BREATHING CICUITS IN ANAESTHESIA PHYSICS BY MOHAMED ANWER RIFKY,SUMMARY FROM A PHYSIC BOOK

1. BreathingandScavengingSystems MOHAMED ANWER RIFKY
THE OPEN SYSTEM: 1-In this system the patient's airway remains open to room air and no tubing, valves or reservoir bag are used >>A, the anaesthetist is inducing anaesthesia in a child by
allowing the anaesthetic gases to fall over the child's face from a cupped hand containing the end of the gas delivery tube. A Schimmelbush mask was used in the past in a similar manner. B, oxygen
is delivered to an Edinburgh mask which has a wide opening to room air >>a negligible increase to the respiratory dead space. In example. C, oxygen is delivered by a cannulae to the patient's
nostrils. Similarly anaesthetic gases >>side tube of a special gag into the mouth of the patient. 2-Open systems advantages >>no added resistance to the patient's breathing, but the
disadvantage>> giving little control over the concentration of oxygen and other gases received by the patient. 3-During peak inspiration, a large inspiratory flow of up to 50 litre min-1 may occur, so
dilution with air is high during this part of the respiratory cycle>>known concentrations of gases can only be administered if the flow of gas to the patient exceeds his peak inspiratory flow>>
'Venturi' masks.
NON-REBREATHING VALVE SYSTEMS: 1- A typical non-rebreathing valve is the 'Ambu E valve'>> a-Moulded rubber one-way inspiratory and expiratory valves
positioned to allow the patient to draw in gas through one connection and expire through the other. b-A reservoir bag. C-The gas supply to this system must equal the
tidal ventilation. 2-An alternative non-rebreathing valve system is found in the Quantiflex machines used in dental anaesthesia in which the inspiratory valve is positioned
immediately after the reservoir bag.3-The Ambu E valve can also be used for controlled ventilation>> Squeezing the reservoir bag>> valve>>occlude the outlet (SO,both as
an inflating valve and as a nonrebreathing).4-The Ambu E2 (used only in resuscitation)the expiratory flap valve is omitted and the nonrebreathing function is then no
longer present.5-Valves should be closely supervised in use lest an excessive flow of fresh-gas maintains the valve in the inflation position>>dangerous build-up of airway
pressure.6-- Resistances can be 100-200 Pa (1-2 cmH20) during peak flow in many non-rebreathing valves>> uncomfortable for long term use (airmen, firemen)>> have
special low resistance flap valves and the Entonox apparatus used for analgesia has a similar low resistance.7-Non-rebreathing valve system is useful for emergencies
outside hospital>>Triservice apparatus ( Inspiratory and expiratory valves are incorporated in the Laerdal valve>> the silicone rubber flaps of the valve occlude the
expiratory ports when the self-inflating bag is compressed). Triservice vaporizer is a modified OMV vaporizer>> extreme lightness and portability.
T-PIECE SYSTEMS: 1-known as Ayre's T-piece after Dr Philip Ayre who first used it in infants in 1937.2- There are no valves in the simplest form of this system, fresh gas being supplied to a T-piece
at the patient who breathes through an open limb of the T-system. This limb must be of suitable diameter to avoid excessive resistance to the flow of gas, e.g. 22 mm internal diameter for an adult
patient.3-A fresh gas flow of 2 to 2.5 times the tidal ventilation (e.g. 14 litre min-1) is supplied to the T-piece and the capacity of the open limb could be the same as the patient's tidal volume. 4-
content of the open limb at the end of expiration, when it contains a mixture of expired and fresh gas>>patient's anatomical dad space are expired first, so these are shown released from the open
end of the tube at A. 5-During mid-expiration, the majority of the alveolar gases are expired at near peak expiratory flow, so they are only slightly diluted by the fresh gas flow as shown in the centre
of the tube at B.6- C, contains more fresh gas. If there is an end expiratory pause, then a further volume of un diluted fresh gas is finally present as at D .7-fresh gas only is inspired until inspiratory flow
becomes greater than the flow of fresh gas to the T-piece. At peak inspiration, the patient may in addition draw upon the gas at D in the open tube. 8-If the fresh gas flow is too low, e.g. below twice the
tidal ventilation, the patient will draw upon the gases at B and C in the open limb during peak inspiration and rebreathe alveolar gases.9-(Figure ) illustrates the effect of the end-expiratory pause upon
the fresh gas flow required to prevent any rebreathing of alveolar gas. . It can be seen from the graph that a flow of 14 litre min-1 (2.9 times the given tidal ventilation) is necessary if there is no end-
expiratory pause. If pause of 1 second is present, then a flow of fresh gas of 8.5 litre min-1 (just under twice the tidal ventilation of 4.8 litre min-1).9-If the open limb is omitted, this leads to inspiration
of air unless the fresh gas flow is equal to the patient's peak inspiratory flow.10-It is possible to add an open-ended reservoir bag it is possible to use lower fresh gas flows. 11-The T-piece system is
particularly >>paediatric anaesthesia>>T-piece connection can be close to the patient's airway with negligible apparatus dead space at this point. it avoids the problems associated with an
expiratory valve(increased resistance and possible malfunction due to sticking). In adults, a disadvantage of the system is the high fresh gas flow >>spontaneous breathing. the added flow increases
expiratory resistance and may raise the risk of pollution from spilt gas.

