Non-rebreathing systems

The simplest way to deliver a consistent fresh gas supply to a patient is with a system that utilizes a non-rebreathing valve (or valves).

Fresh gas enters the system via an inspiratory limb (Fig 5.3A). This is a length of hosing that has a sufficiently wide bore to minimize any resistance to airflow. It is reinforced so as to prevent collapse from sub-atmospheric pressure either by manufacturing it with corrugations or by bonding a reinforcing spiral onto the outside of the hose. Both of these methods also allow the tube to bend without kinking. The fresh gas entering is either sucked in by the patient’s inspiratory effort or blown in during controlled ventilation and enters the non-rebreathing valve. This valve is so constructed that when it opens to admit inspiratory gas, it occludes the expiratory limb of the system (see Fig. 5.3A). When the patient exhales, the reverse occurs, i.e. the valve mechanism moves to occlude the inspiratory limb and opens the expiratory limb to allow expired gasses to escape (Fig. 5.3B).

Figure 5.3 Non-rebreathing valve. A. Inspiration. B. Exhalation. FGF, fresh gas flow.

The inspiratory limb usually includes a bag (1.5–2 litres capacity) that acts as a reservoir for fresh gas. This reservoir contains enough gas to cope with the intermittent high demand that occurs at inspiration. For example, a patient breathing normally (with a minute volume of 7 litres) may well have a tidal volume of 500 ml inhaled over approximately 1 s. This produces an average inspiratory flow rate (not peak flow rate) of 30 l min⁻¹. Without this reservoir in the system, the fresh gas flow rate would have to at least match this figure (probably more, to match the patient’s peak inspiratory flow rate) in order to avoid respiratory embarrassment.

The reservoir bag is refilled with fresh gas during the expiratory phase. It can also be compressed manually to provide assisted or controlled respiration since the non-rebreathing valve works equally effectively in this mode as it does for spontaneous respiration.

In the non-rebreathing system described, the fresh gas flow rate must not be less than the minute volume required by the patient.

Systems where rebreathing is possible

A miscellany of breathing systems was developed by early pioneers (largely intuitively) that allowed the to-and-fro movement of inspiratory and expiratory gasses within the breathing system. Carbon dioxide elimination was achieved by the flushing action of fresh gas introduced into this breathing system, rather than by separation of the inspiratory and expiratory gas mixtures by a non-rebreathing valve as described above. As it is mainly the flushing effect of fresh gas that eliminates carbon dioxide, these systems retain the potential for rebreathing of carbon dioxide when fresh gas flow rates are reduced.

Mapleson (1954) classified these systems (A to E) according to their efficiency in eliminating carbon dioxide during spontaneous respiration. An F system, the Jackson Rees modification of system E (Ayre’s T-piece), was later added to the classification by Willis (1975), (see below).

Mapleson’s classification of breathing systems

Fig. 5.4 illustrates a modified Mapleson classification of breathing systems. These all contain similar components but are assembled in different sequences in order to be used more conveniently in specific circumstances. However, as a result, the efficiency of each system is different. They are catalogued in order (A, B, C, D, E, F) of increased requirement of fresh gas flow to prevent rebreathing during spontaneous respiration. System A requires. 0.8–1 times the patient’s minute ventilation, B and C require 1.5–2 times the patient’s minute ventilation and systems D, E and F (all functionally similar) require 2–3 times the patient’s minute ventilation to prevent rebreathing during spontaneous respiration.

Figure 5.4 A modified Mapleson classification of breathing systems in which there is potential for rebreathing. System A houses the gas reservoir in the afferent limb and is alternatively referred to as an afferent reservoir system. The fresh gas flow (FGF) need only be at or just below the patient’s minute ventilation without functional rebreathing occurring. Systems B and C (junctional reservoir systems) require FGF of 1.5 times the minute ventilation to avoid rebreathing. Systems D, E and F (efferent reservoir systems) require 2–3 times the minute ventilation to avoid rebreathing.

Mapleson A breathing system

The Mapleson A system illustrated in figure 5.5 is the ‘Magill attachment’ as popularized by Sir Ivan Magill in the 1920s. It consists of the following:

Figure 5.5 The Mapleson A system used with spontaneous breathing (see text). FGF, fresh gas flow; APL, adjustable pressure-limiting valve; RB, reservoir bag.

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

A point is reached when the reservoir bag is again full and, as the patient is still exhaling, the remaining exhaled gasses have to pass out through the APL (expiratory) valve, which now opens.

The first portion of exhaled gasses to pass along the corrugated hose from the patient was that occupying the patient’s anatomical dead space and, therefore, apart from being warmed and slightly humidified (a satisfactory state of affairs), is unaltered, not having taken part in respiratory exchange. This is followed by alveolar gas (with a reduced oxygen content and containing carbon dioxide), the first part of which may enter the corrugated hose, and the rest which is expelled through the APL valve when the reservoir bag is full.

During the end-expiratory pause, all the alveolar gasses and some of the dead-space gasses are expelled from the corrugated hose through the APL valve by the continuing fresh gas flow. Thus, during the next inspiratory phase, the gas inspired may well initially contain some of the remaining dead-space gasses from the previous breath, along with fresh gas. As explained above, these dead-space gasses may be re-inspired without detriment to the patient. The fresh gas flow rate may, therefore, be rather less than the patient’s minute volume and rebreathing of alveolar gas is, therefore, prevented.

In theory, provided there is no mixing of fresh gas, dead-space gas and alveolar gas and a sufficient end-expiratory pause, the fresh gas flow rate need only match alveolar ventilation (approximately 66% of the minute volume), as in this situation alveolar gas only will be vented through the APL valve.

In practice, however, a number of factors dictate a higher fresh gas flow rate (70–90%), for instance:

Mapleson A system and controlled ventilation

The mechanical aspects of the Mapleson A (Fig. 5.6) system (Magill attachment) as described above relate to its use in spontaneous respiration. However, if controlled or assisted ventilation is used, with the patient’s lungs inflated by means of squeezing the reservoir bag, a different state of affairs occurs. With the same fresh gas flows as before, we would see the following:

• Inspiratory phase. The APL valve has to be kept almost closed so that sufficient pressure can develop in the system to inflate the lungs. During the first inspiratory phase with the anaesthetist squeezing the bag, some of the fresh gasses are blown out of the valve.

• Expiratory phase. At the end of inspiration, the reservoir bag may be almost empty, and as soon as the anaesthetist relaxes his pressure on it, the patient exhales into the corrugated hose. The exhaled dead space and alveolar gasses pass further back along the breathing hose and may even enter the reservoir bag. The bag rarely fills completely and so there is usually insufficient pressure within the system to open the APL valve during this phase.

Figure 5.6 Mapleson A system with assisted or controlled ventilation: A. at the end of inspiration; B. at the end of expiration; C. during subsequent inspiration; D. at the end of the subsequent inspiration. Note that much rebreathing takes place (see text). FGF, fresh gas flow; APL, adjustable pressure-limiting valve; RB, reservoir bag.

