Functions of Anesthesia Reservoir Bag in a Breathing System

Professional article Zdrav Vestn | May – June 2017 | Volume 86 Metabolic and hormonal disorders id

Professional article

Functions of anesthesia reservoir bag in a breathing system

Miljenko Križmarić

Abstract Faculty of Medicine, This article addresses the different functions of anaesthesia reservoir bag in a circle anesthesia University of Maribor, breathing system. A main purpose of the contribution was to explain complex interaction between Maribor the reservoir bag and fresh gas flow during mechanical ventilation. The anesthesia reservoir bag is a collapsible gas container which is an essential component of most anesthesia breathing system. Correspondence: The anesthesia reservoir bag permits manual ventilation and acts as a visual or tactile indicator of Miljenko Križmarić, e: miljenko.krizmaric@ spontaneous breathing. The bag was excluded from traditional breathing system when the mechani- um.si cal ventilator was in use. Discrepancies between the set and actual tidal volume can occur. However, on some anaesthesia workstation systems, such as the Dräger Primus, the reservoir bag is an integral Key words: part of the breathing system during mechanical ventilation, where it serves as a reservoir for oxygen anesthesia breathing and anaesthetic gases. In mechanically ventilated patients, gases enter the reservoir bag from the systems; circle fresh gas flow during inspiratory phase, when the decoupling valve closes. The safe administration anesthesia breathing of general anaesthesia requires understanding of the technological advances in highly sophisticated systems; anesthesia reservoir bag; fresh gas anaesthetic equipment. decoupling Cite as: Cite as: Zdrav Vestn. 2017; 86:226–35. Zdrav Vestn. 2017; 86:226–35.

Received: 14. 9. 2016 Accepted: 5. 3. 2017 1. Introduction The anaesthesia reservoir bag is an The reservoir bag is most often placed important component in most breath- between the expiratory valve and CO2 ing systems used in anaesthesia. Figure absorber. Such a configuration allows 1 presents a traditional or classical circle for the dust from absorbent beads to be anaesthesia breathing system that com- pushed into the inspiratory limb during prises 7 different components: manual or mechanical ventilation (3). • inflow of fresh gases, As seen in Figure 1, the inflow of • inspiratory and expiratory unidirec- fresh gases at inspiratory phase is di- tional valve; rected to the patient through the inspi- • an inspiratory and expiratory corru- ratory valve, the inspiratory limb (tube) gated tubes (limbs) and the Y-piece. Expired air containing • Y-piece; carbon dioxide (CO2) flows through • adjustable pressure-limiting valve the expiratory limb through the expi- • CO2 absorber with absorbent; ratory valve and fills the reservoir bag. • reservoir bag (1‑2). When the pressure in the bag increases

Functions of anesthesia reservoir bag in a breathing system on line edition Metabolic and hormonal disorders

Figure 1: Traditional dotok svežih plinov anaesthesia breathing (Fresh gas ﬂow) system.

absorber CO2 ven�lator (Ven�lator) inspiracijska zaklopka (CO2 absorber) (Inspiratory valve)

inspiracijski krak nastavek Y (Inspiratory limb) (Y connect) preklopnik (Bag/ven�lator selector switch) ekspiracijski krak bolnik (Ekspiratory limb) (Pa�ent)

dihalni balon (Reservoir bag)

ekspiracijska zaklopka (Ekspiratory valve) razbremenilni ven�l (Adjustable pressure-limi�ng valve)

