Breathing circuits and systems

The definition and classification of the methods for administering inhaled anaesthetics have undergone continuous changes since the onset of anaesthesia. The definitions and classifications described here are the ones currently used in most anaesthesiology literature.

DEFINITIONS

• Rebreathing. Rebreathing in anaesthesia systems conventionally refers to the act of inhaling some or all of the previously exhaled gases, including carbon dioxide and water vapour;
• Dead space of the equipment. This refers to the volume inside the device which contains gas exhaled by the patient which will be rebreathed at the next inspiratory phase.

CLASSIFICATION OF BREATHING SYSTEMS

These can be classified according to function:

• Systems without rebreathing that use non-rebreathing valves;
• Systems in which rebreathing is possiblebut is usually prevented by the flow of fresh gases through the system;
• Systems without rebreathing that use carbon dioxide absorbers;
  • one-way systems (rotary);
  • bi-directional (to and fro);

Systems without rebreathing
The easiest way to provide a constant supply of fresh gases to a patient is to use a system that has one or more non-rebreathing valves. The fresh gases enter the system through the inspiratory limb. The tubes that convey the gas to the patient must be designed to include several features: they must minimise resistance to inspiration; they must not collapse as a result of inspiratory depression; it must be possible to bend them without them kinking; lastly, their inner surface must have no roughness. All these requisites are usually met by using a thin plastic tube to which a spiral is attached to the outer surface. The fresh gases that enter are sucked by the patient's inspiratory effort or insufflated under controlled ventilation. The entry of the gases is enabled by an inspiratory valve that prevents, upon the next expiration, the CO₂-rich gas from mixing with the fresh gas. This same valve, or a different one, acts in such a way that the inhaled gas does not come from the expiratory limb during inspiration.

The use of a device such as the one shown in Fig. 1, requires the presence of a reservoir bag connected to the inspiratory limb which also collects the fresh flow. The bag can also be manually squeezed to provide assisted or controlled ventilation since the non-rebreathing valve is as effective in this modality as it is in spontaneous ventilation. It goes without saying that, to prevent the reservoir bag from collapsing, the fresh gas flow (FGF) must not be less than the minute volume required by the patient.

Systems in which rebreathing is possible
An assortment of breathing systems has been developed by the pioneers of anaesthesia (mostly on an intuitive basis); these circuits allow patients to inhale gases and then eliminate these gases from the circuit. The elimination of carbon dioxide is achieved by the flushing action of the fresh gases introduced into the breathing systems rather than by separating the gas mixtures inspired and expired by a valve as described above. If the fresh flow decreases, the circuit allows rebreathing.

In 1954 Mapleson classified these systems using letters A to E, based on their capacity to remove carbon dioxide (efficiency) during spontaneous ventilation. An F system (the Jackson Rees circuit), which modifies the E system (Ayre’s T-piece ), was later added to the classification by Willis in 1975 (see below).

The Mapleson classification of breathing systems
These systems invariably contain the same components but arranged in different positions: arrival of fresh gas, connection to patient, adjustable pressure limiting (APL) valve and reservoir bag. The circuit has different characteristics depending on how these parts are combined (Fig. 2).

   Fig. 2

The efficiency of each system is, however, different. They are listed (A, B, C, D, E, F) ranked on the basis of the FGF requirement, from the lowest to the highest, to prevent rebreathing during spontaneous ventilation. An A-system requires 0.8 to 1 times the minute ventilation of the patient;  systems B and C require 1.5 to 2 times the minute ventilation of the patient; systems D, E and F (with similar functionality) require 2 to 4 times the minute ventilation of the patient to prevent rebreathing during spontaneous ventilation.

OPERATING PRINCIPLES OF BREATHING SYSTEMS

Mapleson A
Mapleson A is a breathing system that explains the functioning of the Magill circuit, named after Sir Ivan Magill who invented it in 1920. It is commonly found in arrangements for adult human patients, but it also exists for paediatric patients.