2. COAXIAL T-PIECE SYSTEM: 1- More compact >>gas delivery inside the limb of the T >>the Bain (USA) or Penlon coaxial (UK) systems. 2-Gas flows during
spontaneous breathing should be about 2.5 times the patient's tidal ventilation.4- care to ensure that the central tube is correctly attached.
MAGILL SYSTEM: 1-Introduced by Sir Ivan Magill in 1920.2-Comprises >>reservoir bag, delivery tube and expiratory >>popular system for spontaneously
breathing . 3-Gases from the anatomical dead space travel >>at A, while fresh gas inflates the reservoir bag, distending it until the pressure from the bag is sufficient
to open the expiratory valve (50 Pa or 0.5 cmH20) . 4-alveolae gases are then expired through the expiratory valve into an expired gas scavenging system, or into the
atmosphere.5-It conserves the dead space gas>> fresh gas flows of 70% of the patient's tidal ventilation in a spontaneously breathing patient.6- The reservoir bag
can be squeezed to give intermittent positive pressure to the limits set by the valve. Excess gas spills from the valve at the end of inspiration>> fresh the system is
inefficient during controlled ventilation, but may be convenient for short-term use .7-Modern expiratory valves are larger than the former 'Heidbrink' type
expiratory valves as they incorporate a connection for a scavenging tube. However, if the bulk of the valve is inconvenient then it can be positioned remote
from the patient, and remote valve can be convenient if it is decided to use disposable tubing to achieve better sterility, as the valve need not then be changed
with each fresh tubing.
COAXIAL MAGILL SYSTEM: 1-Uses a remote expiratory valve(Lack system). 2-Fresh gas flows from the reservoir bag through the wide tube which contains
a narrower tube leading to the expiratory valve.3- Unlike the coaxial T-piece the patient breathes through both tubes in the coaxial Magill system>> wider coaxial
tubes are required to prevent increased resistance to breathing. 4-Is less suitable for prolonged controlled ventilation.
.
CLOSED SYSTEMS: 1- In the circle system>>gases recirculate through soda lime which absorbs carbon dioxide, while oxygen is added to replace the patient's
metabolic uptake.2-The soda lime used must have granules of a critical size, large enough to avoid excess resistance to breathing and small enough to absorb
carbon dioxide efficiently; a typical size being '4- -
8 mesh' >>a sieve having four and not eight strands per inch.3-The anaesthetic vapour is added in the
concentration needed to meet clinical requirements and the uptake by the patient. 4-Use vaporizers in the circle (VIC), or out of circle (VOC). The latter needs to
be a special type which retains its accuracy at low flows. 5-The variability of the patient's uptake of oxygen and anaesthetic agents needs gas analysers . 6-
Disadvantages (cost of analysers, and their maintenance, difficulty of sterilization, and complextity of the closed system. Advantages(the oxygen uptake at the
patient to be monitored, and the uptake of volatile anaesthetic agents to be demonstrated, the inspired gas is fully humidified at near ambient temperature
,economy in the amount of volatile anaesthetics ,fresh gas flow (for example, 2 litre min-1),and the reduction of pollution.7-closed system with leak, gives better
control of the concentrations but lacks some of the advantages claimed for the fully closed system.as it allows the nitrogen to be replaced by nitrous oxide, at
least for the initial period of use.
