kPa) is needed to

actuate the valve when it is in the

open position.


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4. During positive pressure

ventilation, a controlled leak is

produced by adjusting the valve

dial during inspiration. This

allows control of the patients

airway pressure.

Problems in practice and safety

features

1. Malfunction of the scavenging

system may cause excessive

negative pressure. This can lead to

the APL valve remaining open

throughout respiration. This leads

to an unwanted enormous

increase in the breathing systems

dead space.

2. The patient may be exposed to

excessive positive pressure if the

valve is closed during assisted

ventilation. A pressure relief safety

mechanism actuated at a pressure

of about 60 cmH2O is present in

some designs (Fig. 4.3).

3. Water vapour in exhaled gas may

condense on the valve. The surface

tension of the condensed water

may cause the valve to stick. The

disc is usually made of a

hydrophobic (water repelling)

material, which prevents water

condensing on the disc.


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Adjustable pressure limiting valve

(APL)

n One-way springloaded valve

with three ports.

n The spring adjusts the pressure

required to open the valve.

RESERVOIR BAG

The reservoir bag is an important

component of most breathing

systems.

Components

1. It is made of antistatic rubber or

plastic. Latex free versions also

exist. Designs tend to be

ellipsoidal in shape.

2. The standard adult size is 2 litres.

The smallest size for paediatric use

is 0.5 litre. Volumes from 0.5 to 6

litres exist. Bigger size reservoir

bags are useful during inhalational

induction, e.g. adult induction

with sevoflurane.

Mechanism of action

1. It accommodates the fresh gas

flow during expiration acting as a

reservoir available for the

following inspiration.

Components of the breathing systems

41


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Fig. 4.3 Intersurgical APL valve. In the open position (left) the valve is actuated by pressures of

less than 0.1 kPa (1 cmH2O) with

minimal resistance to flow. A 3/4 clockwise turn of the dial takes the valve through a range of

pressure-limiting positions to the closed

position (centre). In the closed position, the breathing system pressure, and therefore theintrapulmonary pressure, is protected by a

pressure relief mechanism (right) actuated at 6 kPa (60 cmH2O). This safety relief mechanism

cannot be overridden.

High pressure

spring

Green

valve

Gas flow

Light pressure

spring

Scavenged gas

Gas flow Gas flow

Scavenged gas

2. It acts as a monitor of the patients

ventilatory pattern during

spontaneous breathing. It serves

as a very inaccurate guide to the

patients tidal volume.

3. It can be used to assist or control


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ventilation.

4. When employed in conjunction

with the T-piece (Mapleson F), a

0.5 litre double-ended bag is used.

The distal hole acts as an

expiratory port (Fig. 4.4).


features

1. Because of its compliance, the

reservoir bag can accommodate

rises in pressure in the breathing

system better than other parts.

When grossly overinflated, the

rubber reservoir bag can limit the

pressure in the breathing system to

about 40 cmH2O. This is due to

the law of Laplace dictating that

the pressure (P) will fall as the

bags radius (r) increases

{P = 2 tension/r}.

2. The size of the bag depends on the

breathing system and the patient.

A small bag may not be large

enough to provide a sufficient

reservoir for a large tidal volume.

3. Too large a reservoir bag makes it

difficult for it to act as a

respiratory monitor.

Reservoir bag

n Made of rubber or plastic.

n 2 litre size commonly used for


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adults. Bigger sizes can be used

for inhalational induction in

adults.

n Accommodates fresh gas flow.

n Can assist or control

ventilation.

n Limits pressure build-up in the

breathing system.

TUBING

Specific configurations are described

below.


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Fig. 4.4 A 0.5 litre double-ended

reservior.

Classification of breathing systems

One will realize the reason for the failure of the attempts at classification in the 50s to

60s, 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


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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 inclusion of apparatus that are not breathing systems at all into the classification.

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 and

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.

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

displacement afferent reservoir systems.

BREATHING SYSTEMS WITHOUT

CO2 ABSORPTION.

I 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

Bi-directional flow

To and Fro system.


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Mapleson C

Lacks system.

B) Enclosed afferent reservoir systems

Millers (1988)

c) Efferent reservoir systems

Mapleson D

Mapleson E

Mapleson F

Bains system

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.

BREATHING SYSTEMS WITHOUT CO2 ABSORPTION

Unidirectional F low


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a) NONREBREATHING SYSTEMS. They use non rebreathing valves and there is no mixing

of fresh gas and the expired gas.

Functional analysis:When the patient takes a breath, or if the reservoir bag is squeezed, the

inspiratory unidirectional valve opens and the gases flow into the patients lungs(Fig.1). The

expiratory unidirectional valve closes the expiratory port during spontaneous breathing. The

inspiratory unidirectional valve itself closes the expiratory port during controlled ventilation. At

the start 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.

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.


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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.