above the value set on the pressure- the tube is attached with greater force limiting valve, the respiratory gases and thus less likely to slip off. are released from the breathing system ISO 5362 is the international ISO stan- through this pressure-limiting valve dard that defines the characteristics of into the scavenging system. On expira- anaesthetic reservoir bags ISO 5362:2006 tion, the respiratory gases fill the reser- – Anaesthetic reservoir bags) (7). In ac- voir bag, and on inspiration these gases cordance with this standard, the maxi- return to the patient from the reservoir mum permissible gas leakage in reser- bag, thereby passing through the CO2 voir bags of original nominal volume of absorber, which absorbs carbon diox- 1 l is 10 ml/min. In reservoir bags with a ide (CO2). The valves direct the circular volume of more than 1 l, the maximum one-way flow of respiratory gases (4). permissible gas leakage is 25 ml/min. In contrast to the presented traditional The leakage is measured at a pressure of Circle-breathing system, different man- 3 ± 0.3 kPa. Reservoirs can be made of ufacturers may have individual compo- natural rubber and contain latex (often nents placed differently. reusable) or are made of latex-free mate- The disadvantage of the circular rials and are disposable (the note that the breathing system is in a large number of reservoir is latex-free should be indicat- different components where malfunc- ed on the packaging). The force that the tions can occur (5). These occur due to reservoir is required to withstand during incorrectly connected connection sites, the linear load testing of the reservoir leakage of gas, obstruction of the tube, neck is 40 ± 4 N (7). The tolerances for cracks in the filters, or because the tube the nominal volume range within ± 15 %. may accidentally slip off the coupling. Connection to the breathing system is When connecting the tube, it is neces- made by means of two connectors with sary to keep in mind that the tube should internal diameters of 15 mm (15F, female be pushed and rotated (the manufac- conical fitting) and 22 mm (22F, female turer in the user manual talks about the conical fitting). The material of the res- “push and twist” procedure). In this case ervoir must be antistatic (i.e. not prone on line edition Zdrav Vestn | May – June 2017 | Volume 86 Professional article

ration function is then taken over by a mechanical ventilator. 2. Reservoir bag and spontaneous breathing

The most important function of a reservoir bag during patient’s spontaneous breathing is the collection of respiratory gases. In the patient’s inspiration of 500 ml of gas for 1 second, a high average flow of 30 l/min (0.5 l × 60 s) is generated; the maximum values in the middle of inspiration are higher by Figure 2: Breathing to static electricity buildup) and should 30–40 % (12). The inflow of fresh gases reservoir bags of different volumes and not slip when held in hands. The material does not allow for such high flows, so couplings (Intersurgical should also not be permeable to inspira- the gasses must be collected in a respi- Ltd). tion anaesthetics and should not absorb ratory reservoir bag. During spontane- or adsorb them. The upper part of the ous breathing, the patient inhales fresh reservoir with the coupling is called the respiratory gases along with gases from “neck” and the opposite part is the “tail”. the reservoir bag, which are devoid of The tail, which is a tubular extension of CO2 as they pass through the CO2 ab- the reservoir, must be at least 2 cm long. sorber and the inspiratory valve, with When open (i.e. open tail), it functions the expiratory limb being closed via the as a relief mechanism in the linear Ma- expiratory one-way valve. During expi- pleson F breathing system. The reservoir ration, the reservoir bag is filled with ex- bag has an ellipsoid shape, which facili- haled air and the inflow of fresh gases as tates grip (7). the inspiratory valve is closed (Figure 1). Reservoir bags intended for paediat- When the reservoir bag is filled up to its ric use generally have a nominal volune nominal volume, the excess gases are re- of 0.5 or 1 litre (Figure 2, three lower res- leased through the adjustable pressure- ervoir bags), and for adults a volume of 2 limiting valve. In this case, the pressure or 3 l (two upper reservoir bags in Figure limiting outflow valve in the Dräger 2). The lower two reservoir bags in Fig- Primus anaesthesia workstation is in ure 2 have couplings with a standardised the position marked “Spont” (fully open internal diameter of 15 mm (15F) while valve). other reservoir bags have a standardised During spontaneous breathing, the internal diameter of 22 mm (22F). reservoir bag is a visual monitor; it is In the traditional circle breathing sys- slightly deflated on inspiration and re- tem, a reservoir bag has a function only filled on expiration. The reservoir bag is in spontaneous breathing or in manual also a tactile monitor, as its soft structure positive pressure ventilation. During enables us to feel its filling or emptying mechanical ventilation, it is excluded by touch (23). from the breathing system by bag/ven- The monitoring of spontaneous tilator selector switch (Figure 1) and in breathing depends on several factors: the this mode it has no function. The respi- shape of the reservoir bag, its size, the

Functions of anesthesia reservoir bag in a breathing system on line edition Metabolic and hormonal disorders

to that of spontaneous breathing. In as- sisted ventilation, the onset of inspiratory phase can be determined by tactile or visual monitoring of the reservoir bag. 4. Safety pressure

