Breathing Systems "We shape our tools, and thereafter our tools shape us."
Breathing systems connect the continuous flow machine to the intermittently breathing patient's airway. FEATURES OF AN IDEAL BREATIIING SYSTEM
1. Prevents significant fluctuations in pressure 2. Offers low resistance to breathing 3. Has a low dead space 4. Minimizes re-breathing of alveolar gas 5. Prevents breathing from the atmosphere 6. Is economical with regard to gas flows 7. Causes no significant pollution of the atmosphere
Re-breathing of expired gases is always possible when simple systems without absorbers and uni-directional valves are used, and can be prevented by giving appropriate fresh gas flows (FGF).
Re-breathing of expired gases changes the composition of inspired gas by : 1. adding significant carbon dioxide concentrations 2. diluting the oxygen and volatile agent in the fresh gas (FG)
Re-breathing can be used to advantage
1. Dead space gas, which has not been involved in gas exchange, but has been warmed and moisturised, may be re-inspired with benefit to the patient and with economy of gases. This is achieved by selective conservation of dead space gas and venting of alveolar gas.
2. Alveolar gas can be safely re-breathed into the patient's dead space. 3. A controlled amount of alveolar gas may be re-breathed into the alveoli to maintain normocarbia, when the patient is ventilated with large tidal volumes.
THE T PIBCE
The simplest breathing attachment is the T piece which has three limbs (Fig 2.2) 1. Afferent limb for fresh gas flow (FGF) 2. Patient limb (Pt.) 3. Efferent limb for the exhaust The T piece satisfies the requirements 1- 4 of an ideal system.
To prevent breathing of gases from the atmosphere, FGF must equal peak inspiratory flow rate which is at least equal to 30L/min or more depending on the pattern of breathing (Fig. 2.2). This is not economical and will result in heavy pollution of the atmosphere. The T piece is therefore modified (Fig. 2.3). 1. Afferent system where the FGF limb is modified : •
It allows to and fro movement of gases.
• The FGF limb includes the reservoir bag and corrugated tube to provide a reservoir to store fresh gases in inspiration and expiration.
This system is economical with gas flows during spontaneous respiration (SR). e.g. Mapleson A : Magill and Lack circuits (see page 2. 7, 2.10 ).
2. Efferent system where the exhaust limb is modified : •
It allows to and fro movement of gases.
Mapleson classified various arrangements of Non Re-breathing Circuits. These systems are light weight, inexpensive and simple, where rebreathing is prevented by venting exhaled gases through pressure relief valves.
Magill Attachment (Mapleson A)
is an afferent system (reservoir on the afferent limb or l;JGF inlet). The basic principle is the conservation of fresh gas during expiration with selective venting of alveolar gas and conservation of dead space gas.
The modified afferent limb consists of a corrugated tube and reservoir bag which contains fresh and possibly dead space gas if the FGF is inadequate.
The corrugated tube (I metre long) has a volume equal to or more than the tidal volume of the patient (500 ml), so that the expired gases do not reach the reservoir bag where they would mix freely with the fresh gas.
In the corrugated tube, dead space and alveolar gases are compartmentalized allowing selective venting of alveolar gas which is closest to the expiratory valve.
· The reservoir bag has a capacity of 2 L. The reservoir bag acts as :
1. a reservoir for fresh gas 2. a safety device, preventing high pressures in the system due to its special compliant properties which allows expansion to prevent increase in pressure
3. a visual monitor for rate and depth of respiration 4. a device which makes manual ventilation possible
Adjustable Pressure Limiting (APL) valve (Heidbrink valve)
This is a one way spring loaded valve, the setting of which determines the pressure ·within the system and the venting of expired gases.
;
The valve should always be in the upright position so that when the pressure in the system exceeds the pressure set by the valve, it will open and vent alveolar gas.
Even when closed the valve opens at 60 cm pressure to avoid barotrauma. If the valve is in a dependent position it will always be open, and will not selectively vent alveolar gas.
APL valves are often encased in a hood for scavenging.
Note that if the expiratory valve is not competent, the reservoir bag will over fill during expiration indicating that alveolar gas has not been vented adequately.
Apparatus dead space is the volume of the mask and angle piece up to the expiratory valve (the point where fresh gas and expired gases meet).
If all the alveolar gas is not vented during expiration then the functional dead space will extend further than the expiratory valve.
• Expiratory limb includes the reservoir bag and corrugated tube which contains a mixture of fresh, dead space, and expired gases.