The Magill system consists of the following elements:

• The fresh gas inlet connected to a 2 litre reservoir bag. (The exhaled gas should never reach the reservoir bag, if rebreathing is to be avoided!). This is connected to:
• A corrugated breathing tube (minimum length of the circuit for an adult human patient is 110 cm with an internal volume of 550 ml). This represents slightly more than the average tidal volume of an anaesthetised adult who is breathing spontaneously. This volume is critical to prevent alveolar gas from reaching the breathing balloon, which would cause rebreathing. This is in turn connected to:
• An APL valve for the elimination of exhaled gases. This valve is present in proximity to the connection to the patient's tracheal tube.

The system makes efficient use of fresh gas during spontaneous ventilation. This can be explained by examining its function during a breathing cycle consisting of three phases: inspiration, expiration and an end-expiratory pause.

• First inspiration.The reservoir bag and the breathing tube have been filled with fresh gas and the patient is about to take his/her first breath. The entire system is, therefore, full of fresh gas. As the patient inhales, the gases are sucked into the lungs with a FGF flow rate that is higher and so the reservoir bag is partly emptied.
• Expiration. The patient begins to exhale, and since the reservoir bag is not full, the exhaled gases travel back up through the corrugated tube, pushing the fresh gases from the tube to the reservoir bag. However, before the exhaled gases can enter the reservoir bag (hence the importance of the inspiratory tube length), the APL valve must open and vent most of the exhaled gases. In order for this to happen, the reservoir bag must have been at least partially filled by the FGF, so that when the bag was full, the APL valve would open to let out the exhaled breath.

The first part of the exhaled gas that fills the corrugated tube comes from the anatomic dead space which does not, therefore, contain carbon dioxide. This gas is heated and slightly humidified, which is an ideal situation for it to be inspired again. The second part of the exhaled gas, however, is loaded with carbon dioxide and may cause rebreathing. Some of this gas travels into the corrugated tube, and some is eliminated directly by the APL valve; however, during the expiratory pause, the fresh flow pushes the exhaled gases from the corrugated tube to the vent through the APL valve. It follows that the expiratory pause appears to be important, so that the patient does not suffer from rebreathing. When the patient inhales again, the gases coming from the anatomical dead space can be re-inspired without causing rebreathing. This means that the FGF values may be even smaller than the patient's minute volume.

In theory, if fresh gases did not mix with exhaled gases, and if the expiratory pause was long enough, the fresh flows might simply be equivalent to alveolar ventilation (about 66% of the minute volume). In this situation, only the alveolar gas would be eliminated through the APL valve. In practice, however, a number of factors determine the need for higher flow rates (70-90% of the minute volume). For example:

• there is actually a mixing of the different gas interfaces, which reduces the theoretical efficiency of the system;
• occasionally, especially in large dogs, the tidal volume may be greater than the volume of the corrugated tube and it is possible for the exhaled gas to reach the reservoir bag; in this case the carbon dioxide will contaminate the reservoir bag and consequently the gas for the next inspiration;
• a very high respiratory rate will reduce or eliminate the expiratory pause, thereby reducing the potential for elimination of carbon dioxide made possible by the pause.

How does it work? 

Magill (Mapleson A) with controlled ventilation
The dynamics of the above mentioned gases concerns the use of Mapleson A in spontaneous respiration. However, when controlled ventilation is used, there are some changes to the operating dynamics of the circuit:

• Inspiration phase. The APL valve must be kept closed to enable the development of sufficient pressure in the pulmonary system. During the first inspiration phase the anaesthetist squeezes the reservoir bag and some fresh gas is vented by the half-open APL valve.
• Expiratory phase. After inspiration, the reservoir bag may be almost empty and as soon as the anaesthetist releases it, the patient exhales into the corrugated tube. The exhaled gases, whether from anatomical dead space or from the alveoli, travel back up the corrugated tube in a greater amount than in spontaneous ventilation. Part of this carbon dioxide-rich gas can also reach the reservoir bag. In this situation, the  APL valve does not open or  its opening is delayed. When the anaesthetist squeezes the bag again, rebreathing will be very likely. To avoid this, the fresh gas must be increased to eliminate the exhaled gases before the ensuing inspiration but, more importantly, to prevent this gas from reaching the reservoir bag. This increase usually brings a fresh gas flow that is twice the minute volume of the patient. This situation increases the waste of fresh gas and the risk of contamination in operating rooms.