ALTERNATIVE CLASSIFICATION OF SYSTEMS: 1-To avoid confusion the terms semi-open and semiclosed are best avoided.2-Mapleson system: T-piece system
(unmodified) E, T-piece system with open ended bag F, T-piece system with bag and valve D, Magill system A, and systems B and C (seldom used).
RESISTANCE OF BREATHING SYSTEMS: 1-The open system gives no added resistance to breathing but in other systems resistance occurs in the tubings and
valves used.2-large tubing diameter >>allow laminar flow >>no additional pressure needed to overcome the resistance during respiration, as smooth 22 mm
internal diameter with peak flows not exceed ing 22 litre min-1.3-Anaesthetic corrugated tubing >>laminar and turbulent flow. 4- Extra pressure by the patient
must be generated to drive the gases through tubings if it is small, about 25 Pa (0.25 cmH20),though during peak flow>>larger diameter, e.g. 30 mm, may be
used. 5-In simple T-piece systems without any expiratory valve or reservoir bag the resistance is that of the limb of the T through which the patient breathes.6-
Expiratory flow is increased by the fresh gas (resistance is greater during expiration). whereas inspiratory flow in the limb is reduced by the fresh gas flow.7-The
valves are an important source of resistance in other breathing systems>> 50 Pa ,but 100-500 Pa during peak flows with 30 litre min-1. 8- The reservoir bag used in
a system can also affect the pressures which develop during spontaneous breathing.

3. WORK USED IN BREATHING SYSTEMS: 1- The additional work required in breathing systems can be indicated by the area of a pressure-volume loop (pressure at the
patient connector is plotted against the volume change for a tidal volume of 0.4 litre) 2-T-piece system gives very little added work (area of the loop). 3-In the case of the
non-rebreathing valve and circle system or the Magill system the mean pressure swing could be 200 Pa( Additional work done = 200 Pa x (0.4 x 10-3) m3= 0.08 J= 80
mJ>>total mechanical work= 300 mJ (27% increase)>> uncomfortable for conscious individuals. 4-non-rebreathing valve system >> resistance of the inspiratory and
expiratory valves are comparable, so the pressure-volume loop ( equal areas above and below the zero).5-A circle system would give a similar result.6-In the Magill system
>>overcoming the system resistance occurs during expiration.6-Coaxial Magill system performs similarly, but the expiratory resistance in this case is that of the valve with its
central 12.5 mm diameter supply tube.7-T-piece system>> mean expiratory flow through the valve and tubing is increased, >>higher pressures are generated .Positive
pressure during part of an inspiration assisting this phase of the cycle(Pt.uses less E.). 8-Similar tracing could be obtained with the coaxial T-system. 9- Excessive work
>>augment the depression of ventilation. 10- The added expiratory resistance>>reduce venous return and cardiac output.
SAFETY OF BREATHING SYSTEMS: 1-The components of breathing systems must be correctly assembled with no falling aparts.2-Couplings with screw collars with simple cone fitting to allow
rapid changing.3- Plastic connections may distort during autoclaving .4- couplings which do not fit properly >> not be used.5-A taper gauge can be used (dimensions ),and breathing tubes should
not be too narrow and sharp angles avoided.6- valve sticks >>excessive pressures.7- Excess pressure can build up if the scavenging tubing is obstructed. 8- The anaesthetic bag gives a pressure
that is nearly constant, even though it becomes grossly distended>>about 4 kPa (40 cmH20) >>short-term protection to the patient while the anaesthetist identifies and removes the obstruction
in the system.P=2T/R >>LA PLACE law,but some disposable reservoir bags lack elasticity and give no such protection>>pressure of 35 kPa>>valves!!. 9-In Magill system, the expiratory valve>>close
to the patient's >>anatomical dead space!!,and if coaxial tube is detached >>increase dead space >>suffer hypoxia and hypercapnia in consequence.