This system is useful for controlled ventilation, but not economical in SR as fresh gas is lost in expiration. e.g. Mapleson D (Bain circuit), E, and F (see page 13.5).
SPONTANEOUS RESPIRATION a)
EGEl b) EG.E(JY EG.Eu c) • lll!!lf
~ .t- i'G 1
D.S. ALV.
-_ ... ,. Figure 2.5 Magill attachment during spontaneous respiration
Fig 2.5 a) During inspiration the inspiratory flow rate which is approximately 20-30 L/min can be satisfied by fresh gas from the machine, corrugated tube and reservoir bag.
Fig 2.5 b) During early expiration while fresh gas enters the partially empty bag, exhaled gas (dead space and then alveolar gas) passes down the tube towards the bag.
·
Fig 2.5 c) During the latter part of expiration and the expiratory pause, fresh gas having filled the bag, passes down the tube, pushing expired gas down the corrugated tube towards the expiratory valve and out.
All the alveolar gas (closer to valve) is vented during late expiration, thus preventing rebreathing. If FGF equals patient's alveolar ventilation (4L/min), it will vent alveolar gas only. If FGF equals patient's minute volume (MV) approximately 6L/min, it will vent alveolar and dead space gas. A FGF of 6L is recommended, to compensate for leaks and hyperventilation.
SPONTANEOUS RESPIRATION WITH THE MAGILL ATTACHMENT
Very economical during spontaneous respiration To prevent re-breathing during spontaneous respiration : • FGF should be in excess of alveolar ventilation (6 L/min). • Expiratory valve should be open and in a non dependent position. The Magill attachment is unsuitable for children under 20 kg due to the high resistance of the expiratory valve and the increased volume of dead space.
CONTROLLED VENTILATION (CV) a}
F.G.F.--~ ~ b} F.G.F.-~~ ~ c} EGF~~'lrZD"
l Jo
Figure 2.6 Magill attachment during controlled ventilatio
The function during controlled ventilation is completely different from that during spontaneous respiration. Since the expiratory valve is partially closed to allow positive pressure ventilation, gases do not escape from the system unless pressures in the system rise considerably, which only occurs in late inspiration after the lungs are filled
Fig 2.6 a) During expiration the valve remains closed. The expired gas will pass down the corrugated tube and even enter the reservoir bag, mixing with the fresh gas
Fig 2.6 b) During the early part of inspiration, when the bag is squeezed, first alveolar and then dead space gas will be returned to the lung
Fig 2.6 c) During late inspiration, since the lungs are filled, the pressure in the system will rise, causing the valve to open, venting fresh gas which now lies closest to the valve. This results in re-breathing and loss of fresh gas
Disadvantages of the Magill attachment : • Inefficient during IPPV • Scavenging is difficult as the expiratory valve is close to the patien
CONTROLLED VENTILATION WITH THE MAGILL ATTACHMEN
• Very inefficient during IPPV and should not be used except for< S min. • Rebreathing is inevitable resulting in hypercarbia. • Rebreathing is minimized with high FGF and large tidal volumes. • Wastage of fresh gas is inevitable
.Tt....n~Inevitable.
lack circuit (mapleson a)
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 the inner efferent tube placed coaxially and vented through the spill valve placed near the machine end. this facilitates easy scavenging of expired gas and the patient end of the tube is lighter
-1t
'irvw figure 2.7-b lack valv
'°'-~ d u "'" '"'"" figure 2. 7-a the coaxial lack breathing syste
the length of the corrugated tube is 1.5 meters. the fresh gas flows down the wider outer corrugated tube to the patient while the expired air flows in the inner tube towards the valve and is vented. the inner tube should be of sufficient diameter to provide an acceptable resistance to the patient during expiration. inspiration : the valve closes and the patient inspires fresh gas from the outer reservoir tube
expiration : the patient expires into both tubes, while fresh gas from the machine fills the bag and then moves down the outer tube towards the patient end, thus pressurizing the system. towards the end of expiration, positive pressure is sufficient to open the valve, allowing expired gas to escape via the inner tube
expiratory pause: fresh gas washes the remaining expired gas from the outer tube into the inner tube, thus filling the outer corrugated tube with fresh gas only for the next inspiration while the expired gas is vented through the valve via the inner tube. there are two forms of the lack system. 1. the coaxial lack which was the original version (see above) 2. the parallel lack where the inner and outer tubes are replaced with two conventional breathing tubes and a y piece. this avoids the problem of a damaged or disconnected inner tube in coaxial circuits
the lack circui
• the lack system is not suitable for ippv • allows for easy scavenging • the fgf for sr is similar to that of the magill attachment (1-1 .5 x mv
2.10 fgf expired)t...me~.GF expired
Mapleson D (Efferent system)
Mapleson D,E,F, are all efferent systems which share similar configurations. The exhaust limb of the T piece is modified for to and fro movement of gases.