Other circuits that function like Mapleson A

Coaxial parallel Lack circuit (Fig. 3)
This is basically a Magill system in which the APL valve is brought close to the reservoir bag by using a corrugated tube similar to the one that connects the reservoir bag to the patient's mouth. This expedient represents an improvement of the Magill system and is intended to overcome the following drawbacks:

• reduced access to the APL valve by the anaesthetist who must go under the drapes when the patient's  head or neck is being operated on;
• significant traction of the circuit on the tracheal tube due to the weight of the valve and the torsional stiffness that the system can acquire, when the valve is connected to the vent tube

The Lack system overcomes these two problems. The original version was built with a coaxial circuit; the inhaled gas travels in the outer tubing while the exhaled gas travels in the inner tubing. All this is less cumbersome to use and the tracheal tube (if the tubes are not too stiff) is subject to less stress. The valve is placed near the anaesthetist so it is easily accessible in all circumstances and does not drag on the tracheal tube. Due to the (partial) separation of the inspiratory and expiratory flows, the Lack system is deemed marginally more efficient than the Magill system.

How does it work?

      Fig. 3

Parallel Lack system (Parallel Lack)
Coaxial breathing systems carry the risk of the inner tube detaching or breaking without the anaesthetist realising it. Such an event would lead to an extreme alteration of the system's functionality and dramatically increase the dead space. To avoid this risk the parallel Lack is available with two separate tubes, one for inspiration and one for expiration.

   Fig. 4

Systems B and C
In these systems the arrival of fresh gas, the reservoir bag and the APL valve are closer to the patient. One of the circuits that follows the Mapleson C arrangement is the to-and-fro circuit. This circuit is widely used in Italy and since it is compact, it can be taken when necessary from the anaesthesia trolley simply by extending the fresh gas tube and the vent tube. However, as mentioned above, these systems are not as efficient as the Mapleson A. The reasons are outlined below for the to-and-fro circuit:

• The first inspiration. The system is initially filled with fresh gases so that during the first inspiration, the patient breathes only fresh gases.
• Expiration. During expiration, the exhaled gases (gases of the dead space initially and then the first part of the alveolar gas), mixed with the fresh gases, go to the reservoir bag. When it has been filled, the rest of the exhaled gases (the rest of the alveolar gas) and the FGF are eliminated by the APL valve.
• End-expiratory pause. During this phase, the fresh gas flows from the APL valve because it is closer to the reservoir bag.
• The next inspiration. The inhaled gas comes from the reservoir bag and is a mixture of fresh gas, gas from the anatomical dead space and alveolar gas, the proportion of which will be determined by the level of FGF and the expiratory pause. If the FGF is high and the rate of expiration is slow, there will be a greater amount of fresh gas during inspiration.

The “to and fro” circuit is one of the most popular circuits in veterinary anaesthesia in Italy. Its major [11]advantage is its compactness which facilitates its use. This advantage, besides its low cost, is counteracted by a number of disadvantages that should be kept in mind. The reservoir bag provides the gases for inspiration and also collects the alveolar exhaled gas. This means that rebreathing is difficult to avoid even with high flows, and consequently the system is not very efficient (use with vFGF at 2-3 times the minute volume of the patient). Its small size means that the circuit is often covered by drapes and is, therefore, less accessible to the anaesthetist. It is not suitable for smaller patients because of the stress it makes on the tracheal tube. This usually does not depend on the weight of the circuit, which is minimal, but the fact that the exhaled gas tube and the fresh gas tube are often stiff or heavy, thereby increasing the rigidity of the system. In conclusion, the author does not recommend using this circuit, believing that its widespread use is due to a lack of knowledge of other available circuits and not the result of any real benefit that the circuit has to offer.

Mapleson D system 
The Mapleson D system with spontaneous ventilation
This system is best explained if, once again, the three phases of the breathing cycle are considered separately: inspiration, expiration and the end-expiratory pause:

• First inspiration. FGF enters as close as possible to the patient's tracheal tube  (to reduce dead space) and the system is filled in such a way that during the first inspiration, the patient breathes only fresh gases. The gases are inspired from the tube connected to the reservoir bag.
• Expiration. During expiration, the exhaled gases, mixed with FGF, enter the tube connected to the reservoir bag. When it is filled, the rest of the exhaled gases and FGF are released through the APL valve.
• End-expiratory pause.At this stage the FGF pushes the exhaled gases to the reservoir bag and through the APL valve. The longer the pause between the first two above-mentioned phases, the lower the likelihood of rebreathing, FGF being equal. Another important aspect is that the tube connected to the reservoir bag should have a volume greater than the patient’s tidal volume. If the inhaled gas comes from the reservoir bag, rebreathing is inevitable. The FGF adopted in this modality is about three times the patient’s minute volume.