When the anaesthetist squeezes the bag again, the first gasses to enter the patient’s lungs will be the previously exhaled alveolar gasses. The volume of gasses escaping via the APL valve during this second inspiratory phase is initially small (the valve being almost closed), but gradually increases as the pressure in the system rises towards the maximum inspiration. Therefore, the greatest amount of gas will he dumped late in the cycle and will consist mainly of fresh gas. Under these circumstances there is considerable rebreathing (Fig. 5.6 see shading). Furthermore, as alveolar gas will have entered the reservoir bag, there will always be carbon dioxide contamination in any subsequent inspirate. In order to prevent this and thereby minimize the potential for rebreathing alveolar gas, a high fresh gas flow rate is required.

This is usually of the order of 2 times the patient’s minute ventilation. This situation is highly wasteful with regard to fresh gas and also increases the potential for pollution.

Other Mapleson A breathing systems

The Lack co-axial breathing system (Fig. 5.7A)

The traditional layout of a Magill system sites the APL valve as close to the patient as possible. However, this:

Figure 5.7 A. The Lack co-axial breathing system. (i) Working principles. (ii) The actual assembly. The outer corrugated hose is partly transparent so that the inner tube may be seen to be intact. The adjustable pressure-limiting valve (APL) is fitted with a shroud having a 30 mm outlet so that it may be attached to a scavenging system. B. The Lack parallel breathing attachment. The inner (expiratory) limb of the co-axial arrangement has been replaced by one that is parallel to the inspiratory limb. FGF, fresh gas flow.

• reduces the access to the valve in head and neck surgery

• increases the drag on the mask or endotracheal tube when the valve is shrouded and connected to scavenging tubing.

The Lack system overcomes these two problems. The original version was constructed with a co-axial arrangement of breathing hoses. Exhaled gasses are passed into the orifice of the inner hose sited at the patient end of the system and then back towards the APL valve, which is now sited on the reservoir bag mount. The valve is thus conveniently sited for adjustment by the anaesthetist and its weight, and that of any additional scavenging attachment, is now supported by the anaesthetic machine (see Fig. 5.7). The system still functions as a Mapleson A system. The co-axial hosing on early models was criticised as being too narrow and having too high a flow resistance. In later models, the inner and outer breathing hose diameters were subsequently both increased, to 15 and 30 mm respectively, to overcome this problem. Another problem was that the tubing was heavy and stiff, putting a stress on the connection to the facepiece or endotracheal connection.

Lack parallel breathing system (Fig. 5.7B)

Co-axial breathing systems have particular hazards. If the inner hose were to become disconnected or to split, as has been the case, the leak may pass unnoticed. This would drastically alter the efficiency of the system in eliminating carbon dioxide and is therefore dangerous. A version of the Lack system with parallel hoses is now available (see Fig. 5.7B).

Mapleson B and C systems

These place both the fresh gas supply, reservoir bag and APL valve closer to the patient, allowing the anaesthetist easy access to the latter two, whilst at the same time allowing the fresh gas source to be sited at a distance if necessary. However, as mentioned above, neither is as economical as the ‘A’ system for spontaneous respiration. The reasons are outlined below for the ‘B’ system.

Mapleson D system

The Mapleson D system with spontaneous respiration

The system is best explained hypothetically, if again the three phases of the respiratory cycle are considered in sequence: inspiration, expiration and end-expiratory pause.

• The first inspiration. An appropriate fresh gas flow (see later) enters as close as possible to the patient end of the breathing system (so as to reduce any apparatus dead space) and the system including the reservoir bag is filled so that during the first inspiration the patient breathes only fresh gas. As the inspiratory rate is greater than the FGF the bag begins to empty (Fig. 5.8A).

• Expiration. During expiration, the exhaled gasses mix with fresh gas entering the system and these pass down the wide bore hose. They initially displace any fresh gas remaining here and start to fill the reservoir bag (Fig. 5.8B). When the bag is full, the remainder of the exhaled gasses and the FGF are voided via the APL (expiratory) valve. Of the expired gasses, it is those from the patient’s respiratory dead space that are voided first, followed by alveolar gasses.

• End-expiratory pause. During the end-expiratory pause the fresh gas flow entering the system passes down the wide bore hose, displacing some of the mixture of exhaled gas and FGF, which is now vented out through the APL valve (Fig. 5.8C). The amount of fresh gas occupying and thus stored in the patient end of the wide bore hose at the end of expiration, therefore depends on the fresh gas flow rate, the duration of the end-expiratory pause, and the degree of mixing (due to turbulence) of the various gaseous interfaces within the corrugated hose.

• At the next inspiration the inhaled gasses consist initially of this stored fresh gas followed then by the mixture of exhaled gasses and FGF that remain in the tube and possibly some of the mixture from the reservoir bag if the inhaled tidal volume is large (Fig. 5.8D).

Figure 5.8 The Mapleson D system with spontaneous ventilation (see text). FGF, fresh gas flow; APL, adjustable pressure-limiting valve.

However, there are practical problems with this concept, since the expiratory pause may be short, particularly during spontaneous breathing (when volatile anaesthetics only are used with minimal opioid supplementation). In this case the fresh gas flow needs to be sufficiently high to flush the exhaled gasses downstream prior to the next inspiration. In fact, rebreathing of exhaled alveolar gas occurs unless the fresh gas flow is at least 2 times and possibly up to 4 times the patient’s minute ventilation.

It is worthy of note that the Mapleson D system is functionally similar to a T-piece (Mapleson E). However, with a T-piece, the limb through which the ventilation occurs, if used without a reservoir bag, must be of such a length that the volume of gas in it when augmented by the volume of the fresh gas flow being delivered during inspiration is no less than that of the patient’s tidal volume, otherwise dilution of anaesthetic by entrained air will occur.

Mapleson D system with controlled or assisted ventilation

• The first inspiration. As the bag (full of fresh gas) is squeezed, the fresh gas flow entering the system as well as fresh gasses stored in the wide-bore breathing hose pass to the patient. At the same time some gasses from the reservoir bag are lost through the partially open APL (expiratory) valve (Fig. 5.9A).

• Expiration. A mixture of the fresh gas flow and exhaled gasses passes along the hose, eventually entering the now partially deflated reservoir bag, causing it to refill (Fig. 5.9B).

• Expiratory pause. At this point, provided that there is an expiratory pause, the fresh gas supply continues to flow down the hose to replace and drive the mixed gasses out via the APL valve. A longer expiratory pause allows a greater amount of fresh gas to enter the breathing hose (Fig. 5.9C).

• The next inspiration. At the next squeeze of the reservoir bag (Fig. 5.9D) the continuing fresh gas flow, plus the fresh gas now stored in the breathing hose plus any previously expired gasses that may remain in the hose pass to the patient, while some of the mixed gasses within the bag escape via the APL valve. The cycle then repeats itself.

