Breathing systems must fulfil three objectives:
- 1.
delivery of oxygen
- 2.
removal of carbon dioxide from the patient
- 3.
delivery of inhaled anaesthetic agents. These agents are predominantly eliminated by the lungs also, so the breathing system must be able to expel them as necessary.
Several breathing systems are used in anaesthesia. Mapleson classified them into A, B, C, D and E. After further revision of the classification, a Mapleson F breathing system was added ( Fig. 4.1 ). Currently, only A, D, E and F systems and their modifications are commonly used during anaesthesia. Mapleson B and C systems are used more frequently during the recovery period and in emergency situations.
The fresh gas flow (FGF) rate required to prevent rebreathing of alveolar gas is a measure of the efficiency of a breathing system.
Properties of the ideal breathing system
- 1.
Simple and safe to use.
- 2.
Delivers the intended inspired gas mixture.
- 3.
Permits spontaneous, manual and controlled ventilation in all age groups.
- 4.
Efficient, requiring low FGF rates.
- 5.
Protects the patient from barotrauma.
- 6.
Sturdy, compact, portable and lightweight in design.
- 7.
Permits the easy removal of waste exhaled gases.
- 8.
Ability to conserve heat and moisture.
- 9.
Easy to maintain with minimal running costs.
Components of the breathing systems
Adjustable pressure limiting (APL) valve
This is a valve which allows the exhaled gases and excess FGF to leave the breathing system ( Fig. 4.2 ). It does not allow room air to enter the breathing system. It allows control of the pressure within the breathing system and the patienťs airway. The APL valve is an essential component of most breathing systems, except Mapleson E or F (see later). Synonymous terms for the APL valve are expiratory valve, spill valve and relief valve.
Components
- 1.
The APL valve has three ports: the inlet, the patient and the exhaust. The latter can be open to the atmosphere or connected to the scavenging system using a shroud.
- 2.
A lightweight disc rests on a knife-edge seating. The disc is held onto its seating by a spring. The tension in the spring, and therefore the valve’s opening pressure, are controlled by the valve dial.
Mechanism of action
- 1.
This is a one-way, adjustable, spring-loaded valve. The spring is used to adjust the pressure required to open the valve. The disc rests on a knife-edge seating in order to minimize its area of contact.
- 2.
The valve allows gases to escape when the pressure in the breathing system exceeds the valve’s opening pressure.
- 3.
During spontaneous ventilation, the patient generates a positive pressure in the system during expiration, causing the valve to open. A pressure of less than 1 cm H 2 O (0.1 kPa) is needed to actuate the valve when it is in the open position.
- 4.
During positive pressure ventilation, a controlled leak is produced by adjusting the valve dial during inspiration. This allows control of the patienťs airway pressure.
Problems in practice and safety features
- 1.
Malfunction of the scavenging system may cause excessive negative pressure. This can lead to the APL valve remaining open throughout respiration. This leads to an unwanted enormous increase in the breathing system’s dead space.
- 2.
The patient may be exposed to excessive positive pressure if the valve is closed during assisted ventilation. A pressure relief safety mechanism actuated at a pressure of about 60 cm H 2 O is present in modern designs ( Fig. 4.3 ), even when the cap is screwed down and the valve is fully closed.
Fig. 4.3
Intersurgical APL valve. In the open position (left), the valve is actuated by pressures of less than 0.1 kPa (1 cm H 2 O) with minimal resistance to flow. A 3/4 clockwise turn of the dial takes the valve through a range of pressure limiting positions to the closed position (centre). In the closed position, the breathing system pressure, and therefore the intrapulmonary pressure, is protected by a pressure relief mechanism (right) actuated at 6 kPa (60 cm H 2 O). This safety relief mechanism cannot be overridden.
- 3.
Water vapour in exhaled gas may condense on the valve. The surface tension of the condensed water may cause the valve to stick. The disc is usually made of a hydrophobic (water repelling) material, which prevents water condensing on the disc.
- 4.
The valve can add bulk to the breathing system.
- •
One-way spring-loaded valve with three ports.
- •
The spring adjusts the pressure required to open the valve.
- •
When fully open, a pressure of less than 1 cm H 2 O is needed to actuate it.
- •
A pressure relief safety mechanism is actuated at 60 cm H 2 O even when closed.
Reservoir bag
The reservoir bag is an important component of most breathing systems, improving efficiency and allowing manual ventilation.
Components
- 1.
It is made of anti-static rubber or plastic. Latex-free versions also exist. Designs tend to be ellipsoidal in shape.
- 2.
The standard adult size is 2 L ( Fig. 4.4 ). The smallest size for paediatric use is 0.5 L. Volumes from 0.5 to 6 L exist. Bigger size reservoir bags are useful during inhalational induction, e.g. adult induction with sevoflurane.
Fig. 4.4
Intersurgical standard 2 L adult size reservoir bag.
Mechanism of action
- 1.