Bi-Di rectional F low:

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 patients 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 ofcarbon-dioxide as a 5% gas mixture, the alveolar ventilation has to be:

200 x 100 = 4,000 ml.

5


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This 4000 ml or 4 litres is the normal alveolar ventilation. Any breathing system

connected to an adults airway should provide a minimum of 4 litres per minute of carbon-

dioxide free gas to the alveoli 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 200 ml 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

point up to which, to and fro movement of expired gas takes place.

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


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as dead space if there is no FGF. Increasing the FGF above the optimum level will only lead to

wastage of FG.

Sub-Classif ication Of Bi -Di rectional F low Systems:

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 afferent limb is that part of the breathing system which delivers the fresh gas from the

machine to the patient. If the reservoir is placed in this limb as in Mapleson A, B, C and Lacks

systems, they are called afferent reservoir systems (ARS).

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.

AFFERENT RESERVOIR (AR) SYSTEMS

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.4). This limb is

coaxially placed inside the afferent limb.


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These AR systems work efficiently during spontaneous breathing provided the expiratory valve is

separated from the reservoir bag and FGF by at least one tidal volume of the patient and

apparatus dead space is minimal. They do not function efficiently during controlled ventilation.

If the FGF is close to the expiratory valve as in Mapleson B & C, the system is inefficient both

during spontaneous and controlled ventilation. The efficiency is determined in terms of carbon-

dioxide elimination and FGF utilization.

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 enbloc. 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.

Mapleson A/Magills system:

Functional analysis:


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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.5a). 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.5b).

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

the atmosphere (Fig.5c). During the expiratory pause, alveolar gas that had come into the

corrugated tube is also pushed out through the valve, depending on the FGF. The system is filled

with only fresh gas and dead space gas at the start of the next inspiration when FGF is equal to

the alveolar ventilation (Fig.5d). The entire alveolar gas and dead space gas is vented through the

valve and some FG also escapes, if the FGF is higher than the minute ventilation. Some amount

of alveolar gas will 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 (which has not taken part in gas exchange) is allowed to be

rebreathed and utilized for alveolar ventilation.


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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.6a) 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.6b). 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.6c). This leads to considerable rebreathing, as well as excessive waste of fresh gas. Hence

these systems are inefficient for 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 patient, is carried by an efferent tube placed coaxially and vented through the

valve placed near the machine end (Fig.4). This facilitates easy scavenging of expired gas.


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Mapleson B & C systems:

In order to reduce the rebreathing of alveolar gas and to improve the utilization of FGduring 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.

ENCLOSED AFFERENT RESERVOIR (EAR) SYSTEMS

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.


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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.


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EFFERENT RESERVOIR (ER) SYSTEMS:

Bain system

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 Ayres 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.


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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 atleast one tidal volume of the patient. They are not

economical during spontaneous breathing.

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 asit 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 patients minute ventilation in order to minimise rebreathing to acceptable levels.


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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 (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.


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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.

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


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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.

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

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.

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

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. For efficient


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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


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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

Total ly 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, 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.


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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.

A number of classifications exist and the one introduced in 1954 by Professor W W Mapleson is

most commonly used in the UK (Figure 1). It does not however, include systems with carbon

dioxide absorption.

The Mapleson A (Magill) system was designed by Sir Ivan Magill in the 1930's and remains an

excellent system for spontaneous ventilation (Figure 2). Fresh gas enters the system at the fresh

gas outlet of the anaesthesia machine. The expiratory valve (Heidbrink valve) is very close to the

patient to reduce the dead space.
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The respiratory cycle has three phases during spontaneous breathing; inspiration, expiration and

the expiratory pause. During inspiration gas is inhaled from the 2 litre reservoir (breathing) bag

which partially collapses giving a visual confirmation that breathing is occurring.

This breathing system is popular and

widely used in the UK.

Components

1. Corrugated rubber or plastic

tubing (usually 110180 cm in

length).

2. A reservoir bag mounted at the

machine end.

3. An APL valve situated at the

patient end.

Mechanism of action

1. As the patient exhales (Fig. 4.5C),

initially the gases from the

anatomical dead space are

channelled through the tubing

back towards the reservoir bag

which is filled continuously with

fresh gas flow.


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2. Pressure builds up opening the

APL valve and expelling the

alveolar gases first (Fig. 4.5D). By

that time the patient inspires again

(Fig. 4.5B) getting a mixture of

fresh gas flow and the rebreathed

anatomical dead space gases.

3. It is a very efficient system for

spontaneous breathing. Because

there is no gas exchange in the

anatomical dead space, the fresh

gas flow requirements to prevent

rebreathing of alveolar gases is

theoretically equal to the patients

alveolar minute volume (about 70

ml/kg/min).

4. The Magill system is not an

efficient system for controlled

ventilation. A fresh gas flow rate

of three times the alveolar minute

volume is required to prevent

rebreathing.


features

1. It is not suitable for use with

children of less than 25 to 30 kg

body weight. This is because of

the increased dead space caused

by the systems geometry at the

patient end. Dead space is further

increased by the angle piece and


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face mask.