The reservoir bag has a safety function which protects the patient’s lung from barotrauma. It is the most flexible part of the breathing system. During gas filling, it is the reservoir bag that first shows evi- dence of filling. When the pressure limiting outflow valve is closed, the reservoir Figure 3: A protective level of inflation, the flow of fresh gases, bag is filled beyond the nominal volume, cage on the inside of the 22F coupling. and the patient’s respiratory capacity. but nevertheless it must maintain a safe- The accurate assessment of the pa- ty pressure within the limits below 60 cm tient’s respiratory volume cannot be H2O (7). Laplace’s law shows the correla- done simply by observing the reservoir tion between the tension on the surface bag (8). This can provide only a rough of the reservoir bag (T), the inside pres- estimate of the respiratory volume (12). sure (P) and the reservoir bag’s diameter On the neck of the reservoir bag, at (r). Mathematically, it can be presented the site of the tube connection, there is with the following equations (9‑11): a special plastic cage (Figure 3), which P · r 2 · T prevents that the upward turned and T = 2 ; P = r ; folded reservoir bag would clog up the connector and thus prevent gas filling. When the reservoir bag is filled with respiratory gases over the nominal vol- 3. Reservoir bag and ume, the pressure inside it starts to in- manual ventilation crease. Pressure rising (unit of force per surface:N/m2) is counteracted by the During manually controlled or as- opposite force, called surface tension or sisted ventilation, the reservoir bag is tension of the reservoir bag’s wall and pressed to create pressure in the circle is expressed as unit of force per length breathing system, which results in the (N/m). flow of respiratory gases from the reser- Figure 4 shows how the reservoir bag voir bag to the patient. In this ventilation has been inflated over the nominal vol- mode, the adjustable pressure limiting ume with a fully closed pressure limit- valve is only partly closed to provide the ing outflow valve (70 cm H2O). During pressure required for efficient ventila- inflation, the radius (r) of the reservoir tion. During inspiration and expiration, bag increases along with the surface ten- the excess gas is discharged through the sion (T), while the pressure remains con- adjustable pressure limiting valve (the stant, as can be seen from the Laplace’s pressure in the breathing system is then law equation. In this case, the pressure higher than the pressure set on the ad- measured in the Dräger Primus work justable pressure limiting valve). The station amounted to 34 cm H2O. Prior pattern of gas flow direction is similar to the experiment, we slightly stretched on line edition Zdrav Vestn | May – June 2017 | Volume 86 Professional article

nal value is used. The same pressure limits are required (between 3 kPa and 6 kPa) (7). Figure 5 shows the approxi- mate course of pressure change in the reservoir bag when it is being filled with anaesthetic respiratory gases (12‑13). The curve is divided into three phases. In the first phase we observe the initial filling up to the nominal value (2 l) with only a slight increase in pressure. When the volume reaches the nominal volume, the pressure starts to increase rapidly and soon achieves the maximum value (small radius of the reservoir bag, high pressure). In the third phase that fol- lows, the pressure reaches a plateau and is relatively constant. After an initial high pressure, all reservoir bags undergo pressure release after 1–5 seconds (13). This pressure that protects (the lungs) against barotrauma must not exceed 40–50 cm H2O (14). The reservoir bag bursts when the surface tension is too high for the material to withstand it. Physical characteristics of reservoir bags slightly dif- fer among producers (13). Some achieve high initial pressure values (Figure 5, curve a), others are more flexible and therefore the pressures are lower (Figure Figure 4: Reservoir bag’s safety function – the maintenance of a constant 5, curve b). The reservoir bag however pressure in the respiratory system (example in the figure: pressure inside the does not meet the standard if it does not reservoir bag: 34 cm H2O, diameter: 66 cm, volume: 150 litres). reach the minimum required pressure of the Intersurgical’s reservoir bag, so that 30 cm H2O (Figure 5, curve c). we inflated it to several times its nomi- Modern anaesthesia workstations nal volume (2 l). Before the reservoir bag have an extra safety automatic mecha- burst its diameter extended to 66 cm, nism against barotrauma in manual op- which corresponded to a volume of ap- eration. The Dräger Primus anaesthesia proximately 150 litres. workstation activates an alarm of the In establishing compliance with ISO highest priority (pressure high) if on 5362 standard, a reservoir bag is filled continuous inflation of the reservoir with water at four times the nominal bag, the pressure set as the upper limit value. At this experiment, the pressure for alarm by the user is exceeded (15). should not be lower than 3 kPa = 30 hPa If the pressure is below the upper level (approximately 30 cm H2O) and neither of alarm, this triggers an alarm of con- higher than 6 kPa = 60 hPa (approxi- tinuous pressure that also represents mately 60 cm H2O). When filling a res- the highest priority (red colour), thus ervoir bag with air, two times the nomi- requiring immediate action. Accord-