The Mapleson D system with controlled ventilation
Controlled ventilation makes this system more efficient because the respiratory rate and the expiratory pause can be set and optimised. A low respiratory rate, increased tidal volume and prolonged expiratory pause increase the efficiency of the system, a fact demonstrated by the  decrease in FGF which prevents rebreathing when it reaches twice the minute volume (there are some reports of the use of FGF at 0.7 times the minute volume). The system can be optimised in such a way as to occupy the anatomical dead space with exhaled gases. Obviously this would not cause rebreathing, however, the boundary between fresh and exhaled gas is not clear and a certain amount of mixing could also be caused by the turbulence of the flow.

The Mapleson D circuit can, therefore, be used advantageously in controlled ventilation both because of its greater efficiency and because its connection to the ventilator is very simple. The reservoir bag can be removed and replaced by a corrugated tube of sufficient capacity to accommodate the air or oxygen delivered by the ventilator, thereby preventing this gas from reaching the patient and diluting the anaesthetic gases.

The Bain system
The Bain breathing system (Fig. 6)  is similar in function to the Mapleson D system. The only difference is that the FGF is conveyed by a tube inside the corrugated flexible tube (coaxial circuit). This type of Mapleson D has the advantage of being compact, but if the inner tube is torn or detaches from the distal connection, it can have serious consequences. In this case the whole volume of the circuit between the disconnection and the tracheal tube  becomes dead space. In theory, since the inner tube is narrow, it might kink and stop the FGF to the patient.

Several breathing systems have been described which have a toggle switch that converts the system from a Mapleson A to a Mapleson D or E, allowing a choice of the system that is best suited to the type of procedure or patient. The Humphrey ADE system (Anaequip UK) seems to be one of the most popular versions in the Anglo-Saxon world.

The Humphrey system
This circuit consists of a metal block with an APL valve, two connectors, one for the inspiratory limb and one for the expiratory limb, plus two additional connectors to attach to a reservoir bag or the outflow of a ventilator. A lever with two positions connects the various components of the block in different ways so as to give rise to a Parallel Lack or Mapleson D system, which makes it easy to provide controlled ventilation. A canister of soda lime with one-way valves can be connected to the connections for the inspiratory and expiratory limbs, thereby converting the system into a rotary one. The advantage of this solution is that the lever allows a rapid change in the configuration from reservoir bag use to ventilator use. The system is flexible and adapts well to different clinical needs. The major drawbacks are the exorbitant cost and, in the configuration with soda lime, the fact that if the circuit is fixed to a fresh gas outlet, it must necessarily bear a substantial weight.

Mapleson E breathing system with spontaneous ventilation
With this system, fresh gases arrive close to the tracheal tube while the patient inhales and exhales into a corrugated tube with a larger diameter. In the original version, this tube lacked the breathing bag. The system, which follows the classic arrangement described above, was the Ayres T-piece, a circuit for paediatric use. This circuit allowed the gas at the end of the cycle to be released directly into the operating room. Nowadays, such a solution would obviously be unacceptable and, therefore, it makes more sense to refer to it as a modified T-piece [16]  (Fig. 7) which, with the addition of a reservoir bag and an APL valve, exactly follows the Mapleson D arrangement. For a description of how it works the reader is, therefore, referred to the description given for the Mapleson D system. In addition, it should be kept in mind that, unlike the Bain circuit, the T-piece can be used in neonatal or paediatric patients. The salient features are a reduced dead space, the gentle traction exerted on the tracheal tube, compactness, the presence of a reservoir bag far from the patient and, like the Bain system, ease of use with ventilators. The name T-piece comes from the fact that the connection between the patient, the inspiratory/expiratory limb and the fresh gases form an actual T, very close to the patient. The arrival of fresh gas constitutes the perpendicular limb of the T, and this solution prevents excessive expiratory pressure in cases in which the gases are directed towards the connection with the patient, or depression due to the Venturi effect in cases in which they are travelling in the opposite direction. As with the other circuits described above, the volume of the inspiratory/expiratory limb must be greater than the patient's tidal volume to avoid rebreathing.