Figure 5.9 Mapleson D system with manual ventilation. A. The first inspiration; note that the APL valve is forced open. B. Early exhalation; the APL valve is closed and the partially collapsed reservoir bag is filling. C. Late exhalation/expiratory pause; mixed gas is vented from the system. D. Subsequent inspiration.

Thus, to prevent rebreathing in the Mapleson D system during both spontaneous and controlled ventilation, the fresh gas flow must be sufficiently high enough to:

• purge the breathing hose of exhaled gasses

• supplement the stored fresh gas in this breathing hose so that any mixed gas in the reservoir bag is prevented from reaching the patient. The amount of fresh gas required will always be greater than the patient’s minute volume and will depend largely on the expiratory pause. The longer the pause, the more effective will be the ability of the fresh gas to purge the breathing hose of expired gas.

However, in practice during controlled ventilation, deliberate use is often made of functional rebreathing. Theoretically, if slow ventilation rates (with long expiratory pauses) and large tidal volumes are chosen, then sufficient expiratory time will elapse to allow a modest fresh gas flow to fill the proximal part of the system with sufficient fresh gas to provide alveolar ventilation. This will enter the lungs first, followed by a mixture of previously expired gasses which will then occupy the patient’s anatomical dead space. Hence, theoretically, it should be possible to reduce the fresh gas flow to the volume required for alveolar ventilation.

In practice, there is turbulent mixing of the various gaseous interfaces so that alveolar gas is widely distributed (and diluted). Even so, provided sufficiently large controlled minute ventilation is delivered so that most of the fresh gas flow reaches the alveoli, adequate alveolar ventilation will occur with fresh gas flow rates of 70% of the anticipated minute ventilation since, as mentioned above, some rebreathing is acceptable. Fig. 5.10 demonstrates this as well as the fact that the arterial carbon dioxide tension remains fairly constant for any given fresh gas flow rate despite alterations in minute ventilation.

Figure 5.10 Isopleths showing the relationship between fresh gas flow (V_F), minute ventilation (V_E) and various alveolar carbon dioxide tensions.

Mapleson D systems are thus able to make efficient use of fresh anaesthetic gasses during controlled ventilation and could have considerable cost saving benefits. Fig. 5.11 shows how the Mapleson D system may be employed with an automatic ventilator. The reservoir bag is removed and replaced with a standard length of corrugated hose of sufficient capacity to accommodate the air or oxygen that is delivered by the ventilator, and therefore prevents it reaching the patient in place of the intended anaesthetic gasses.

Figure 5.11 Controlled ventilation with a Mapleson D system using a ventilator. Note that with the employment of a ventilator that operates as a ‘gas piston’, the former must be separated from the breathing system by a suitably long piece of breathing hose (with an internal volume of at least 500 ml). This prevents the driving gas from reaching the patient and diluting the anaesthetic mixture. APL, adjustable pressure-limiting valve; FGF, fresh gas flow.

The Bain system

The Bain breathing system (Fig. 5.12) is similar in function to the Mapleson D system. The only difference is that the fresh gas flow is carried by a tube within the corrugated hose (a co-axial arrangement). In the earlier models in particular, there was a risk that the inner tube could become disconnected at the machine end; if this happened a very big dead space was introduced. It could also become kinked, so cutting off the supply of fresh gasses.

Figure 5.12 The Bain breathing system. A. Working principle. B. Intersurgical disposable. FGF, fresh gas flow; APL, adjustable pressure-limiting (expiratory) valve.

Hybrid systems

A number of breathing systems have been described that, by means of a lever switch, can convert the system from a Mapleson A to a Mapleson D or E, allowing a system to be chosen and used in its most efficient mode (i.e. system A, spontaneous respiration; system E, controlled respiration). The Humphrey ADE system (Anaequip) seems to be the most popular version (see Fig. 5.13A). A ‘Y’ shaped length of corrugated breathing hose is attached to H and F in the diagram with the short limb of the Y at the patient.

Figure 5.13 A. The Humphrey ADE system. A, lever; B, fresh gas input; C, overpressure relief valve; D, APL valve with spindle; E, scavenging shroud; F, 22 mm breathing hose port from patient; G, ventilator port; H, 22 mm breathing hose port to patient; I, bag mount. B. An exploded view; with the lever set upright, the system functions in its Mapleson A mode for spontaneous respiration with a recommended fresh gas flow (FGF) of 70 ml kg^–1 or less if used with capnography. Manual ventilation is easily instituted by pressing on the spindle to close the valve during inspiration and releasing it during exhalation. C. For mechanical ventilation the lever is positioned downwards, converting the breathing system to the Mapleson E mode by isolating the reservoir bag and APL valve but incorporating the ventilator port. The fresh gas flow may be kept the same.

With the lever up in the A mode (Fig. 5.13B), the reservoir bag on the ADE block is connected to the inspiratory pathway as in a Mapleson A system. The breathing hose connecting the block at H to the patient is now designated as the inspiratory limb. Expired gas is carried back along the other limb to the block at F and then is vented through an APL valve which is shrouded to facilitate scavenging. In practice it appears to function more efficiently than a traditional Magill attachment. The improved efficiency is thought to relate to the position and design of the components at the patient end of the system. Towards the end of exhalation in a Magill attachment, the exhaled dead-space gas which has passed up the breathing hose is now returned towards the APL valve by the flushing action of the fresh gas flow entering the system. At the APL valve, it meets and mixes with alveolar gas in turbulent fashion, and a mixture of both is discharged from the valve. However, in the Humphrey (and Lack) systems alveolar gas is diverted in a more laminar fashion into a physically separate expiratory limb, which minimizes any potential for mixing of the two gas phases in question. This arrangement, and the removal of the APL valve assembly away from the patient end of the system, also reduces apparatus dead space, so that with further modification (see below) it may be suitable for infants and neonates.

With the lever down in the D/E mode (Fig. 5.13C), the reservoir bag and APL valve are isolated from the breathing system. What was the ‘inspiratory’ limb in ‘A’ mode now delivers gas to the patient end of the system as in a T-piece (see below). The breathing hose returning gas to the ADE block now functions as the reservoir limb of a T-piece. This hose vents to atmosphere via a port adjacent to the bag mount. As this mode does not incorporate a reservoir bag, it is strictly a Mapleson E system. The port described above is usually connected to a ventilator of the ‘bag-squeezer’ type (see Chapter 9), so that the system can be used in its most efficient mode for controlled ventilation.