The far end of the exhaust system may be open (E) for SR, end in a valve (D) or a double ended open bag (F). The system functions similarly for both spontaneous and controlled ventilation.
·~ ~ .(5 4 {)
Inspiration Early expiration • .,dx FGF Expiratory pause Expired gas Figure 2.8 Functioning of Mapleson D
The FGF enters at the patient end of the system and the exp. valve is at machine end. The system should be filled with fresh gas before connecting it to the patient.
a) During inspiration the patient inhales fresh gas from the machine and the balance gas is taken from the corrugated tube and bag to satisfy the inspiratory flow rate. This limb contains only fresh gas in the first breath but with subsequent breaths contains a mixture of mixed expired (alveolar and dead space gas) and fresh gas.
b) During early expiration fresh gas continues to enter the corrugated tube, mixing with the expired gases as it passes down towards the far end and the bag. No compartmentalization of dead space, alveolar and fresh gas takes place because of turbulence during this time.
c) During the expiratory pause FGF fills the patient end of the corrugated tube. The fresh I expired gas mixture moves down the tube and pressurizes the system, venting the contents (mixture or expired air and fresh gas through the valve) The degree of filling with fresh gas depends on the duration of the expiratory pause and the FGF. Mixing of gases precludes preferential venting of pure alveolar gas as in the Magill attachment.
The degree of rebreathing depends on : 1. Fresh gas flow rate in relation to the inspiratory flow rate
2. Tidal volume, rate, inspiratory : expiratory ratio, and length of expiratory pause
SPONTANEOUS VENTILATION
Rebreathing has to be prevented with increased FGF, as ventilation is often inadequate. •
If the FGF is> 3 x MV, (i.e. it exceeds the peak inspiratory flow rate), the inspired gas will all be fresh gas.
• If the FGF is 2 x MV, and the expiratory pause is long enough to allow filling of the corrugated tube with fresh gas,, the inspired gas will be predominantly fresh gas.
The system is therefore not economical during spontaneous breathing. CONTROLLED VENTILATION
Though the behaviour of the system is no different from that during spontaneous ventilation, the recommendations for FGF are different and allows partial re-breathing. Normocarbia is maintained by hyperventilating the patient to wash out the extra C02" This system works efficiently and economically for controlled ventilation as long as the FG entry and the expiratory valve are separated by a volume equivalent to at least one tidal volume of the patient.
FGF
The elimination of C02 is a function of the FGF and ventilation (Fig. 2.10).
Ventilation determines the removal of C02 FGF determines the removal of C02
from the lungs. from the breathing system. If the FGF is high no rebreathing occurs and the PaC02 is dependent on ventilation.
This is the principle used during spontaneous respiration. In Fig. 2.10, the right horizontal limb of the curves represent high FGF (VF) with no rebreathing.
If the FGF is low relative to ventilation, re-breathing occurs and PaC02 depends on
FGF which determines the amount of re-breathing, as shown by the vertical limb. This is controlled rebreathing with hyperventilation (which eliminates C02
).
Thus various combinations of FGF and MY are used to control the PaC02 The ventilation and FGF are therefore interdependent in controlling C02
• In SR, MY is usually depressed and the FGF must be sufficient to avoid rebreathing.
In IPPV, large tidal volumes are recommended to minimize alveolar collapse and this tends to cause significant hypocarbia. This is prevented by reducing the FGF to allow rebreathing in a controlled fashion.
The following recommendations applies to Mapleson D, E, and F which are used in paediatrics (see page 13.4 ). They allow a degree of controlled re-breathing to provide a predicted PaC02
• • This allows the use of lower FGF than that used for SR, but it must be
combined with increased MV and oxygen concentration to compensate for rebreathing. CONTROL OF PaC02
SR FGF of 200-300 ml/kg or 2-3x MY prevents significant re-breathing. MV
FGF
70 ml/kg/min 100 ml/kg/min
140 ml/kg 140 ml/kg
IPPV Oxygen 40% 40%
DISCONNECTION
If the fresh gas tube gets detached from the machine in the D,E,F circuits there will be increased rebreathing, which may not be easily detected. During controlled ventilation : In electrically driven ventilators the airway pressure monitor will indicate a slight lowering of airway pressure but may not alarm because the low pressure alarm is often set at< lOcm Hp. In gas driven ventilators however, it will give an indication immediately as the bag will not rise to the top of the "bottle".