Controlled ventilation with a T-piece
Before introducing the aforementioned changes, controlled ventilation was performed by the anaesthetist who used his or her thumb to occlude the inspiratory/expiratory limb during inspiration and then release it during expiration. Using the modified circuit, the anaesthetist performs the same manoeuvres as those discussed for the Bain circuit. As with the Bain circuit, controlled ventilation makes it possible to set the ventilation times, thereby making the system more efficient. There are many reports that the FGF can vary from more than three times the tidal volume during spontaneous breathing to less than twice the tidal volume during controlled ventilation. All paediatric circuits that have an APL valve should have it at a maximum opening pressure of 30 cmH₂O. This reduces the risk of barotrauma. It is worth remembering that the APL valves fitted on circuits intended for adult human patients have an opening pressure of about 60 cmH₂O.

T-piece with Jackson-Rees modification (Mapleson F system)
This system is only mentioned here for historical completeness. It was designed as a Mapleson E with a reservoir bag added to the end of the inspiratory/expiratory tube. The balloon was cut at the narrowest part to allow the gases to vent. By occluding this opening, the anaesthetist could manually ventilate the patient. The opening of the balloon was occluded by a valve connected to a tube carrying the vented gases. The bag also made it possible to check the patient's breathing activity visually.

     
           Fig. 8

Rotational (or circular) system
The circular system makes it possible to re-use most of the exhaled gas containing oxygen, water vapour, anaesthetic gas and heat. This means that the inhaled gases are partly fresh gases and partly gases already exhaled. The benefits include using less anaesthesia and the inspiration of gases that are already humidified and heated; moreover, the dead space is usually limited by the presence of one-way valves. The disadvantages are the overall size (although modern  anaesthesia trolleys  have built-in rotational circuits), the higher initial cost, the presence of moving parts (one-way valves) that could malfunction, the need to replace the soda lime periodically and a certain unpredictability in the percentage of inhaled anaesthetic gases in relation to the settings of the vaporizer. A rotational circuit is composed of two corrugated tubes (inspiratory and expiratory limbs) connected with a Y-connector, two one-way valves (inspiratory and expiratory), a canister containing material that removes the carbon dioxide from the exhaled gas, an expiratory valve and a reservoir bag.

How does it work?

   Fig. 9

Each of the components described above can have different characteristics depending on the intended use of the circuit. In general, an increase in the internal volume of the circuit decreases the resistance to the inspiratory and expiratory flows, decreases the time between replacing the carbon dioxide absorbent and increases the amount of anaesthetic required to achieve the balance between patient and circuit. This means that any variation of the concentration of anaesthetic will occur more slowly and that the anaesthetist must, therefore, increase the percentage of anaesthetic contained in the fresh gases and/or increase the FGF.

Another critical aspect of circular systems are the valves. These allow the gases to move in a single direction in the circuit and also make it possible to separate exhaled gas from gas with no carbon dioxide. They are also the only moving components of the circuit (besides the gases), so their size, the material they are made of and whether they are vertical or horizontal makes them more or less suitable for different patients. The mechanical characteristics of the valves are important precisely because they cause inertia to movement and hence delay in closing or opening the valves, resulting in an effort by the patient's respiratory muscles to activate them.  Any situation that makes the valves less mobile may result in serious consequences for the proper functioning of the circuit, with a more or less severe onset of rebreathing. Each rotational circuit should be properly designed to allow the movement of the valves to be controlled during their use and whenever an intervention to a valve is necessary it should be promptly disassembled.