The Anaequip version is supplied with 15 mm smooth bore non-kinking breathing hose and a unique APL valve (see later in chapter). Interestingly, the use of this smooth bore hose reduces turbulence in the range of flows seen in quietly breathing adults, so that its performance is little different from that of 22 mm corrugated hose. The narrower bore hose also reduces the internal volume of the system to an extent that it is now also suitable for use with infants. A low internal volume is important in a paediatric breathing system in order that:

Mapleson E and F systems

The T-piece system

When what are now termed APL valves were first made, they offered a resistance to exhalation, which was deemed unacceptable in certain anaesthetic techniques such as those used for neonatal and infant anaesthesia. This distinction has become less important (see Chapter 12). To avoid this resistance, the T-piece system was designed by Philip Ayre in 1937. In Fig. 5.14 the fresh gasses are supplied via a small-bore tube to the side arm of an Ayre’s T-piece. The main body of the T-piece is within the breathing system and must, therefore, be of adequate diameter. One end of the body is connected by the shortest possible means to the patient. (The volume of this limb makes up apparatus dead space.) The other end is connected to a length of tubing that acts as a reservoir.

Figure 5.14 A. The Ayre’s T-piece connected to an endotracheal tube; B. during inspiration; C. during expiration; D. during the expiratory pause. FGF, fresh gas flow.

In the case of spontaneous ventilation, the FGF rate must be high. During inspiration the peak inspiratory flow rate is higher than the FGF, so some gasses are drawn from the reservoir limb. During expiration both the exhaled air and the fresh gasses, which continue to flow, pass into the reservoir limb and are expelled to the atmosphere. During the end-expiratory pause the FGF flushes out and refills the reservoir limb. The dimensions of the reservoir limb and the FGF rate are governed by the following considerations:

• The diameter of the reservoir limb must be sufficient to present the lowest possible resistance (not more than 0.075 kPa (0.75 cm H₂0) for a neonate and not more than 0.2 kPa (2 cm H₂0) for an adult at the appropriate flow rates).

• The volume of the expiratory limb should be not less than the patient’s tidal volume. Too great a volume would matter only in that the greater length would lead to increased resistance. Too great a diameter would lead to altered mixing of the fresh gasses with alveolar gas and to inefficiency of the system. For an adult a standard 110 cm length of corrugated hose is satisfactory.

• The optimum FGF rate depends not only on the patient’s minute volume and respiratory rate, but also on the capacity of the reservoir limb. If the latter is at least that of the patient’s tidal volume, then a rate of 2.5 times the patient’s minute volume is sufficient. This is the most satisfactory arrangement. However, if the capacity of the reservoir is reduced, the flow rate must be increased. If the capacity of the reservoir is reduced to zero, the flow rate must be in excess of the peak inspiratory flow rate so as to reduce the possibility of ingress of air.

The shape of the T-piece is also important. Normally the side arm is at right angles to the body. If it is at an angle pointing towards the patient, there is continuous positive pressure applied which would act as a resistance during expiration; similarly, if the gasses were directed towards the reservoir, a sub-atmospheric pressure would be caused by a Venturi effect. As mentioned previously this continuous positive airways pressure is thought to be beneficial in minimizing the fall in functional residual capacity (FRC), especially in neonates.

The Rees T-piece (Mapleson F system)

A great improvement to the T-piece was made by Jackson Rees, who added a small double-ended bag to the end of the reservoir limb (Fig. 5.15).

Figure 5.15 The Rees T-piece. FGF, fresh gas flow.

However, this limb should still approximate to the patient’s tidal volume, or rebreathing could occur as a result of the mixing of expired and fresh gasses in the attached reservoir bag. During spontaneous ventilation small movements of the semi-collapsed bag demonstrate the patient’s breathing. During the inspiratory phase of manual ventilation, the bag is squeezed with the open end of the bag partially or totally occluded by the anaesthetist. During exhalation, the open end is released to allow the gas in the system to escape. This simple method is extremely efficient for infants and small children. Several mechanical devices are available which can be placed in the tail of the bag so as to provide a variable restriction similar to that described above. However, these may all be accidentally turned off, occluding the expiratory limb completely. As the system does not contain an over-pressure safety valve, a dangerously high pressure could build up which could damage the lungs of any infant connected to it. These devices cannot, therefore, be recommended.

Other paediatric breathing systems are described in Chapter 12.

Alternative classification for Mapleson type systems

An alternative nomenclature has been proposed for the systems described above which relates to the position of the reservoir bag within the breathing system. The Mapleson A system in which the reservoir bag stores fresh gas is described as an afferent reservoir system. System D, in which the reservoir bag stores mixed expired gasses, is described as an efferent reservoir system, and systems B and C, in which the reservoir bag stores mixed inspired and expired gasses, are called junctional reservoir systems.

The aforementioned Mapleson systems may also be termed by some as ‘partial rebreathing systems’ owing to their facility for allowing rebreathing of part of the volume of exhaled gasses.

Carbon dioxide absorption

Carbon dioxide can be removed from exhaled gasses by a chemical reaction with various metallic bases (hydroxides). This reaction requires the presence of water in order that these bases and carbon dioxide (as carbonic acid) can exist in ionic form.

Other constituents

Granule size

The size of the granules is important. Too large a granule size produces large gaps in a canister of stacked granules, leading to poor contact with gasses passing through and consequent inefficient absorption of carbon dioxide. Too small a granule may provide an unacceptably high resistance to gas flow, along with the increased possibility of dust formation. The optimum size of granule is thought to be between 1.5 and 5 mm in diameter.

Production

The ingredients are mixed along with added water into a paste. This is treated in a number of ways. Originally the paste was dried and then crushed between rollers so that it formed granules. The product was then sieved through various meshes to retain the sizes quoted above. The dried granules were then sprayed with water to give the right amount of water content to allow optimal reactivity without making the granule soft or sticky. Mesh standards differ between countries due to variations in thickness of the wire used to construct the mesh. In the USA soda lime granules are supplied at between 4 and 8 Mesh USP (2.36–4.75 mm). In the UK the granules are supplied to a BP (British Pharmacopoeia) standard of 1.4–4.75 mm (3–10 Mesh). More recently, production methods have been introduced that produce a more uniform size. The paste may be squeezed through a sieve like spaghetti and chopped into little pieces. It may be passed through a mangle that has thousands of dimples on its surface. This produces tiny similar-sized spheres that are blown off the roller by a high-pressure jet of air. The paste may also be placed on a dimpled belt that is passed through a smooth roller. This produces little hemispheres that are removed from the belt in a manner similar to removing an ice cube from its tray.

The dust is caustic and can produce burns in the respiratory tract if inhaled. This was a problem with the older type of system with ‘to-and-fro’ absorption (Waters’ Canister, see below), where the absorber was placed in close proximity to the patient’s airway. However, circle absorbers are usually separated from the patient by at least 1 m of breathing hose, which normally hangs in a loop. This allows the dust, if present, to fall out before it call get to the patient. Furthermore, a breathing filter separating the patient from the breathing system would protect against this.

Absorptive capacity

Traditional soda lime is capable of absorbing 25 L of carbon dioxide per 100 g, and barium lime 27 L of carbon dioxide per 100 g. However, in continuous use, the absorbent appears exhausted (as indicated by the colour change) before these capacities are reached, because the outside of the granule is exhausted before the whole granule is used up. Furthermore, the contact time between the absorbent and carbon dioxide affects efficiency. Smaller canisters containing 500 g of absorbent appear exhausted at a carbon dioxide load of 10–12 L per 100 g. ‘Jumbo’ absorbers containing 2 kg of absorbent, which allow a longer contact time between it and the carbon dioxide, appear exhausted at a carbon dioxide load of 17 L per 100 g.