Functions of anesthesia reservoir bag in a breathing system on line edition Metabolic and hormonal disorders

tlak [cmH2O] anaesthesia workstation and METI HPS 60 a simulator were used. 50 The higher the flows of fresh gases are set, the higher the pressure in the breath- 40 ing system is achieved when the pressure b relief valve is closed. Flows exceeding 8 l/min can create a high dangerous pres- 30 c sure in 20–30 seconds (16). 20 5. Reservoir bag and mechanical ventilation 10 In the traditional circle anesthesia breathing system, during mechanical ventilation, the reservoir bag is isolated 2 4 6 8 10 12 with a special selector switch (Figure 1). volumen [l] In such a positioning of the components, we note a discrepancy between the set Figure 5: The course of pressure in filling reservoir bags with different levels breathing volume and the actual one that of flexibility. The most flexible one (C) does not reach the required pressure as that with low material flexibility (a). Generally reservoir bags do not exceed the occurs due to the ventilator-fresh gas pressure within the range of 40to 50 cm H2O). (Source: adapted from 13). coupling. Fresh respiratory gases from the inflow of fresh gases are pooled with ing to the standards, this alarm must gases from the mechanical ventilator. be triggered after 15 seconds of con- Example: at a set tidal volume of 500 ml, tinuous pressure (23). As the last safety a respiratory rate of 10 min-1 and the in- mechanism, the device itself reduces spiration/expiration rate I:E = 1 : 2, inspi- the pressure by automatically relieving ration lasts 2 seconds. If the flow of fresh the breathing system. At that time, the gases is 6 l/min (100 ml/second), the de- device alerts us with a medium level sired tidal volume (500 ml) is enhanced alarm (blue colour) and a “pressure re- by additional 200 ml on inspiration. lief” display. Unlike the previous alarm, Thus, a pooled volume of 700 ml is de- this one takes twice as long before it livered to the patient (200 ml more than goes off, i.e. it is triggered after 30 sec- we set on the ventilator). In most mod- onds. The duration of relief depends ern anaesthetic ventilators, the volume on the preset flow of fresh gases. With delivered to the patient is independent a flow rate between 14 l/min and 18 l/ of the flow of fresh gases. There are three min, the relief lasts 30 seconds, then it approaches to solving this problem: is interrupted and the pressure slowly • fresh gas decoupling; increases again for 30 seconds. This is • fresh gas compensation; followed by reopening of the valve. The • fresh gas interruption (17). system is relieved in 30 seconds. With lower flow rates (below 14 l/min), the Hereinafter, we will limit our discus- relief duration was shorter, i.e. 1–3 sec- sion to fresh gas decoupling, which is onds. This quantitative experiment was schematically shown in Figure 6. This carried out with multiple repetitions mode of ventilation is the only one in the Simulation Centre of the Medi- among the mentioned three approaches cal Faculty of Maribor. Dräger Primus that influences the reservoir bag during on line edition Zdrav Vestn | May – June 2017 | Volume 86 Professional article