2. One of its disadvantages is the

heaviness of the APL valve at the

patients end, especially if

connected to a scavenging system.

This places a lot of drag on the

connections at the patient end.


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Fig. 4.5 Mechanism of action of the Magill breathing system during spontaneous

ventilation; see text for details (FGF = fresh gas flow) (reproduced with permission from

Aitkenhead R and Smith G; Textbook of Anaesthesia 3rd edition, 1996, Churchill

Livingstone)

Fig. 4.6 (A) The coaxial Lack breathing system. (B) The parallel Lack breathing system.

During expiration the bag and tubing are initially refilled with a combination of exhaled dead

space gas (containing no carbon dioxide) and fresh gas flowing from the anaesthetic machine.

Once the bag is full the pressure within the breathing system rises and the expiratory valve near

the patient opens allowing the alveolar gas (containing carbon dioxide) to be vented from the

system. During the expiratory pause more fresh gas enters the system driving any remaining

alveolar gas back along the corrugated tubing and out through the valve. If the fresh gas flow is

sufficiently high all the alveolar gas is vented from the circuit before the next inspiration and no

rebreathing will take place. With careful adjustment the fresh gas flow can be reduced until there


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is only fresh gas and dead space gas in the breathing system at the start of inspiration. When the

system is functioning correctly, without any leaks, a fresh gas flow (FGF) equal to the patients

alveolar minute ventilation is sufficient to prevent rebreathing. In practice however, a FGF closer

to the patients total minute ventilation (including dead space) is usually selected to provide a

margin of safety. An adult's minute volume is approximately 80mls/kg /min and thus for a 75kg

man a FGF of 6 litres per minute will prevent rebreathing. This is an efficient system for

spontaneously breathing patients if carbon dioxide absorption is not available.

During controlled ventilation the Magill circuit works in a different way and becomes wasteful

and inefficient, requiring high fresh gas flows to prevent rebreathing. The inspiratory force is

provided by the anaesthetist squeezing the reservoir bag after partly closing the expiratory valve

next to the patient. During lung inflation some of the gas is vented from the circuit and at the endof inspiration the reservoir bag is less than half full. During expiration, dead space and alveolar

gas pass down the corrugated tubing and may reach the bag which will then contain some carbon

dioxide. During the next inspiration when the bag is compressed alveolar gas re-enters the

patients lungs followed by a mixture of fresh, dead space and alveolar gas. A FGF of two and a

half times the patient's minute volume is required to vent enough alveolar gas to minimise

rebreathing (FGF of about 12-15 litres /min) which is obviously very inefficient. In practice the

Magill circuit should not be used for positive pressure ventilation except for short periods of a

few minutes at a time.

Modifications of the Mapleson A system

A simple modification of the Mapleson A circuit is required to make it more efficient for

controlled ventilation. This is achieved by substituting a non-rebreathing valve (such as an Ambu

E valve) for the Heidbrink valve at the patient end of the circuit. Not only does this arrangement

prevent rebreathing, but during manual ventilation the delivered minute volume will be the same

as the desired FGF which should be set at the rotameters. It is, however, a dangerous

arrangement for spontaneous respiration because the valve may jam if the fresh gas flow is

greater that the patient's minute volume.

The Lack circuit


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A disadvantage of the Magill system is that the expiratory valve is attached close to the patient

making it awkward to use (particularly when a scavenging circuit is added). The Lack circuit

(Figure 3) is a Mapleson A system in which the exhaled gases travel down a central tube located

within an outer corrugated tube towards the expiratory valve (co-axial system).

The inner tubing is wide enough to prevent an increase in the work of breathing and the

expiratory valve is placed next to the reservoir bag, by the common gas outlet. The fresh gas

flows required for both spontaneous and controlled ventilation are as described for the standard

Mapleson A system.

Components

1. 1.8 m length coaxial tubing (tube

inside a tube). The fresh gas flows

through the outside tube, and the

exhaled gases flow through the

inside tube (Fig. 4.6A).

2. The inside tube is wide in

diameter(14 mm) to reduceresistance to expiration. The outer

tubes diameter is 30 mm.

3. The reservoir bag is mounted at

the machine end.

4. The APL valve is mounted at the
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machine end eliminating the drag

on the connections at the patient

end, which is a problem with the

Magill system.

Mechanism of action

1. The mechanism is similar to that

of the Magill system except the

Lack system is a coaxial version.

The fresh gas flows through the

outside tube whereas the exhaled

gases flow through the inside tube.

2. A fresh gas flow rate of about 70

ml/kg/min is required in order to

prevent rebreathing. This makes it

an efficient breathing system for

spontaneous ventilation.

3. Since it is based on the Magill

system, it is not suitable for

controlled ventilation.