The reservoir bag as well as the APL valve and the arrival of fresh gases can be placed in different positions within the circuit. This means that the characteristics of the various circuits can vary slightly depending on the choices made by the manufacturer. To cite an example, if the respiratory bag is placed between the expiratory valve and the soda lime canister, the resistance to expiration will be low, but during the inspiratory phase patients must increase their efforts to ensure that the gases pass through the canister. The opposite situation occurs when the bag is placed between the inspiratory valve and the canister. The APL valve should be located near to the expiratory valve so that the exhaled gas escapes rather than the gas filtered through soda lime. However, not all circuits have this device and consequently the gases already purified of carbon dioxide gas can escape from the APL valve and sometimes the fresh gases too. Even the entrance of fresh gases can have different effects depending on where it is located. For example, if the gases were to cross the soda lime to reach the reservoir bag, the likelihood of drying the lime increases with all the consequences described below. Even the amount of fresh gas can alter the functioning of the circuit. Examples of malfunctioning are listed below:

• A high FGF rate will increase the flow in the inspiratory part of the system, thus reducing any resistance to inspiration, but the resistance to expiration will increase because of greater resistance to the opening of the expiratory valve and the APL valve.
• The opposite occurs with low gas flow rates. A low FGF will also increase the relative humidity, thus increasing the “viscosity” of the one-way valves due to condensation of the water vapour, thereby further increasing the flow resistance.
• High inspiratory flow rates can significantly increase the resistances exerted by the circuit; this can lead to the development of considerable inspiratory depression with negative consequences of various kinds. These factors may have an especially deleterious effect in small patients.

Typically, high FGF are used in the early stages after connecting the patient to the circuit; the high flows at this stage have various purposes:

• to eliminate the air from the system and fill the circuit with anaesthetic agents and the desired percentage of oxygen;
• to provide a sufficient amount of inhaled agents for alveolar uptake (which is initially high).  

“To-and-fro” circuit with soda lime
The to-and-fro circuit with soda lime is basically a Mapleson C in which a small soda lime canister has been placed between the unit for incoming fresh gases, the APL valve and the connection to the patient on the one side and the reservoir bag on the other side (Fig. 10). This circuit is designed to reduce the consumption imposed by a to-and-fro valve which works like a Mapleson C, thereby avoiding the clutter of a circular circuit, to reduce potential problems arising from the use of valves and to provide a low cost solution. Despite these theoretical advantages, on the whole the circuit has many limitations and potential risks, which, in a context of maximising safety, mean that the use of this circuit cannot be recommended. Indeed, the risks of using this system are numerous: the canister inserted in the circuit may weigh too heavily on the tracheal tube and this is unacceptable because it could injure the patient's larynx or trachea; the soda lime powder could reach the airways, with caustic effects, given the proximity of the airways and the canister; small canisters could undergo tunnelling which would cause re-breathing; and the progressive exhaustion of the lime causes an increase in the dead space of the circuit. Their potential risks and the availability of less problematic alternatives make the use of these circuits unadvisable.

CARBON DIOXIDE ABSORPTION

Chemical composition of absorbents
The main ingredient of the absorbents made by different manufacturers is calcium hydroxide. Other components are added to enhance the reactivity of this compound. These include:

Sodium hydroxide/potassium  hydroxide.Small amounts of sodium hydroxide (1.5-5%) are usually added to improve the reactivity and hygroscopic properties (water-binding capacity) of the mixture, hence the absorbents are often referred to as “soda” lime. Some manufacturers added potassium hydroxide for similar reasons, although this practice is no longer in use (see below).

Barium. Baralyme consists of 85% calcium hydroxide, 11% barium hydroxide and 4% potassium hydroxide. Barium hydroxide in combination with calcium hydroxide has never required additional hardening agents. However, products containing barium were taken off the market in the autumn of 2004.

Water content.The optimum moisture content of the absorbent mixture is between 14-16%. This is essential for the ensuing chain reaction:

• CO₂ + H₂O = H⁺ + HCO₃^-
• 2NaOH + 2 (H⁺ + HCO₃^-)= Na₂CO₃+ 2H₂O + h(heat)
• Na₂CO₃ + Ca (OH)₂ = CaCO₃ + 2NaO

In the second equation, sodium can be replaced by potassium for the absorbents containing potassium hydroxide.

The reaction is interesting because:

• it produces thermal energy (it is an exothermic reaction); 
• it changes the pH of the lime carbonate which allows the use of indicator dyes that show when the lime carbonate is used up; 
• it produces more water than that used in the reaction. In fact, for each mole of carbon dioxide absorbed one mole of water is produced.