mechanical ventilation. Figure 6 shows A few minutes after the replacement the flows of respiratory gases during in- it was found that the reservoir bag was spiration, when the decoupling valve of empty, and the concentration of oxygen, fresh gases is closed. Gases from the ven- nitrogen oxide and inspiration anaes- tilator are directed into the inspiratory thetic had decreased. The error in the limb to the patient. Due to the closed reservoir bag was detected when they decoupling valve, the flow of fresh gases, switched from mechanical ventilation to instead of being added to the flow from manual ventilation. During expiration, the ventialtor, is directed into the reser- the surrounding air entered into the res- voir bag through the absorber. Thus, the ervoir bag and diluted the respiratory reservoir bag functions as a collector of gases, so the proportions of O2, N2O and fresh gases, similarly as in manual mode anaesthetics decreased accordingly. But of ventilation. During the expiration despite the hole in the reservoir bag, the phase, the engine in the electric ventila- alarm on the device was not triggered. tor moves the piston upwards and pulls Therefore, we must pay attention to the into the cylinder fresh gases together values obtained from the monitoring of with gases from the reservoir bag and respiratory gases. the expiratory arm of the breathing sys- A similar case was described by Ku- tem. ruma et al. in 2013, where problems due And where can problems arise? Two to the change of CO2 absorber occurred scenarios are possible. In the first, the res- during surgery (20). Recent versions of ervoir bag that is damaged or punctured, absorbers, such as Dräger CLIC, allow it so that gases leak out of it, can unexpect- to be replaced during operation of the edly detach from the system. The inflow appliance. In this case, the inflow of fresh of fresh gases that fills the reservoir bag gases was set at 4 l/min, and the concen- escapes into the surroundings during in- tration of sevoflurane to 1.5 %. When the spiration and thus contaminates the op- absorber was replaced, the proportion of erating theatre with anaesthetics. During oxygen in the inhaled air decreased from expiration, the ventilator draws air from FIO2 = 38 % to 34 %, the proportion of the operating theatre through the dam- sevoflurane decreased from 1.4 % to 1.1 %, aged reservoir bag. The respiratory gases and the bispectral index (BIS) increased, are diluted because they are mixed with which indicated a lower depth of anaes- the air from the atmosphere. The patient thesia. In the operating theatre, the staff thus receives a lower fraction of oxygen noted an anaesthetics smell. This case on inspiration (FIO2) and a lower pro- also shows that attention should be paid portion of inhaled anesthetic. In 2004, to the reservoir bag during mechanical Sandberg and Kaiser (18) described a ventilation. case that occurred in a Dräger Fabius GS The second potential scenario deals anaesthesia workstation (19), where an with a case whee the reservoir bag is operation was already underway when under-filled. In such a case, problem oc- the patient was found to be allergic to curs during expiration when the ven- latex, and therefore the latex reservoir tilator tries to pull the gases out of the bag was replaced with a latex-free one. empty reservoir bag. This scenario was The replacement bag had a hole, but the simulated at the Simulation Centre us- leakage through it could not be detect- ing Dräger Primus anaesthesia worksta- ed, because the device cannot perform tion and a METI HPS-simulator (CAE checking during mechanical ventilation. Healthcare, Sarasota, Florida), where

Functions of anesthesia reservoir bag in a breathing system on line edition Metabolic and hormonal disorders

Figure 6: Fresh gas decoupling during motor inspiration. dotok svežih plinov (Fresh gas ﬂow) ven�lator bat (Ven�lator) razdruževanje svežih cilinder plinov (Fresh gas decoupling)

absorber CO2 (CO2 absorber)

inspiracijska zaklopka (Inspiratory valve)

nastavek Y (Y connect) bolnik (Pa�ent) dihalni balon (Reservoir bag) ekspiracijska zaklopka (Ekspiratory valve)

the flow of fresh gases was adjusted breathing. Negative pressure would also to the lowest possible value of 200 ml/ be obtained with spontaneous breathing, min of air in manual mode of spontane- in the event that the tube to which the ous breathing (42 ml/min of oxygen). reservoir bag is connected is either fold- The simulator had oxygen consump- ed or obstructed. In the case of mechani- tion set to 250 ml/min, while the deliv- cal ventilation, only the alarm (FGLOW ery was six-fold lower (below metabolic OR LEAK) would be detected, as the pis- flow). Due to greater oxygen consump- ton of the electric ventilator would stop tion than delivery, the partial pressure when pulling gasses from the reservoir of oxygen in the reservoir bag was de- bag and thus would not create a negative creasing and consequently the reservoir pressure (this applies to the Dräger Pri- bag shrank. The appliance emitted an mus anaesthesia workstation). alarm of low fresh gases flow (FG LOW In 2015, Vinay et al. described a case OR LEAK) and low proportion of oxy- when using the Dräger Primus anaesthe- gen in inhaled breath (INSP O2 LOW). sia workstation (21), the position of the When the simulator tried to pull gases head and neck of a nine-year old child from the empty reservoir bag at inspi- was changed during surgery, and as a ration, it caused negative pressure and result, the tracheal tube moved. Due to triggered an alarm (PRESSURE NEGA- this movement, the tracheal tube cuff TIVE). In this alarm, the manufacturer was no longer functioning and the respi- recommends increasing the flow of fresh ratory gases started to leak. The respira- gases and, if necessary, using the Oxygen tory volume (VT) decreased from 280 ml flush. The negative pressure occurred as to 200 ml. An alarm (LOW FRESH GAS a result of the simulator’s spontaneous FLOW) was triggered. The patient was