4. Instead of the coaxial design, a

parallel tubing version of the

system exists (Fig. 4.6B). This has

separate inspiratory and

expiratory tubing and retains the

same flow characteristics as the

coaxial version.


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Fig. 4.7 The Bain breathing system (reproduced with permission from Aitkenhead R

and Smith G; Textbook of Anaesthesia 3rd edition, 1996, Churchill Livingstone).

The Mapleson B and C breathing systems (Figure 1) are similar in construction, with the fresh

gas flow entry and the expiratory valves located at the patient end of the circuit. They are not

commonly used in anaesthetic practice, although the C system is used on intensive care units.

High flows of gases are needed to prevent rebreathing of CO2 and this system was at one time

combined with a canister of sodalime to absorb CO2 (Waters' "To and Fro" Circuit). However the

cannister proved too bulky for practical use and there was a risk of the patient inhaling soda lime

dust.

Components

1. A reservoir bag. In the B system

corrugated tubing is attached to

the bag, and this also acts as a

reservoir.

2. An APL valve at the patients end.

3. Fresh gas flow is added just

proximal to the APL.

Mechanism of action

1. Both systems are not efficient
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during spontaneous ventilation. A

fresh gas flow of 1.5 to 2 times the

minute volume is required to

prevent rebreathing.

2. During controlled ventilation, the

B system is more efficient because

of the corrugated tubing acting as

a reservoir. A fresh gas flow of

more than 50% of the minute

ventilation is still required to

prevent rebreathing.

Bain system

(Mapleson D)

The Bain system is a coaxial version

of the Mapleson D System (Fig. 4.7).

It is lightweight and compact at the

patient end. It is useful where access

to the patient is limited, such as

during head and neck surgery.

A Manley ventilator which has

been switched to spontaneous

ventilation mode is an example of a

non-coaxial Mapleson D system.

Components

1. A length of coaxial tubing (tube

inside a tube). The usual length is

180 cm but it can be supplied at

270 cm (for dental or ophthalmic

surgery) and 540 cm (for MRI

scans where the anaesthetic

machine needs to be kept outside


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the scanners magnetic field).

Increasing the length of the tubing

does not affect the physical

properties of the breathing system.

2. The fresh gas flows through the

inner tube while the exhaled gases

flow through the outside tube. The

internal lumen has a swivel mount

at the patients end. This ensures

that the internal tube cannot kink,

so ensuring delivery of fresh gas to

the patient.

3. The reservoir bag is mounted at

the machine end.

4. The APL valve is mounted at the

machine end.

Mechanism of action


the patients exhaled gases are

channelled back to the reservoir

bag and become mixed with fresh

gas (Fig. 4.8B). Pressure build-up

within the system will open the

APL valve allowing the venting of

the mixture of the exhaled gases

and fresh gas (Fig. 4.8C).

2. The fresh gas flow required to

prevent rebreathing (as seen in

Fig. 4.8D) during spontaneous

ventilation is about 1.5 to 2 times

the alveolar minute volume. A


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flow rate of 150 to 200 ml/kg/min

is required. This makes it an

inefficient and uneconomical

system for use during spontaneous

ventilation.

3. It is a more efficient system for

controlled ventilation. A flow of

70 to 100 ml/kg/min will maintain

normocapnia. A flow of 100

ml/kg/min will cause moderate

hypocapnia during controlled

ventilation.

4. Connection to a ventilator is

possible (Fig. 4.9). By removing

the reservoir bag, a ventilator such

as the Penlon Nuffield 200 can be

connected to the bag mount using

a 1 metre length of corrugated

tubing (the volume of tubing must

exceed 500 ml if the driving gas

from the ventilator is not to enter

the breathing system). The APL

valve must be fully closed. A

newly designed modification

incorporates a switch knob at the

valve end to switch between

spontaneous and controlled

ventilation. This allows the

ventilator to be connected to the

breathing system all the time.

5. A parallel version of the D system


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is also available.


features

1. The internal tube can kink

preventing fresh gas from being

delivered to the patient.

2. The internal tube can become

disconnected at the machine end

causing a large increase in the

dead space, resulting in

hypoxaemia and hypercapnia.

Movement of the reservoir bag

during spontaneous ventilation is

not therefore an indication that

the fresh gas is being delivered to

the patient.

Fig. 4.8 Mechanism of action of the Mapleson D breathing system during

spontaneous ventilation (reproduced with permission from Textbook of Anaesthesia 2nd


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edn, Aitkenhead and Smith 1990, Churchill Livingstone).

Fig. 4.9 The Bain breathing system connected to a ventilator (e.g. Penlon Nuffield

200) via tubing connected to the bag mount (reproduced with permission from Textbook

of Anaesthesia 2nd edn, Aitkenhead and Smith 1990, Churchill Livingstone).