When the system is allowed to stand for a few hours, absorbent appears to ‘regenerate’ as the surface carbonate is diluted by hydroxide ions migrating from within the granule. The colour of fresh absorbent depends on which indicator dye is added by the manufacturer. For example if ‘Titan yellow’ is added, the product turns from deep pink when fresh to off-white when exhausted, whereas if ‘ethyl violet’ is used as the indicator dye, the colour changes from white when fresh to purple when exhausted. It is, therefore, important to know which colour change to expect. Some absorbents (LoFloSorb) have a green pigment added to the ethyl violet so that it is pale green when fresh, but violet when exhausted.

The exothermic reaction

The heat and water produced by the reaction of absorbent on carbon dioxide has been considered to be beneficial in that (at low flows) they warm and partially humidify the inspiratory gas. The temperature and humidity of the inspired gas is related to a number of factors:

The heat produced, however, is not necessarily all beneficial. The chemical reaction between volatile anaesthetic agents and absorbents containing sodium or potassium increases in proportion to the temperature within the system:

Dry barium lime, which contains potassium hydroxide, has a much greater tendency to produce carbon monoxide than dry soda lime that contains sodium hydroxide. Fresh absorbent, which has an approximately 15% water content, appears to prevent carbon monoxide formation. In fact significant production takes place only when the water content drops below 2%. The problem can occur when the absorber is left unused for long periods, or when large amounts of dry gas are allowed to pass through it between cases, particularly overnight or at weekends. Even with the flow meters turned off, this is seen albeit after a long period of time in some anaesthetic machines, if:

Carbon monoxide is not easily measured as an exhaled gas as it binds to haemoglobin in the blood first. Its production in a system using absorbents may be difficult to detect. There is not necessarily a colour change in absorbents containing strong bases (potassium/sodium) if they dry out! Suspicion should be raised if there appears to be excessive heat production from the absorbent. This phenomenon can easily be avoided by:

Sevoflurane may undergo degradation within the absorber, to non-toxic fluorinated metabolites in humans (mainly a sevo-olefin called ‘compound A’). Levels rise, as would be expected, with increased concentration of the agent, prolonged anaesthesia, low fresh gas flows and increased operating temperature within the absorber. However, there is no evidence of any danger to humans. In the presence of dried absorbent (especially barium lime) there may be an extreme temperature rise and a number of other breakdown products are produced including formaldehyde and methanol. There have been isolated reports of fire or extreme heat in circle systems resulting in at least one fatality. This prompted the manufacturer to issue a letter to healthcare professionals in the USA warning of the danger. In the cases investigated the following signs were noted:

In summary the recommendations for preventing the problem are as follows:

It is worth noting that as a result of increased awareness and the availability of safer absorbents, there have been no reported incidents of any of the problems discussed above since the year 2000.

’To-and-fro’ absorption systems

The Waters’ canister (Fig. 5.16)

Here the patient breathes in and out of a closed bag, which is connected to the facemask or endotracheal tube via a canister containing soda lime. The part of the system between the patient and the soda lime is dead space and, therefore, its volume must be kept to a minimum. This means that the soda lime canister must be close to the patient’s head, and this leads to practical difficulties. A length of wide-bore tubing may, however, be interposed between the canister and reservoir bag without detriment. The fresh gasses are introduced at the patient end of the system, and the expiratory valve is usually mounted close by, though it may equally well be put at the bag end. The canister is usually placed in the horizontal position for convenience, and it is most important that it is well packed, since if there were a space above the soda lime, ‘channelling’ would occur and absorption would be incomplete (Fig. 5.17A).

Figure 5.16 A ‘to-and-fro’ system incorporating a Waters’ canister. FGF, fresh gas flow; APL, adjustable pressure-limiting valve.

Figure 5.17 A. Channelling in a Waters’ canister. If the canister is not completely filled with soda lime and is placed in a horizontal position, the gasses can pass through the void at the top and therefore fail to come into adequate contact with the soda lime. B. The prevention of channelling by the insertion of a spacer to compress the soda lime. Note also the filter at the patient end that prevents particles of soda lime reaching the patient.

Furthermore, the absorbent at the patient end of the system becomes exhausted first and so increases the functional dead space of the system.

Canisters are available as pre-packed, disposable units. In those intended for reuse, the absorbent may conveniently be compressed to prevent gaps by the insertion of a spongy ‘spacer’ at one end (Fig 5.17B). When the canister is closed, the sealing washer should be checked to ensure that it is in the correct position and any soda lime on the threads of the canister or the sealing washer should be carefully removed as these may cause leaks. The whole system should be tested before use.

Apart from being cumbersome, the ‘to-and-fro’ system has the disadvantage that the patient could inhale absorbent dust. A breathing filter should be inserted in the patient end of the canister to prevent this.

Circle absorption systems

Here the disadvantages of the absorbent canister being so close to the patient are avoided. The patient is connected to the absorber by two corrugated hoses, one inspiratory and the other expiratory, as shown in Fig. 5.18A. The one way or ‘circle’ flow of gasses through the system is determined by two unidirectional valves V₁, and V₂, which are accommodated in transparent domes so that their correct action may be observed.

Figure 5.18 A. Schematic diagram of a circle breathing system with absorber. FGF, fresh gas flow; V₁ and V₂, one-way valves; R, reservoir bag; S, soda-lime canister. The diagram highlights the alternative siting of the adjustable pressure-limiting (APL) valve at points A, B or C. B. ADU circle system, Datex-Ohmeda. C. Blease 1 kg absorber. D. 2 kg absorber assembly with attached ventilator bellows (Spacelabs).

The fresh gas port and the reservoir bag may be sited in the inspiratory pathway close to the inspiratory valve V₁ or in the expiratory limb of the system downstream of the valve V₂. There are claimed advantages for each arrangement. Positioning the FGF and bag in the inspiratory pathway may reduce the resistance to inspiratory effort and prevents desiccation of the absorbent if the fresh gas flow is left on for long periods when the system is not in use. This has implications for the efficiency of the absorbent and its potential for producing unusual breakdown products (see above). However, positioning the FGF and bag in the expiratory limb so that the FGF passes through the absorbent before reaching the patient allegedly warms and humidifies this gas. This also seems to remove some of the excess moisture often seen in absorbers that might increase resistance through the system. The APL valve is usually mounted downstream of the valve V₂ (position ‘B’) in the expiratory limb, but before gas entry into the absorber. Here, it can dump excess exhaled gas prior to entry of gas into the absorber. At one time it was considered that an APL valve was best positioned in the breathing system at position ‘A’ for use with spontaneous respiration (Fig. 5.18A). This would preferentially dump alveolar gas during exhalation, thus increasing carbon dioxide elimination upstream of the absorber and conserving soda lime. However, as scavenging assumed a greater importance, the inconvenience of connecting a cumbersome scavenging hose to a valve in this position has limited its usefulness.

