AMIR SALAH
MODERN ANAESTHETIC
MACHINE
4 of 4
CO-AXIAL CIRCUITS
It is the form of circuits in which the inspiratory
and expiratory tubes are inside each other in
order to make the APL valve far a way from the
patient.
It was used in war time for life jackets . Revival
of co-axial circuits in anaesthesia as version of
semiclosed Mapleson’s was done by :
1.Canidian BAIN & SPOEREL in 1972
2.LACK in 1976
1.BAIN’S CIRCUIT
It was designed to work by 70 ml/kg but by testing it
was found to need 2 to3 MV ( D ).
The FGF goes through central tube and expiratory gas
along outer sleeve.
Its length is 1.8 m ,outer and inner diameter are
22,7mm
ADVANTAGES :
1.Reduced wt.
2. Scavenging attachment.
3.Adjustment of valve easily.
4.Warming of inspiratory gas by exhales.
5. Longer.
6.Ventilator attachment instead of bag (PENLON )
2.LACK’S CIRCUIT
1.It is classified as Mapleson’s A
2.FGF from outer sleeve and exhales through the
inner tube.
3.Unlike Bain’s the patient breathes through both
tubes ,so diameters must be wider ; 28 & 14
Disadvantages
Bulky & no attachment to scavenging or ventilator.
Problems with co-axial circuits.
1.Resistance to inspir or expir flow, Lack’s needs
outer sleeve at least 500 ml to decrease resistance
2.More serious inner tube KINK or DETACHMENT
CLOSE CIRCUIT
LOW FLOW
1. Totally closed system:
Use only in veterinary practice
2. Basal flow system :
FGF is reduced to basal level (250 ml) FGF
must be
100%. It needs accurate rotameters
and trained persons.
How to give nitrous ?
3. Low-flow system:
FGF set above basal but below the alveolar
ventilation with CO2 absorber.
Close Circuits
Two basic pattern of closed circuit
absorption apparatus are present :
I. TO & FRO
II. CIRCLE SYSTEM
To & Fro
Water’s 1924
1.In the To & Fro system there is no valves.
2.The patient breath to & fro from a rebreathing bag
filled with CO2 absorber.
3.FGF is introduced into the circuit near patient , the
reservoir bag can be separated by a suitable length
of wide bore tubing but absorber must be near
patient.
4.This system is bulky , restricts surgeon field ,
increases dead space and most important risk of
inhalation of soda lime (dusting).
CIRCLE SYSTEM
In the circle system the direction of gas is
controlled by two unidirectional valves , so that
expired gas passes through an absorber into RB
and then back to the patient.
Advantages:
1.Economic in the consumption of gases and
anaesthetic vapor
2.Conservation of heat and water.
3.Limitation of fire and explosion.
4.Less pollution . 5.Education!
Disadvantage of closed circuit
1.However an over-riding danger of closed
c. is that of varying anaesth uptake.
2.Varying efficiency of CO2 absorption.
3.Difficulty of accurately calculate basal O2
consumption.
4.It is never to predict conc. Of gases
within circuit especially during induction.
5.It is consists of 7 parts & 10 connections
CO2 Absorber
1.SODA LIME:
Ca OH 94 %
Na OH
5%
K OH
1%
Silica as a hardener material (no dusting)
Co2+H2OH2CO3+Ca OH Caco3+heat+
water
2.BARALYME
Ca OH 80%
Barium (octo-hydrate)20%
Less caustic ,produce less heat than soda
lime , no silica is necessary to produce
hardening , it contains mimosa Z and ethyl
violet as indicator ,and used in space craft.
Evaluation of CO2
absorber
1.
2.
3.
4.
Absorptive Capacity
Granules size &hardness
Indicators
Interaction with volatile agents
1.Absorptive Capacity
1.In soda lime approx. 25L/100g , while in
baralyme is 27L/g
2.Ineffective removal occurs because outside of
granules becomes exhausted before the
whole granules is used up.
3.Another complicating factor is the size of
canister , as larger provide a greater
absorptive area and allow slower reaction
4.When system allows to stand regeneration
occurs , it is important to shake canister after
filling to increase its capacity
2.Granules size & Hardness
1.A fully packed canister consists of 50%
granules and 50% inter-granular space .
2.Large granules make less resistance but more
channeling and less surface area .
3.Granule size is measured by mesh size the
optimum size is 4 to 8 mesh
4.To prevent fragmentation silica and clay is
added to soda lime .
5.The tendency for dusting is measured by
hardness number (> 75)
3. Indicators
1. A chemical indicator: is attached to granules
which changes its color in acid media.
2. Capnography: is the key to the early detection
of CO2 rebreathing.
3. Signs of hypercapnia: Increase in pulse rate ,
BP ,RR & oozing.
Hypercapnia must be considered first and the
easiest way to deal is to increase FGF
4. Interaction with volatile agents
It occurs because of heat and moisture.
1.Trichlortheylene: Trilene
At 60 c◦ decomposed to di- then to phosagene
(toxic).
2.Production of CO:
Increase level of carboxyhemoglobin (COHb) occurs
if water content below 3.2%. The major danger
occurs immediately when the circuit is used for the
first time after a period of disuse, especially if
there is a basal flow of O2 left on dehydration of
granules especially baralyme. ( Monday morning).
CO produced with enflurane, iso,sevo & esp.desflur
3.Compound A:
Can be produced from modern anaesth drugs
which can be absorbed by dry lime.
4.Fires from interactions of anesthetics with
desiccated absorbent:
In (2003) Holak &others concluded that large amount of
CO may be generated and flammable gases may be
produced and ignited when used sevo with baralyme.
In (2004) Laster & others in lab work proved that
baralyme with sevo leads to temp of 200 C, while with
des & iso reached 100 C.
Measures to prevent :
1. Monitoring of absorbent temp to 50 C in canister
2. In late (2004) manufacture withdraw baralyme from
market.
3. Amsorb is a new absorbent that contains no strong
base.
4. Soda lime seems to be less in hazard.
MONITORING OF CIRCUITS
1.Pressure:
Low <10 or high>50 pressure alarm
Continuous press alarm >10 cm for > 15”
Sub atmospheric press alarm < -10.
PEEP alarm >25 cm or active.
2.Volume :
Spirometry placed in the vicinity of the expiratory
valve ,detecting low or high volume.
3.Gas composition :
O2,CO2,Anaesthitics conc. I&E ,N2