T-piece system

(Mapleson E and F)

This is a valveless breathing system

used in anaesthesia for children up to

25 to 30 kg body weight (Fig. 4.10).

It is suitable for both spontaneous

and controlled ventilation.Components

1. A T-shaped tubing with three open

ports (Fig. 4.11).

2. Fresh gas from the anaesthetic

machine is delivered via a tube to


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one port.

3. The second port leads to the

patients mask or tracheal tube.

The connection should be as short

as possible to reduce dead space.

4. The third port leads to reservoir

tubing. Jackson-Rees added a

double-ended bag to the end of the

reservoir tubing (making it

Mapleson F).

5. A recent modification exists where

an APL valve is included before a

closed-ended 500 ml reservoir

bag. A pressure relief safety

mechanism in the APL valve is

actuated at a pressure of

30 cmH2O (Fig. 4.12). This design

allows effective scavenging.

Mechanism of action

1. The system requires a fresh gas

flow of 2.5 to 3 times the minute

volume to prevent rebreathing

with a minimal flow of 4 litre/min.

2. The double-ended bag acts as a

visual monitor during

spontaneous ventilation. In

addition, the bag can be used for

assisted or controlled ventilation.

3. The bag can provide a degree of

CPAP during spontaneous

ventilation.


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4. Controlled ventilation is

performed either by manual

squeezing of the double-ended bag

(intermittent occlusion of the

reservoir tubing in the Mapleson

E) or by removing the bag and

connecting the reservoir tubing to

a ventilator such as the Penlon

Nuffield 200.

5. The volume of the reservoir tubing

determines the degree of

rebreathing (too large a tube) or

entrainment of ambient air (too

small a tube). The volume of the

reservoir tubing should

approximate to the patients tidal

volume.


features

1. Since there is no APL valve used in

the standard breathing system,

scavenging can be a problem.

2. Patients under 6 years of age have

a low functional residual capacity

(FRC). Mapleson E was designed

before the advantages of CPAP

were recognised for increasing the

FRC. This problem can be

partially overcome in the

Mapleson F with the addition of

the double-ended bag.


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Fig. 4.10 A T-piece breathing system.

Fig. 4.11 Mechanism of action of the Tpiece


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breathing system.

Fig. 4.12 Intersurgical T-piece incorporating an APL valve and closed reservoir bag to

enable effective scavenging.

The Mapleson D, E and F systems are all functionally similar(Figure 1).

They act as T pieces with the FGF delivered to the patient end of the circuit and differ only in the

presence of valves or breathing bags at the expiratory end of the circuit. These systems are all

inefficient for spontaneous respiration (Figure 4).
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During expiration exhaled gas and fresh gas mix in the corrugated tubing and travel towards the

reservoir bag. When the bag is full the pressure in the system rises and the expiratory valve

opens venting to the atmosphere a mixture of fresh and exhaled gas. During the expiratory pause

fresh gas continues to push exhaled alveolar gas down the tubing towards the valve. However,

unless the FGF is at least twice the patient's minute volume, rebreathing of alveolar gas will

occur. A FGF of at least 8-10 litres/min (150mls/kg/min) is required to prevent rebreathing in an

adult.

When used for controlled ventilation the Mapleson D system functions more efficiently. During

expiration the corrugated tubing and reservoir bag fill with a mixture of fresh and exhaled gas.

Fresh gas fills the distal part of the corrugated tube during the expiratory pause prior to

inspiration. When the bag is compressed this fresh gas enters the lungs and when the expiratory

valve opens a mixture of fresh and exhaled gas is vented. The degree of rebreathing that occurs

depends on the FGF. A FGF of 70ml/kg/min is usually adequate for controlled ventilation;

100mls/kg/min will result in a degree of hypocapnia (lowered CO2 level in the blood).

Modifications of the Mapleson D system

The Bain Circuit (Figure 3) is the most commonly used form of the Mapleson D system. It is a

co -axial circuit which was introduced in 1972 by Bain and Spoerel. Unlike the Lack co-axial

circuit described above, fresh gas flows down the central narrow bore tubing (7mm i.d.) to the

patient and exhaled gases travel in the outer corrugated tubing (22mm i.d.). The reservoir bag

may be removed and replaced by a ventilator such as the Nuffield Penlon 200 for mechanical

ventilation. Before use the Bain circuit should be carefully checked by the anaesthetist. The outer
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tubing of a Bain circuit is made of clear plastic and the inner green or black. If a leak develops in

the inner tubing or it becomes detached from the fresh gas port, a huge increase in apparatus

dead space occurs. In order to check for this, the lumen of the green tubing should be occluded

with a finger or the plunger of a 2ml syringe when a rise in gas pressure within the anaesthetic

circuit should be observed.

The degree of rebreathing that occurs suring IPPVwill depend on the FGF. In an adult, fresh gas

flows of 70-80mls/kg/min (6-7litres/min) will maintain a normal arterial carbon dioxide tension

(normocapnia) and a flow of 100mls/kg/min will result in mild hypocapnia.