Circle breathing systems are manufactured in many different designs and sizes. Most are two part systems with one part comprising a fixed body containing the gas pathways, switches, valves and the other a detachable canister that contains the absorbent. Figs 5.18B, C and D show commercial versions of the system described. Fig. 5.18B (circle system for ADU anaesthetic machine, Datex-Ohmeda) has a container for 1 kg of absorbent that may be either disposable or refilled. During replacement, self-sealing valves on the body of the unit close to prevent escape of patient gas from the rest of the system. Figs 5.18C and D show other types of 1 kg and 2 kg absorber respectively. The rationale for larger absorbers is discussed below.

The system shown in Figs 5.19A and B is a disposable version. The valve V₁ is sited in the breathing hose and as close to the patient as possible. In this position it is alleged to have a faster response to pressure changes caused by exhalation and closes earlier, although, as it is exhaled dead space gas that enters the system first, it is immaterial as to which limb this enters initially. Unlike many circle systems that are now an integral part of the anaesthetic workstation, the APL valve is not automatically excluded from the system in mechanical ventilation mode. Therefore, when a ventilator is attached to the reservoir bag port and is in use, the APL valve must be closed fully or gas will be lost from the system.

Figure 5.19 The Intersurgical disposable circle system. FGF, fresh gas flow; V₁ and V₂, one-way valves; APL, adjustable pressure-limiting valve; RB, reservoir bag; S, soda lime canister.

The use of ventilators with circle systems

Any ventilator deemed suitable for use with a circle system must have the following features:

• It must be able to reproduce the effect of manual compression and refilling of the reservoir bag, i.e. it must have a single breathing hose inlet/outlet to allow to and fro movement of gas.

• It must have a valve to release any excess gas in the circle system during the exhalation phase.

• It must have a power source that is independent of the fresh gas entering the circle system so that a minute ventilation can be delivered that is larger than the fresh gas flow (especially when this is set at low flows).

• If the ventilator is gas powered, this driving gas should not be able to contaminate the respirable gas reaching the patient.

There are three types of ventilator that are used to drive circle systems: ‘bag squeezer’, pneumatic piston and turbine.

‘Bag squeezer’

Here, the reservoir bag is replaced by a bellows that may be compressed mechanically (see Figs 9.7E and F) or pneumatically (Fig. 5.20). In the latter the bellows is enclosed in a gas-tight container and the driving gas that enters the container then compresses the bellows, but does not mix with the patient gas contained within it.

Figure 5.20 ‘Bag squeezer’ ventilator and circle system. A. Inspiratory phase; B. expiratory phase. A, ventilator; B, bellows; C, absorber; V₁ and V₂, one-way valves; E, exhaust; FGF, fresh gas flow.

‘Pneumatic piston’

Here, the reservoir bag is replaced by a suitably long length of breathing hose. The driving gas from the ventilator is passed through a special valve (Figs 5.21A and B) into the breathing hose. The latter is sufficiently long so that its internal volume is at least 1.5–2 times greater than the patient’s tidal volume and prevents the driving gas from entering the circle and contaminating the patient gas. This is especially important in many circle systems where the reservoir limb is sited in the inspiratory pathway. Here, driving gas could dilute the inspiratory anaesthetic gas sufficiently to cause an inadequate level of anaesthesia to be maintained.

Figure 5.21 Pneumatic piston ventilator and circle system. A. Inspiratory phase; B. expiratory phase. A, ventilator; C, absorber; V₁ and V₂, one-way valves; E, exhaust; FGF, fresh gas flow.

Turbine

An electronically driven and controlled turbine may be fitted within the circle system to act as a ventilator (Dräger Zeus; see Chapter 4).

Oxygen concentrations in circle systems at low fresh gas flows

As the fresh gas flow in a circle is decreased, exhaled gas that is allowed to recirculate exerts an increasing influence on the subsequent inspired gas mixture. The oxygen concentration of this exhaled gas depends on:

• its original inspired concentration

• the alveolar oxygen extraction, which may be unpredictable.

Fig. 5.22 shows the decrease in alveolar oxygen concentrations of a 50% nitrous oxide and 50% oxygen mixture under controlled conditions. It can be seen that at a FGF of 1 l min⁻¹ if there is no longer any nitrous oxide uptake, the oxygen concentration has dropped to 27% and drops even further at a FGF of 0.5 l min⁻¹. Therefore, in clinical practice the oxygen concentration in a circle at low flows is unpredictable and monitoring of inspired oxygen with an analyzer is essential. In fact, monitoring of all gasses and anaesthetic vapours should be considered mandatory for circle systems at low flows.

Figure 5.22 Predicted variations in alveolar oxygen concentrations (F_AO₂) produced by decreasing fresh gas flow (V_F) and assuming different levels of nitrous oxide uptake in a 50 : 50 mixture of oxygen in nitrous oxide. A constant oxygen consumption of 200 ml min^–1 and a constant ventilation required to produce an alveolar CO₂ concentration of 5% have been assumed. The heavy line shows the fall in alveolar oxygen concentration with decreasing fresh gas flow when the inspired and body levels of nitrous oxide have reached equilibrium (zero uptake).

(From Scurr C, Feldman SA, Soni N, editors. Scientific Foundations of Anaesthesia. London: Butterworth/Heinemann; 1990, with permission.)

Rebreathing and reservoir bags

Rebreathing and reservoir bags (Fig. 5.23) are identical, the distinction being solely in the use to which they are put, as explained previously. The commonly used size in adult breathing systems is 2 L (i.e. that which when fully, but not forcibly, distended has a capacity of 2 L; in clinical practice it is seldom filled beyond this capacity). They are also available in 1 L and 0.5 L sizes for paediatric anaesthesia. Larger bags are sometimes used as reservoir bags in ventilators.

Figure 5.23 2 L reservoir bag showing the cagemount connector.

In adult breathing systems, the capacity to which the bag may easily be distended must exceed the patient’s tidal volume. A larger capacity, bag (2 L), however, is safer as it more easily absorbs pressure increases. The neck of the bag is stretched over a female 22 mm metal or plastic tapered connector. A metal or plastic cage is often attached to the part of the connector that fits inside the bag. This prevents the inlet from being occluded if the bag were folded.

In the Jackson Rees T-piece paediatric attachment, a double-ended bag is added to the expiratory limb (Fig. 5.24) and the smaller end acts as an expiratory port, the aperture of which can be controlled by the anaesthetist.

Figure 5.24 Double-ended bag for paediatric anaesthesia.