Additional ingredients
Zeolite. Zeolites are natural or synthetic, microporous and crystalline substances that contain aluminium, silicon and oxygen in their structure; cations and water are in the pores. Zeolites have voids (cavities or channels) that can accommodate other molecules. They can be added to calcium hydroxide to increase:

• The porosity of the mixture. The more porous the mixture, the more efficient the absorption becomes, because the available surface area is increased.
• Hardness. The harder the compound, the less likely it is for powder to form. The powder is caustic if inspired. Powder can also blend with condensation on the various structures, become wet and settle on the valves in the breathing system, thereby making the valves sticky.
• Water content. Zeolite also helps to prevent the absorbents from drying in adverse conditions. However, if this happens, the zeolite can begin to absorb volatile anaesthetics, thereby reducing the concentration of the anaesthetic in the breathing system, especially at the beginning of anaesthesia. Up to 80% of the agent can be absorbed in this situation.

Silica. Sodium and potassium hydroxide have always been added to increase the reactivity of  absorbent compounds and provide hygroscopic properties (water retention). However, these are the main cause of anaesthetic agents breaking down in the presence of dry soda lime (isoflurane/desflurane), which leads to the production of carbon monoxide, and the degradation of sevoflurane into  various other compounds, including compound A, formaldehyde and methanol. Calcium hydroxide was originally thought to be sufficiently reactive by itself and therefore suitable for binding carbon dioxide alone. However, new production methods have achieved more porous granules (see above), so that calcium hydroxide alone is indeed now sufficient as an absorbent. Nevertheless these granules are much less hard and require the addition of hardening materials such as silica. The new absorbents have proven to minimise production of the undesirable compounds described above, even when dry.

Calcium chloride. The addition of 3% calcium chloride instead of sodium or potassium has a similar, but less potent, effect. This product also contains 1% of calcium sulphate to improve the hardness of the granule. Unlike absorbents that contain strong bases, there may also be a change in colour (similar to the depleted state) in samples that become dry. Even if this substance becomes dry, carbon monoxide (with isoflurane and desflurane) or compound A (with sevoflurane) does not form.

Size of granules
The granule size is important. Granules that are too large produce large gaps in the canister, resulting in poor contact with the passing gases and consequent inefficient absorption of carbon dioxide. Granules that are too small can produce a high resistance to the gas flow and a greater chance of powder formation. The optimal size of the granule is thought to be between 1.5 and 5 mm in diameter.

Production
The ingredients are mixed into a paste by adding water. This is treated in different ways. At first the paste is dry and is crushed between rollers to form granules. The product is then dried and sifted through various meshes to keep the above-mentioned size. The granules are then sprayed with water to obtain the proper water content which enables optimal reactivity without making the granule soft or sticky. In the United States soda lime is supplied in granules between USP mesh 4 and 8 (2.36- 4.75 mm). In the United Kingdom granules are supplied at a British Pharmacopoeia (BP) standard of 1.4-4.75 m (3-10 mesh). More recently, production methods that achieve a more uniform granule size have been introduced. The paste can also be squeezed through a sieve like spaghetti and then chopped into small pieces. It can be passed through a roller, which has thousands of dimples on its surface. This produces tiny spheres of similar size that are separated from the roller by a high pressure jet of air.

Absorption capacity of soda lime
Soda lime is able to absorb 25 litres of carbon dioxide per 100 g while barium lime absorbs 27 litres of carbon dioxide per 100 g. However, when used continuously, lime carbonate is exhausted (as indicated by the colour change) before these capacities are reached because the external part of the granule is exhausted before the whole granule is completely exhausted. In addition, the contact time between the absorbing compound and the carbon dioxide affects the efficiency of the absorbent itself. Several canisters containing 500 g of soda lime appear to be exhausted by a carbon dioxide load of 10-12 litres per 100 g, while large (jumbo) absorbers containing 2 kg of soda lime, which allow contact between the carbon dioxide and the absorbent for a longer time, seem to exhausted by a carbon dioxide load of 17 litres per 100 g.