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maintained on mechanical ventilation ative pressure that pulls gases from the with a low flow of fresh gas set at 1 l/min. reservoir bag. In an ascendant ventilator On the onset of alarm, the proportions (standing bellows) fresh gas decoupling of oxygen, nitrogen oxide and inspira- is not feasible because expiration ends tion anaesthetics did not change. They with a positive pressure (5,22). detected an error in the tracheal tube Extreme caution is required when us- cuff and re-inflated it. They also found ing high oxygen flush during mechani- that the reservoir bag was emptied. Oxy- cal ventilation. Oxygen flush activation gen flush activation followed, and due to (35–75 l/min) during inspiration can mixing of respiratory gases with 100 % cause barotrauma when using anaesthe- oxygen, the fraction of inspired oxygen sia workstation that do not have a built- (FIO2) increased from 40 % to 69 %. At in fresh gas decoupling system. In fresh the same time, the proportion of isoflu- gas decoupling systems, the high flow rane decreased from 1.0 % to 0.7 %. A during inspiration is directed to the res- decrease in the proportion of nitrous ox- ervoir bag and not to the patient. In this ide was noted too. In the discussion, the case, oxygen flush can also be used dur- authors summarise the incorrect conclu- ing inspiration (23). sions that due to the empty reservoir bag Fresh gas decoupling systems are in- during expiration the ventilator creates tegrated in the following anaesthesia a negative pressure that opens the valve workstations: Dräger Fabius GS, Dräger on the ventilator itself and allows the Narkomed 6000, Dräger Narkomed entry of external air from the operating 6400, Dräger Primus, Dräger Apollo, theatre. Therefore they expected that af- Dräger Zeus and Datascope Anes- ter mixing with air the gases would be tar (22,24). diluted, which however did not happen. A fresh gas compensation system for The anaesthetic ventilator on the Dräger solving the problem of discrepancy be- Primus system does not have a valve tween the set tidal volume and the actual that would open if a negative pressure one is used in the following anaesthesia was generated (the authors apparently workstations: GE Datex-Ohmeda Aesti- believed that it had). On expiration, the va/5, GE Datex-Ohmeda ADU Caresta- electric piston does not move to the val- tion, GE Aisys CS2, GE Avance CS2, and ues that would create a negative pressure. GE Aespire 7100/7900 SmartVent. These The protection against negative pres- systems carry out continuous measure- sure on the Dräger Fabius GS devices ment of volume and flows of respiratory is slightly different (NEGATIVE PRES- gases during inspiration and expiration, SURE RELIEF); there the valve opens and continuously adjust the volume gen- when the negative pressure reaches a erated by the ventilator (22). In these sys- value between -7.5 and -9 cm H2O. Then tems, the reservoir bag has no function air is released from atmosphere into the in controlled mechanical ventilation. ventilator cylinder, which dilutes the re- The third option for solving the prob- spiratory gases (22). lem of discrepancy in the respiratory Fresh gas decoupling is possible only volumes is fresh gas interruption (17). when using a ventilator with an electric This method is used in the Dräger Julijan drive (piston and cylinder) or a descen- anaesthesia workstation (25). In this case dant ventilator (hanging bellows). In too, the reservoir bag has no active role both cases, it is possible to achieve a neg- in controlled mechanical ventilation.

Functions of anesthesia reservoir bag in a breathing system on line edition Metabolic and hormonal disorders

6. Conclusion tems the reservoir bag is an active component of the breathing system during When using modern anaesthetic mechanical ventilation and it functions workstations, we have to know to some as a respiratory gas collector unit. It is extent their function in order to ensure important to monitor it, particularly the patient’s safety. Modern systems in the case that NEGATIVE PRES- have built-in new technologies that SURE or FRESH GAS LOW alarms are ensure accurate respiratory volumes, triggered. The reservoir bag also has however, there are differences between a monitoring function in mechanical traditional systems. It should be point- ventilation. ed out that in fresh gas decoupling sys-

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