The material of which a breathing system bag is constructed is important. Where ventilation is spontaneous, the opening pressure of the expiratory valve must exceed that required to prevent the bag from emptying spontaneously owing to its weight or resistance to distension. Therefore, to maintain a low expiratory pressure, the bag must be ‘soft’. This was achieved easily when natural latex rubber bags were in common use. The increase in latex allergy in the general population and in healthcare workers has had a major impact on the provision of equipment that had previously contained natural latex rubber. All anaesthetic equipment in the UK is now supplied with natural latex-free parts. These include ventilator bellows and reservoir bags. The latter are made from synthetic compounds such as polychloroprene (neoprene, a synthetic latex rubber). Some manufacturers make their bags sufficiently pliant to mimic the elasticity of natural latex. These will distend well-beyond their normal filling capacity until they burst, but the pressure in the bags will stay below 60 cm H₂0. This is thought to be a safety feature that prevents barotrauma to a patient’s lungs should the exhalation pathway in a breathing system containing this bag become occluded. This would be caused most commonly by an APL valve inadvertently screwed shut. Some, however, are made from less compliant material and the pressure may rise to over 60 cm H₂0. These bags should be fitted only to breathing systems that have a pressure relief valve attached either as a separate feature or built into the APL valve (see below).

The observed movement of the bag depends on several factors, such as its shape, size, degree of filling, the tension of the expiratory valve and the fresh gas flow rate, as well as on the patient’s tidal volume. An accurate estimate of the patient’s tidal volume cannot be made simply by watching the bag.

Adjustable pressure limiting (APL) valves

The purpose of this valve is to allow the escape of exhaled (expired) and surplus gasses from a breathing system, but without permitting entry of the outside air, even during a negative phase. Usually it is desirable that the pressure required to open the valve should be as low as possible, in order to minimize resistance to expiration. It must, however, present sufficient resistance to prevent the reservoir bag from emptying spontaneously, particularly when a scavenging system is employed that exerts a slight sub-atmospheric pressure upon it.

The operating principles of APL valves are based on the Heidbrink valve (Fig. 5.25). The valve disc is as light as possible, and rests on a ‘knife-edge’ seating that presents a small area of contact. This lessens the tendency to adhesion between the disc and seating due to the surface tension of condensed water from the expired air, or after washing or sterilizing. The disc has a stem that is located in a guide, in order to ensure that it is correctly positioned on the seating, and a lightweight coiled spring, which promotes closure of the valve.

Figure 5.25 The Heidbrink valve: 1, male tapers at both ends; 2, retaining screws; 3, disc; 4, spring; 5, valve top.

The spring is a delicate coil and is of such dimensions that when the valve top is screwed fully ‘open’ there is minimal pressure on the disc when seated. However, during the ‘blow-off’ phase the disc rises and shortens the spring so that the pressure it exerts on the disc is greater and will close it at the appropriate time. Screwing down the valve top produces progressively increasing tension in the spring. When the top is screwed down fully, the valve is completely closed. The valve should always be used in the vertical position so that the disc is seated by gravity.

The Humphrey APL valve

An interesting addition to the standard APL valve is the modification seen on the version (Figs 5.13 and 5.26) that is part of the Humphrey ADE system. Here, the valve disk is attached to a red-coloured spindle that extends through the valve top. When the valve is fully open, the spindle is seen to bob up and down as the disk is lifted up and down during respiration. The valve top is made concave and shiny so that it reflects and magnifies the spindle colour so as to detect even the smallest movement when used in paediatric anaesthesia. The inside of the valve body has a small funnel through which the disk has to move before significant gas can escape. This initial movement of 5 mm accentuates the bobbing action of the spindle which is useful particularly in paediatric anaesthesia.

Figure 5.26 ‘Humphrey’ APL valve. A. Inspiration (with valve disc closed); B. beginning of exhalation (valve disc just clearing small funnel). (1) Valve top; (2) valve body; (3) exhaled gas scavenging shroud; (4) ‘O’-ring seal between the valve body and shroud; (5) valve spindle; (6) valve disc; (7) valve spring; (8) screw thread to secure valve to the breathing system; (9) 5 mm funnel which accentuates movement of the spindle.

When the ADE system is used in the Mapleson ‘A’ mode, the valve spindle may be held down with a finger if switching from spontaneous to manual ventilation is required. This has a number of advantages:

• As there is no leak from the valve during inspiration, the patient’s lung compliance may be more accurately assessed.

• It becomes as efficient as the Mapleson ‘A’ mode in spontaneous respiration (i.e. the valve is shut during inspiration and open during expiration).

• The valve top may be kept in the ‘open’ position at all times and does not require repeated adjustment when switching between spontaneous and manual ventilation.

It was originally thought that the increased respiratory work produced by the expiratory resistance of APL valves was detrimental to anaesthetized patients. This is without doubt true when the valve resistance is high (due to sticky valves, narrow valve apertures) or where the respiratory effort is severely compromised (e.g. in neonates). However, modern valve design (with wider valve apertures, lighter valve discs, more delicate springs, better screw threads) minimizes this resistance. Furthermore a small PEEP (positive end expiratory pressure) effect that these valves may produce is now thought to be positively beneficial, reducing the potential for the functional residual capacity of the lungs to fall below the closing volume in supine anaesthetized patients.

APL valves with in-built overpressure safety devices

Any unexpected pressure rise in a breathing system was made relatively safe by the compliance of the latex rubber of the reservoir bag. This could still fail, for instance if the bag were trapped under the wheel of an anaesthetic machine, a dangerously high pressure could develop within the breathing system and be passed on to the patient. Now that latex rubber is no longer used owing to the increase in latex allergy, new materials are used that may not have the same elasticity and so are not as compliant. This safety feature can no longer be relied on. APL valves are now available in which an overpressure safety device has been incorporated.

An example, is shown in Figure 5.27.

Figure 5.27 An APL valve with an overpressure safety device. (1) Valve control knob; (2) high-pressure spring; (3) valve spindle; (4) light-pressure spring; (5) the part of the asymmetric valve body that rotates during valve closure and occludes the expiratory limb. The figure shows A. the valve open during exhalation; B. the valve closed; and C. the valve closed with the overpressure safety device in action. D. Dräger calibrated APL valve with overpressure relief valve.

It has two valves:

• the inner (3), which is tensioned with a weak spring (4)

• the outer (5), which is tensioned with a more powerful one (2).

When the valve top (1) is unscrewed fully (Fig. 5.27A), the outer valve is permanently open, but the inner one is closed until exhaled gas forces it open. The pressure required to do this is small (0.15 kPa /1.5 cm H₂O). As the valve is gradually closed, the expiratory flow resistance increases and in sophisticated examples this resistance can be calibrated and displayed on the valve body. When the valve top is screwed down fully, both valves are closed and in this position the outer one is pushed against the inner so that is has no movement of its own (Fig. 5.27B). An excess pressure is now required to move the more powerful spring on the outer valve which will begin to open at 3 kPa (30 cm H₂O) and be fully open between 6 and 7 kPa (60–70 cm H₂O) when the gas flow is 50 l min⁻¹ (Fig. 5.27C). Fig. 5.27D is an example of a calibrated APL valve incorporating an overpressure safety valve set at 70 cm H₂O.