BAIN ATTACHMENT (MAPLESON D)
This is a coaxial modification of the Mapleson D system. It is the most popular breathing system for controlled ventilation as it is light weight and streamlined. The inner tube is connected to the machine and delivers fresh gas to the patient, while the expired gases enter the outer corrugated tube and are vented via the spill valve at the machine end which is convenient for scavenging. The corrugated tube is 1.8 meters long, but this distance from the machine can be extended by connecting the inner tube to the machine via a small diameter tube of required length. This is convenient for head and neck surgery and transport of patients. In the functioning of this system there is more mixing of fresh and expired gases due to turbulence at the patient end, resulting in a lowered ETC02
reading on the capnograph. The reservoir bag can be replaced by the ventilator. (a) •. J!fl.
f~*l..-.Ar~-!v\;'M'0hl.~ .. · . "'fo'IV\"l>Ml )'l (b)
u'.J ' vX) {.·.·.· ... : .. · .•.... ." <iJ FGf ~::t-/V:\.~;vw~1,Y1rffrvn ..
•"'P""""" m
CN,. ._. _______ (~) O\,·;..qr1;)\b) During the end of expiration and expiratory pause, the fresh gas continues to enter the corrugated tube, and pushes the mixed gas towards the valve where it is vented when the system becomes pressurized.
Figure 2.11 Functioning of the Bain Attachment
c) When the next inspiration is initiated, the patient gets ventilated first with fresh gas from
the inner tube and proximal part of the corrugated tube, and then with the mixture of fresh gas, alveolar gas and dead space gas. Many factors influence the composition of the inspired mixture. They are FGF, respiratory rate, expiratory pause, tidal volume and co2 production in the body (see page 2.13). During controlled ventilation unlike SR, most of the fresh gas could be utilized for alveolar ventilation without wastage, with a low respiratory rate and a long expiratory pause, large tidal volumes and FGF of 2 x MV. Importance of checking the inner tube Correct functioning of the Bain system depends on the integrity of the inner tube. The dead space of this system extends from the ETT to the patient end of the inner tube as long as it is intact. If the inner tube is damaged, the dead space will extend up to the point of damage resulting in an increased dead space and considerable re-breathing with hypoxia and hypercarbia. The inner tube must be checked before use. Visual check is not sufficient. The Pethick test must be performed.
2.14 F~r.J~ ... ~ .. ·:\Im"~ t:.·.·i.'l \C7
During controlled ventilation The system is initialy filled with fresh gas so the initial breath will be fresh gas.
a) During expiration, the expired gas mixes continuously with the fresh gas that is flowing into the corrugated tube at
PETHICK TEST FOR INTEGRITY OF THE INNER TUBE
• •
Fill the bag with the patient port occluded. Press on the oxygen flush whilst removing the occlusion
If the inner tube is intact, the flush of gas from the inner tube causes collapse of the reservoir bag by the Venturi effect
• If the inner tube is not intact, the reservoir bag will fill further ..•her.
Circle Circuit
The circle circuit allows re-breathing of expired gases by absorption of C02 •
One way flow prevents inspiration from the expiratory limb and expiration into the inspiratory limb. This is achieved by pressurising the inspiratory limb (by FGF) and de-pressurising the expiratory limb (APL valve and bag) as in the following arrangement
Uni directional valve
Fresh gas inlet
COi, y-~ ~~/ Hmb
Uni dlrectlon11I Villve
Figure 2.12 Components of the Circle system
I. Unidirectional valves on inspiratory (I) and expiratory (E) limbs (on either side of reservoir bag.
2. FGF inlet between absorber and (I) valve pressurising the inspiratory limb. If downstream from (I) valve, it bypasses patient in expiration and is wasted. If between (E) valve and absorber, FG is absorbed, and diluted by re-circulating gas.
3. APL valve on (E) limb, downstream from the (E) valve, just before the absorber, preferentially eliminates alveolar gas, conserves sodalime and reduces loss ofFG
4. Reservoir bag on the (E) limb reduces resistance to expiration. Bag compression during IPPV will vent alveolar gas through the spill valve, conserving absorbent in the cannister.