When the system is left to rest for a few hours, the lime carbonate seems to regenerate, given that the carbonate surface is diluted by the migration of hydroxide ions from inside the granule. The colour of fresh soda lime depends on which indicator dye is added by the manufacturer. For example, if ‘titan yellow’ is added, the soda is bright pink when it is fresh and white when it is exhausted. If “ethyl violet” is used as the indicator dye, the calcium carbonate, in this case, changes from white when fresh to purple when exhausted. Some absorbents have a green pigment added to the ethyl violet, so that it is light green when fresh, but purple when exhausted.

The exothermic reaction
The reaction between soda lime and carbon dioxide  produces heat and water and has been considered advantageous because (at a low flow rate) it supplies a humidified, warmed inspiratory gas, as well as moisture. The temperature and humidity of the inhaled gas are related to a number of factors:

• if the FGF rate is high, the dry gas entering the system reduces both the humidity and temperature of the re-circulating gas
• with a low FG, if the circulation time of the gas is high, the humidity and  temperature rise;
• the longer the system is in use at a low FGF, the greater the humidity and the higher the temperature of the circulating gases; the heat produced, however, is not necessarily always beneficial. The chemical reaction between volatile anaesthetics and traditional absorbents is increased in proportion to the temperature inside the system: trichloroethylene can be decomposed to dichloroacetylene (which is neurotoxic) and then phosgene if the temperature inside the soda lime calcium exceeds 60°C. Anaesthetic agents with a CHF2 fraction (desflurane, enflurane and isoflurane) react with hot, dry soda lime to produce varying amounts of carbon monoxide.

The so-called baralyme, containing potassium hydroxide, has a much greater tendency to produce carbon monoxide than soda lime containing sodium hydroxide. The formation of carbon monoxide appears to be prevented when soda lime has a water content of about 15%.

In fact, significant production occurs only when the water content is less than 2%. This problem can occur when the absorbent is left to dry. This happens when the breathing system is left unused for long periods, or when large amounts of dry gas pass through the lime for a long time. This can occur with certain anaesthesia machines, even when the flow meters are turned off, if:

• the machine has remained connected to the gas supply line;
• there is a minimum baseline flow of oxygen through the machine;
• the absorber is constructed in such a way that the fresh gas can flow through the absorbent before entering the inspiratory tube.

Carbon monoxide is not easily measured as an exhaled gas by the anaesthesia machine. Its production and absorption by the patient is difficult to measure. The colour of absorbents containing strong bases (potassium/sodium) does not necessarily change once the absorbents have become dry. Suspicion should be raised if there is excessive heat production by the soda lime. This phenomenon can easily be avoided by:

• using small canisters (so that they are changed regularly);
• disconnecting the anaesthesia machines or canisters, particularly when it is presumed they will not be used for extended periods;
• avoiding that circuits are designed in such a way that the fresh gases flow through the soda lime before being used.

Sevoflurane can be broken down in the absorber, giving rise to non-toxic fluorinated products (especially “compound A”). The levels of this compound increase when the concentration of sevoflurane is high, in prolonged anaesthesia, when the FGF is low and when the operating temperature within the absorber is increased. There is, however, no obvious danger to the patients.

Suggested readings

1. ConwayCM (1986). Gaseous homeostasis and circle system. BJA 58(3): 337-44.
2. Cook LB (1996) Mapleson breathing systems. The importance of the of the respiratory pause. Anaesthesia 51: 435-460.
3. Davey AJ (2005). Breathing systems and their components. In Ward’s (5thEd): Anaesthetic equipments. Elsevier Limited pp: 131-64.
4. Dorrington KL (1996). The Mapleson breathing systems. Anaesthesia 51(10): 988.
5. Dorrington KL, Lehn JR (1989). Rebreathing during spontaneous and controlled ventilation with “T” piece breathing systems: a general solution. Anaesthesia 44: 300-2.
6. Mapleson WW (1998) The elimination of rebreathing in various semi-closed anaesthetic systems. BJA 80(2): 263-269.
7. Miller DM (1996). Brathing systems reclassified. Anaesthesia in Intensive Care 23(3): 281-3.
8. Murray JM, Renfrew CW, Bedi A, McCrystal CB, Jones DS, Fee JP Amsorb. A new carbon dioxide absorbent for use in anaesthetic breathing systems. Anesthesiology 91(5): 1342-8.
9. Spoerel WE, Bain JA (1986). Anaesthetic breathing systems. BJA 58(7): 819-21