Alternative APL valve design

Many breathing systems are now supplied as single use items. This includes the APL valve (Fig. 5.28A). It is now possible to simplify the design so that the spring and the valve disc are replaced by a neoprene flap valve (1) (Fig. 5.28Bi). This opens and shuts in the normal manner during spontaneous respiration. When positive pressure is required, the valve top (2) operates a screw threaded insert (3) that lowers an overpressure relief valve (4). As the valve top is screwed shut, the insert lowers the second valve (4) onto the flap valve housing, gradually occluding the expiratory pathway until complete occlusion occurs (Fig. 5.28Bii). The second valve is fitted with a spring (5) which is strong enough to maintain the occlusion up to a pressure of 60 cm H₂O, but weak enough for the valve to lift (Fig. 5.28Biii) and allow gasses at a higher pressure to escape.

Figure 5.28 A. A single-use APL valve (Intersurgical). B. Operation of an Intersurgical single use APL valve. (1) Neoprene flap valve; (2) valve top; (3) screw threaded insert; (4) overpressure relief valve; (5) spring. (i) Valve in open position. (ii) Valve shut. (iii) Overpressure relief valve operated.

Breathing hoses

The hoses connecting the components of a breathing system must be of such a diameter as to present a low resistance to gas flow. Its cross-section must be uniform to promote laminar flow where possible and, although it should be flexible, kinking should not occur.

The most commonly used type was for a long-time, corrugated hose of rubber or polychloroprene (neoprene). The corrugations allow acute angulations of the hose without kinking. The advantage of these materials is that the ends are more easily stretched, and will make a good union with other components of different diameters. They may be sterilized by steam autoclaving and be reused in countries where single-use alternatives are impractical. The disadvantages are that the irregular wall must cause turbulence and being opaque may harbour dirt and infection unseen. They are also heavy and, if unsupported, may drag on a facemask or endotracheal tube.

Various other materials such as silicone rubber and plastics (polyethylene) are in use, both in corrugated and smooth form (Fig. 5.29). Smooth bore breathing hose produces less turbulence than the corrugated variety at similar gas flows. It can also be produced so that it resists kinking (by the attachment of a reinforcing spiral of a similar material to its external surface). With smooth bore hosing, a smaller diameter (e.g. 15 mm) may well be acceptable for use with adult breathing systems (see Humphrey ADE breathing system).

Figure 5.29 From top to bottom: 15 mm corrugated plastic breathing hose; 15 mm smooth-bore hose; 22 mm smooth-bore hose; 22 mm corrugated hose.

Plastic hosing has become very popular because it is lightweight, cheap to manufacture, and, therefore, disposable. Some are supplied as complete breathing systems (Figs 5.12 and 5.19) or in long coils, the appropriate length of which may be cut off at one of the frequent intervals where the corrugations (Fig. 5.30) give way to a shaped connector. More recently, the addition of small quantities of silver mixed with the plastic (Silver Knight) has been used to make breathing systems bactericidal both on the inside and the outside of the hosing.

Figure 5.30 Plastic hosing showing a section that may be cut to produce a suitable length.

Silicone rubber hosing is autoclavable, unlike many plastics that would melt if so treated. Plastic apparatus intended for single use only is normally sterilized at manufacture by gamma irradiation.

There are several standard sizes of corrugated hose, both ends of which have smooth walls for about 2–3 cm. These ends are designed to fit either tapered connectors (see below) or tapered components of a breathing system. The breathing hose for adult use is normally 22 mm wide so as to reduce the resistance to breathing to a minimum. Paediatric breathing hose has a narrower bore (15 mm) to reduce its internal volume (see Chapter 12) and to make it less cumbersome.

Tapered connections (adapters)

Tapered connections (adapters) provide a useful way of joining rigid tubes or other components together in such a manner that the joint will not leak. The joint is described as having a male half and a female half that are pushed together with a slight twist to form a gas-tight fit. The joint may be easily dismantled and reassembled, a feature that makes it useful for the interconnection of breathing systems, catheter mounts and endotracheal connectors.

A leak-proof joint relies on its components being completely circular and having the same angle of taper so that the maximum contact between the components of the joint will occur. The standard also requires that male tapers for breathing systems be fitted with a recess behind the taper so that when rubber and plastic hosing is pushed on to a male connector, its leading edge can contract into this recess to provide a more secure fitting.

Examples of tapers are shown in Figs 5.31A and B.

Figure 5.31 Plastic (disposable) 22 mm tapered adaptors. (Top) Female to male; the male part is normally inserted into each end of a breathing hose. (Bottom) Male to male; this is inserted into the distal end of a piece of corrugated tubing when used as an extension for a reservoir bag (Fig. 5.32C).

The current ISO and BS recommendations on the sequence of tapers in various breathing systems are set out in Fig. 5.32.

Figure 5.32 Current sequence of tapers in anaesthetic breathing systems as recommended by the ISO and BSI (BS 3849) as from 1988 and maintained in BS EN ISO 5356-1: 2004. Typical layout of A. Mapleson A system; B. to-and-fro absorption system; and C. circle absorption system. M, male conical fitting; F, female conical fitting; APL, adjustable pressure-limiting valve.

Prior to the introduction of any standards, manufacturers were free to decide the size and angle of tapers used with their equipment. However, there is now an internationally agreed size for tapered connections for use with anaesthetic breathing systems and endotracheal tubes so that there is compatibility between equipment from different manufacturers.

The International Standards Organization (BS EN ISO 5356-1: 2004, Anaesthetic and respiratory equipment – Conical connectors – Part 1: Cones and sockets) specifies the use of:

Some 22 mm male breathing hose connections are so manufactured that they incorporate a 15 mm female taper for direct connection to an endotracheal tube.

It is worthy of note that the current British Standard requires that a reservoir bag should have a female inlet to fit the male outlet for bag mounts on all breathing systems. However, should a length of breathing hose be required between the bag mount and reservoir bag, a problem arises. The breathing hose is manufactured with two female ends. One will fit the bag mount (male to female) but the other will not fit the bag (female to female). A male to male 22 mm tapered adapter is required (see Fig. 5.31).

Reuse of breathing system components

Items that are intended for reuse (i.e. to be used on different patients) are normally made from materials (rubber, neoprene, silicone and metal) that withstand repeated autoclaving.

Some items are designated as single use (i.e. to be used on one person only) if they are made from materials such as plastics that are not easily sterilized. Also, if the tapers (see above) are made from materials that are easily distorted with repeated connection they will be designated for single use only.

A third category has been described. Some items that might be designated as single use because they are not easily sterilized may be acceptable for repeated use. Provided that these items are protected by a high-quality bacterial filter, some manufacturers will accept product liability if the item is used on a number of patients as long as the item is thoroughly checked between uses.