The Mapleson E system performs in a similar way to the Mapleson D, but because there are no

valves and there is very little resistance to breathing it has proved very suitable for use with

children. It was originally introduced in 1937 by P Ayre and is known as the Ayre's T-piece. The

version most commonly used is the Jackson-Rees modification which has an open bag attached

to the expiratory limb (classified as a Mapleson F system although it was not included in the

original description by Professor Mapleson). Movement of the bag can be seen during

spontaneous breathing, and the bag can be compressed to provide manual ventilation. As in the

Bain circuit, the bag may be replaced by a mechanical ventilator designed for use with children.

This system is suitable for children under 20kg. Fresh gas flows of 2 - 3 times minute volume

should be used to prevent rebreathing during spontaneous ventilation, with a minimum flow of 3

litres/minute, eg a 4 year old child weighing 20kg has a normal minute volume of 3 litres/min

and would required a FGF of 6-9litres/min. During controlled ventilation in children

normocapnia can be maintained with a fresh gas flow of 1000mls + 100mls/kg. e.g. a 4 year old

weighing 20kg would need a total FGF of around 3litres/min. systems are all functionally

similar.

Combination of the Mapleson A, D and E Systems - The Humphrey A D E Circuit

The Mapleson A circuit is inefficient for controlled ventilation as is the Mapleson D circuit for

spontaneous ventilation. David Humphrey has designed a single circuit (Figure 5) that can be

changed from a Mapleson A system to a Mapleson D by moving a lever on the metallic block

which connects the circuit to the fresh gas outlet on the anaesthetic machine. The reservoir bag is


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situated at the fresh gas inlet end of the circuit, and gas is conducted to and from the patient

down the inspiratory and expiratory limbs of the circuit.

Depending on the position of the control lever at the Humphrey block, gases either pass through

the expiratory valve or the ventilator port. When the lever is "up" the reservoir bag and the

expiratory valve are used, creating a Mapleson A type circuit. When the lever is in the "down"

position the bag and valve are by-passed and the ventilator port is opened creating a Mapleson D

system for controlled ventilation. If no ventilator is attached and the port is left open the system

will function like an Ayre's T piece (Mapleson E ).

Like all pieces of equipment, it is essential that the anaesthetist fully understands the function ofa particular circuit. If the lever on the Humphrey block is moved from "up" to "down" whilst

gases are flowing the breathing bag will remain full of gas but manual ventilation of the patient`s

lungs by compressing the bag will be impossible and may resemble complete obstruction of the

breathing circuit. This has led to anaesthetists ocasionally concluding that their endotracheal tube

required changing.

The Humphrey ADE

breathing system

This is a very versatile breathing

system which combines the

advantages of Mapleson A, D and E

systems. It can therefore be used


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efficiently for spontaneous and

controlled ventilation in both adults

and children. The mode of use is

determined by the position of one

lever which is mounted on the

Humphrey block (Fig. 4.13). Both

parallel and coaxial versions exist

with similar efficiency. The parallel

version will be considered here.

Components

1. Two lengths of 15 mm smoothbore

tubing (corrugated tubing is

not recommended). One delivers

the fresh gas and the other carries

away the exhaled gas. Distally

they are connected to a Yconnection

leading to the patient.

Proximally they are connected to

the Humphrey block.

2. The Humphrey block is at the

machine end and consists of

a) an APL valve featuring a visible

indicator of valve performance

(Fig. 4.14)

b) a 2 litre reservoir bag

c) a lever to select either

spontaneous or controlled

ventilation

d) a port to which a ventilator can

be connected, e.g. Penlon

Nuffield 200


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e) a safety pressure relief valve

which opens at pressures in

excess of 60 cmH2O

f) a new design incorporating a

soda lime canister is also

available.

Mechanism of action

1. With the lever up (Fig. 4.15A) in

the spontaneous mode, the

reservoir bag and APL valve are

connected to the breathing system

as in the Magill system.

2. With the lever down (Fig. 4.15B)

in the ventilator mode, the

reservoir bag and the APL valve

are isolated from the breathing

system as in the Mapleson E

system. The expiratory tubing

channels the exhaled gas via the

ventilator port. Scavenging occurs

at the ventilators expiratory valve.

3. The system is suitable for

paediatric and adult use. The

tubing is rather narrow, with a low

internal volume. Because of its

smooth bore there is no significant

increase in resistance to flow

compared to 22 mm corrugated

tubing used in other systems. Small

tidal volumes are possible during

controlled ventilation and less


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energy is needed to overcome the

inertia of gases during spontaneous

ventilation.

4. The presence of an APL valve in

the breathing system offers a

physiological advantage during

paediatric anaesthesia, since it is

designed to offer a small amount

of PEEP (1 cmH2O).

