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