VENTILATORS: Manley Pulmovent type: 1-A fresh gas flow of, for example, 7 litre min-1 inflates a reservoir bellows>> 10-12 kPa by springs.2-When the bellows are
inflated to the preset volume, the cycling mechanism opens a valve>> bellows inflate the patient's lungs, and full pressure of 10 kPa is not reached >>flow control valve
restricts the flow >>a safety valve at the patient's side of the cycling valve limits pressure to a maximum of 7 kPa. 3- The expiratory valve is closed during inspiration and
open at expiration.4-It is convenient to classify ventilators as 'constant pressure generators'( constant pressure during inspiration),though neither pressure nor flow is
constant at the patient's airway.5-Regarded as a 'volume cycled' ventilator(cycling valve). 6- The ventilator is sometimes referred to as a minute volume divider>>7
litre min-1 in the example , tidal volume measured by a Wright respirometer is 0.5 litre>> the ventilation rate is 14.7- During spontaneous breathing>>the breathing
system is converted to a T-piece system.8-A ventilator alarm usually has two adjustable pressure sensors.
SCAVENGING: 1-Incidence of spontaneous abortion has been reported in(early reports) suggested>>exposure to trace quantities of anaesthetic agents, but subsequent studies have not
substantiated this suggestion.2-Systems can be classified as active, in which an external source of power such as a pump draws away the scavenged gas, or passive in which gases are driven to the
exterior of the building by the pressure generated by the patient during expiration.3-The simplest technique for active scavenging is an open technique, sometimes referred to in Scandinavia as
'local scavenging'.4-British standards are specified according to the system used for the transfer, receiving and disposal of the gases.
OPEN SCAVENGING TECHNIQUE: 1-Suction is applied to the waste gases is open to the atmosphere without any intervening reservoir bag or valves, which is less effective than the other
systems. As a funnel is positioned near the expiratory valve and mask ( aspirate spilt agents, e.g. in anaesthesia for dental extractions in which an airtight mask fit is difficult to achieve, and during
Entonox analgesia ).Alternatively, the funnel can be inverted in the form of a dish and used to collect gases released from the end of a T-piece breathing system.2-Open scavenging systems lack
control. A high scavenging flow is needed and the funnel must be very close to the point of release of the gases to be effective.
COMPONENT SYSTEMS OF STANDARD TECHNIQUES: The term 'transfer system' is used for the wide bore 30 mm diameter tubing with its 30 mm male and female conical connectors which
transfers the gases collected from the exhaust port of the anaesthetic breathing system or ventilator to the 'receiving system'>>exterior of the building.
RECEIVING SYSTEM: Which incorporates an open ended reservoir usually a cylinder( incorporate channelling and the open end ).Designed to prevent any risk of obstruction at this air
break otherwise excess negative or positive pressure could be applied to the patient's airway ( flow, typically 80 litre min-1, removes all expired gas and a flow control and indicator).

4. DISPOSAL SYSTEM: 1-use the same system as the centralized vacuum system but an independent system is now recommended. This may incorporate a fan
instead of a pump to remove the scavenged gases>>not necessarily safer as even a low vacuum will give obstruction and risks of pulmonary oedema if it is
applied via the breathing system directly to the patient's lungs>> port must not exceed 50 Pa (0.5 cmH20) when tested with a flow of 30 litre min-1 into the
system exhaust .2-May cause problems due to their effect on the functioning of the expiratory valve or the ventilator.
PASSIVE SCAVENGING SYSTEM:1-Wide bore tube conducts the expired gases outside the building. 2-problems(excessive positive or subatmospheric
pressures >>wind movements at the outlet. and if the outlet is above roof level>>re-entry of the scavenged gases to the building (denser vapour such as nitrous
oxide gives a back pressure in the patient's breathing system).3-The collection system with bag and valves has been used without the injector as a basis for a
passive scavenging system. 3-To avoid the pressure problems due to air movement at the outlet of the passive system, the expired gases can be conducted to the
exit grill (ventilation must recirculates the air).THEATRE VENTILATION: Efficient theatre ventilation, e.g. 15 air changes per hour, is therefore recommended in
areas where volatile anaesthetics are used.
Ambu E valve. (A) During expiration. (B)
During positive inspiration