5. During spontaneous ventilation

a) a fresh gas flow of about 50 to

60 ml/kg/min is needed in

adults

b) the recommended initial fresh

gas flow for children weighing

less than 25 kg body weight is 3

litre/min, this offers a

considerable margin for safety.

6. During controlled ventilation

a) a fresh gas flow of 70 ml/kg is

needed in adults

b) the recommended initial fresh

gas flow for children weighing

less 25 kg body weight is 3

litres/min. However,

adjustment may be necessary to

maintain normocarbia.


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Fig. 4.13 The parallel Humphrey ADE

breathing system.

Fig. 4.14 The Humphrey ADE breathing

systems APL valve (reproduced with

permission from Dr D. Humphrey).


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Fig. 4.15 Mechanism of action of the parallel Humphrey ADE breathing system. With the lever

up (A) the system functions in its

Mapleson A mode for spontaneous ventilation. For mechanical ventilation the lever is down (B)

and the system functions in its

Mapleson E mode (reproduced with permission from Dr D. Humphrey).

Circle Systems

An alternative to using high flow circuits is to absorb CO2 from the expired gases which are then

recirculated to the patient. These circuits are known as circle systems, were first devised by

Brian Sword in 1926 and require smaller amounts of fresh gas each minute.

Carbon dioxide is removed from the expired gas by passage through soda lime, a mixture of 94%

calcium hydroxide and 5% sodium hydroxide, and 1% potassium hydroxide which reacts with

CO2 to form calcium carbonate. Soda lime also contains small amounts of silica to make the

granules less likely to disintegrate into powder and a chemical dye which changes colour with

pH. As more carbon dioxide is absorbed the pH decreases and the colour of the dye changes


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from pink to yellow/white. When around 75% of the soda lime has changed colour it should be

replaced . The soda lime canister should be mounted vertically on the anaesthetic machine to

prevent the gases passing only through a part of the soda lime (streaming).

Fresh soda lime contains 35% water by weight which is necessary for the reaction between

carbon dioxide and soda lime to take place. This generates considerable heat. The soda lime may

rise in temperature to 40 centigrade. There are therefore additional advantages of using circle

systems in that the gases within the circle are warmed and humidified prior to inspiration.

(Baralyme is a commercially available CO2 absorber which contains 5% barium hydroxide

instead of sodium hydroxide.)

Design of Circle Systems

A circle system (Figure 6) is composed of two one way valves (one inspiratory and one

expiratory), a reservoir bag, a fresh gas inlet, a canister of soda lime and an expiratory spill

valve. Although there may be slight differences in the positioning of these components, all the

systems function in the same way.

Vaporiser Position. The vaporiser may be placed either outside the circle (VOC) on the

anaesthetic machine in its conventional position, or rarely within the circle itself (VIC). Normal

plenum vaporisers, with high internal resistance, cannot be used within the circle and a low

internal resistance type vaporiser (such as the Goldman) is required. Drawover vaporisers such as


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the OMV are not recommended for use within the circle because of the risk of over-dosage.

Since the gases are recirculated, if the vaporiser is placed in the circle, gas already containing

volatile anaesthetic agent will re-enter the vaporiser and the resulting output will exceed the

vaporiser setting. This is a particular danger during controlled ventilation when dangerously high

concentrations can build up. Vaporisers should only be placed inside the circle (VIC) when

inspired volatile anaesthetic agent monitoring is available. It is safer to use conventional plenum

vaporisers mounted on the anaesthetic machine outside the circle. In this case the maximum

volatile anaesthetic agent concentration achievable within the circle cannot exceed that set on the

vaporiser.

Practical Use of Circle Systems. During the first 5 - 10 minutes of an inhalational anaesthetic

using a volatile anaesthetic agent in oxygen and nitrous oxide, large amounts of the anaestheticagent and nitrous oxide will be taken up by the patient, and the nitrogen contained in the patient's

lungs and dissolved in their body will be washed out. If low fresh gas flows are used

immediately the patient is connected to the circuit the nitrogen will not be flushed out of the

circle system and will dilute the anaesthetic agent concentration. This may be prevented by using

conventional fresh gas flows of 6litres/min for the first 5-10 minutes of each anaesthetic before

reducing the flow rates.

Reducing the fresh gas flow rates. Inspired anaesthetic gases should contain no carbon dioxide

and a minimum of 30% oxygen. Exhaled alveolar gas contains a lower concentration of oxygen

and around 5% carbon dioxide which is removed from the exhaled gas on passage through the

soda lime. A small amount of fresh gas is added before the next breath. At low fresh gas flow

rates (


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These comments are less important if only oxygen and a volatile agent is being used in the circle.

Under these circumstances there is no risk of oxygen dilution and the flows may be reduced to

1000mls/min.

With flows of >1500mls/min the inspired concentration of volatile agent will be similar to that

set on the vaporisers. With flows


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gas flow to the patient and the

other to receive exhaled gases

from the patient, each port

incorporates a unidirectional

valve

c) an APL valve connected to a 2

litre reservoir bag.