2.15
APL valve
-cenlsler ~ir -bag Expired air Inspired
Advantages of the circle circuit
• Economy of gases and volatile anaesthetic • Conservation of heat and humidity • Decreased pollution
Disadvantages of the circle circuit
• Complex system with multiple connections • When using low flows with a vapouriser outside the circuit o Concentrations of gases delivered to the patient are not those set by the flowmeters, and cannot be predicted due to varied uptake by soda lime and rubber, and dilution with expired gases.
o Monitoring of inspired gas concentrations is needed especially with low flows. o Anaesthetic gases are degraded and metabolites which accumulate during non use must be flushed before use.
o Hypoxia and awareness are possible. o Depth of anaesthesia cannot be changed rapidly.
C02 ABSORPTION "Soda lime" consists of 90% Ca(OH)2 5% NaOH, and 1 % KOH, with silicates to , prevent powdering. For effective absorption of C02 , moisture ( 14-19%) must be
incorporated within the granule. Soda lime granules are size 4-8 mesh to minimize resistance and allow plenty of surface for absorption. (4 mesh is 4 quarter-inch openings per inch and 8 mesh is 8 one eighth-inch openings per inch)
It should be packed well to avoid channelling (50% volume is granules and 50% air). Vertical positioning prevents channelling of gas flow down the edges.
Canister volume should be equal to or greater than the tidal volume. pH indicator for Durasorb is pink to white and for Baralime pink to purple.
Heat produced may increase the temperature to 60°C. Hot dry soda lime degrades anaesthetic agents (e.g. sevoflurane). 500g absorbs 125L of C02
and is exhausted in 2h. (large canister has 2kg).
Regeneration of soda lime occurs on standing due to migration of OH ions to the surface of granules.
FRESH GAS FLOWS HIGH GAS FLOWS (>SL /minute) Re-breathing is minimal and the absorber is unnecessary. SR : open the relief valve close to the patient (corresponds to Mapleson A) CV: open the valve close to the machine (corresponds to Mapleson D)
BASAL FLOWS(< lL) Absorber is essential. Fresh gas flow should equal the uptake of oxygen and volatile agent
Requirements • Low flow rotameters
Efficient vaporizers to supply high concentration
• Leak proof circuits • Intensive monitoring of the inspired gases and the patient to avoid hypoxia, hypercarbia, over-dosage, and awareness
LOW FLOW
Low flows < 2L I min Overcomes problems of special equipment and compensates for leaks. Should be monitored as for basal flows
Low flows < 3L I min The patient rebreathes more than 50% of expired gas. The concentration of inspired ga~es is intermediate between the concentration of the FGF and the exhaled gas that has passed through the absorber
LOW FLOW OR BASAL FLOW ANAESTHESI
I. Flush the circuit before use with the oxygen by-pass to vent nitrogen and all toxic gases which could accumulate during non-use
2. Give an initial FGF of at least 6L I min for 10 minutes at the start of anaesthesia, to stabilize the depth before reducing the FGF to low or basal flows
3. When reducing the flow rate: • Give an oxygen concentration of 40%. • Increase vaporizer setting according to the reduction ofFGF
4. If there is evidence of awareness, increase FGF temporarily for rapid correction, till the increase in vaporizer setting takes effect.
Rapid pre-oxygenation with 4/8 vital capacity breaths (fast track)
1. Flush the circuit by closing the patient end, opening the relief valve at the patient end and activating the oxygen flush. Open the rotameter to the maximum flow.
2. Apply the mask tight with no leaks, and ask the patient to take 4 maximum breaths in 30 sec or 8 in 60 sec.
3. Use the oxygen flush if the bag collapses during inspiration, till the increase in vapouriser setting takes effect.
CHECKING THE CIRCLE CffiCUIT Check integrity of the one way valves (Two bag test).
Attach a 2 litre bag to the patient end of the circle system. With a FGF of 6L/min :
1. Squeeze the bag on the machine : the bag at the patient end should fill, one way valve on the inspiratory limb should open, and one way valve on the expiratory limb should remain shut.
2. Squeeze the bag at the patient end : the bag at the machine end should fill, one way valve on the expiratory limb should open, and one way valve on the inspiratory limb should remain shut.
Leaks in the circuit with the absorber in the "on" position only, are due to leaks in the canister, due to overfilling or loose connections. If the leak cannot be corrected the circuit can still be used, but with high flows (5Umin) as given above. Check the efficiency of the soda lime by the colour of the indicator.