2. Inspiratory and expiratory tubings

connected to the canister.

3. A vaporizer mounted on the

anaesthetic machine back bar

(vaporizeroutside circleVOC)

or a vaporizer positioned on the

expiratory limb within system

(vaporizerinside circleVIC).

4. Soda lime consists of 94% calcium

hydroxide and 5% sodium

hydroxide with a small amount of

potassium hydroxide. Silica is

added to prevent disintegration of

the granules into powder. A dye is

added to change the granules

colour when the soda lime is

exhausted. Colour changes can be

from white to violet or from pink

to white.

5. The size of soda lime granules is 4

to 8 mesh. Strainers with 4 or 8

mesh have four and eight openings

per inch respectively. Therefore,

the higher the mesh number, the


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smaller the particles that are

retained. Soda lime can also be

made to a uniform shape of 3 to 4

mm spheres allowing a more even

flow of gases and a reduction in

channelling. This results in a

longer life with lower dust content

and lower resistance to flow. One

kilogram of soda lime can absorb

more than 120 litres of CO2.

Mechanism of action

1. Exhaled gases are circled back to

the canister where carbon dioxide

absorption takes place and water

and heat are produced. The

warmed and humidified gas joins

the fresh gas flow to be delivered

to the patient (Fig. 4.17).

CO2 + 2NaOH Na2CO3 + H2O

+ heat

Na2CO3 + Ca(OH)22NaOH +

CaCO3

2. The direction of gas flow is

controlled via the unidirectional

disc valves. These are mounted in

see-through plastic domes so that

they can be seen to be working

satisfactorily.

3. The canister is positioned

vertically to prevent exhaled gas

channelling through unfilled


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portions.

4. The initial fresh gas flow

required is several litres per

minute for a period of 5 to 10

minutes. This high flow washes

any nitrogen from both the circle

system and the patients

functional residual capacity. This

can later be reduced to 0.5 to 1

litre/min.

5. The circle system can be used for

both spontaneous and controlled

ventilation.

6. Disposable circle breathing

systems exist. They feature coaxial

inspiratory tubing. The inner

tubing delivers the fresh gas flow

from the anaesthetic machine and

the outer tubing delivers the

recircled gas flow. Both gas flows

mix distally. This allows a more

rapid change in the inhalational

gas and vapour concentration at

the patient end.

USE OF VAPORIZERS IN THE

CIRCLE BREATHING SYSTEM

VOC vaporizers (Fig. 4.18A) are

positioned on the back bar of the

anaesthetic machine. They are high

efficiency vaporizers that can deliver

high output concentrations at low


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flows. They have high internal

resistance.

1. The vaporizer should be able to

deliver accurate concentrations of

inhalational agent with both high

and low fresh gas flows. This is

easily achieved by most modern

vaporizers (e.g. the Tec series).

2. The volume of the circle system is

large in relation to the low fresh

gas flow used. Rapid changes in

the concentration of the inspired

vapour can be achieved by

increasing the fresh gas flow to the

circle system. Delivering the fresh

gas flow distally, using a coaxial

inspiratory tubing design, allows

faster changes in inspired vapour

concentration compared to

conventional circle systems at low

flows.

VIC vaporizers (Fig. 4.18B) are

designed to offer minimal resistance

to gas flow and have no wicks on

which water vapour might condense

(e.g. Goldman vaporizer). The VIC is

a low efficiency vaporizer adding

only small amounts of vapour to the

gas recirculating through it.

1. Fresh gas flow will be vapour free

and thus dilutes the inspired


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vapour concentration.


respiration is depressed with

deepening of anaesthesia. Uptake

of the anaesthetic agent is

therefore reduced. This is an

example of feedback safety

mechanism. The safety

mechanism is lost during

controlled ventilation.


features

1. Adequate monitoring of inspired

oxygen, end-tidal carbon dioxide

and inhalational agent

concentrations are essential.

2. The unidirectional valves may

stick and fail to close because of

water vapour condensation. This

leads to an enormous increase in

dead space.

3. The resistance to breathing is

increased especially during

spontaneous ventilation.

4. Compound A is produced when

sevoflurane is used in conjunction

with soda lime. Newer designs of

soda lime claim lesser production

of compound A.

5. The circle system is bulkier, less

portable and more difficult to


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clean.

6. Soda lime is corrosive. Protective

clothing, gloves and eye/face

protection can be used.

7. Because of the many connections,

there is an increased potential for

leaks and disconnection.

Fig. 4.16 The circle breathing system.


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Fig. 4.17 Mechanism of action of the circle breathing system.

Fig. 4.18 Diagrammatic representation of the circle system with (A) vaporizer outsidethe circle (VOC) and (B) vaporizer inside the circle (VIC)

Breathings Systems

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