The reservoir bag accommodates the FGF during expiration, acting as a reservoir available for the following inspiration. Otherwise, the FGF must be at least the patienťs peak inspiratory flow to prevent rebreathing. As this peak inspiratory flow may exceed 30 L/min in adults, breathing directly from the FGF will be insufficient.
- 2.
It acts as a monitor of the patienťs ventilatory pattern during spontaneous breathing. It serves as a very inaccurate guide to the patienťs tidal volume.
- 3.
It can be used to assist or control ventilation.
- 4.
When employed in conjunction with the T-piece (Mapleson F system), a 0.5 L double-ended bag is used. The distal hole acts as an expiratory port ( Fig. 4.5 ).
Fig. 4.5
Intersurgical 0.5-L double-ended reservoir.
Problems in practice and safety features
- 1.
Because of its compliance, the reservoir bag can accommodate rises in pressure in the breathing system better than other parts. When grossly overinflated, the rubber reservoir bag can limit the pressure in the breathing system to about 40 cm H 2 O. This is due to Laplace’s law dictating that the pressure ( P ) will fall as the bag’s radius ( r ) increases: P = 2(tension)/r.
- 2.
The size of the bag depends on the breathing system and the patient. A small bag may not be large enough to provide a sufficient reservoir for a large tidal volume.
- 3.
Too large a reservoir bag makes it difficult for it to act as a respiratory monitor.
- •
Made of rubber or plastic.
- •
2-L size commonly used for adults. Bigger sizes can be used for inhalational induction in adults.
- •
Accommodates FGF.
- •
Can assist or control ventilation.
- •
Limits pressure build-up in the breathing system.
Tubings
These connect one part of a breathing system to another. They also act as a reservoir for gases in certain systems. They tend to be made of plastic, but other materials such as silicone rubber and silver-impregnated bactericidal plastics are available.
The length of the breathing tubing is variable depending on the configuration of the breathing system used. They must promote laminar flow wherever possible and this is achieved by their being of a uniform and large diameter. The size for adults is 22-mm wide. However, paediatric tubing is 15-mm wide, to reduce bulk. The corrugations resist kinking and increase flexibility, but they produce greater turbulence than smooth-bore tubes.
Specific configurations are described as follows.
Mapleson classification
In 1954, Mapleson classified the breathing systems into five configurations (A to E) and a sixth (F) was added later. The classification is according to the relative positions of the APL valve, reservoir bag and FGF. Mapleson systems need significantly higher FGF to prevent rebreathing compared to the circle breathing system and therefore the expensive use of volatile agents. Their use in modern anaesthesia is very limited with the wide spread of the circle breathing system.
Magill system (Mapleson A)
This breathing system was popular and widely used in the UK.
Components
- 1.
Corrugated rubber or plastic tubing (usually 110–180 cm in length) and an internal volume of at least 550 mL.
- 2.
A reservoir bag, mounted at the machine end.
- 3.
APL valve situated at the patient end.
Mechanism of action
- 1.
During the first inspiration, all the gases are fresh and consist of oxygen and anaesthetic gases from the anaesthetic machine.
- 2.
As the patient exhales ( Fig. 4.6C ), the gases coming from the anatomical dead space (i.e. they have not undergone gas exchange so contain no CO 2 ) are exhaled first and enter the tubing and are channelled back towards the reservoir bag which is being filled continuously with FGF.
Fig. 4.6
Mechanism of action of the Magill breathing system during spontaneous ventilation; see text for details. FGF, Fresh gas flow.
- 3.
During the expiratory pause, pressure build-up within the system allows the FGF to expel the alveolar gases first out through the APL valve ( Fig. 4.6D ).
- 4.
By that time the patient inspires again ( Fig. 4.6B ), getting a mixture of FGF and the rebreathed anatomical dead space gases.
- 5.
It is a very efficient system for spontaneous breathing. Because there is no gas exchange in the anatomical dead space, the FGF requirements to prevent rebreathing of alveolar gases are theoretically equal to the patienťs alveolar minute volume (about 70 mL/kg/min).
- 6.
The Magill system is not an efficient system for controlled ventilation. An FGF rate of three times the alveolar minute volume is required to prevent rebreathing.
Problems in practice and safety features
The Magill system is not suitable for use with children of less than 25–30 kg body weight. This is because of the increased dead space caused by the system’s geometry at the patient end. Dead space is further increased by the angle piece and face mask.
One of its disadvantages is the heaviness of the APL valve at the patienťs end, especially if connected to a scavenging system. This places a lot of drag on the connections at the patient end.
- •
Efficient for spontaneous ventilation. FGF required is equal to alveolar minute volume (about 70 mL/kg/min).
- •
Inefficient for controlled ventilation. FGF three times alveolar minute volume.
- •
APL valve is at the patienťs end.
- •
Not suitable for paediatric practice.
Lack system (Mapleson A)
This is a coaxial modification of the Magill Mapleson A system.
Components
- 1.
1.8-m length coaxial tubing (tube inside a tube). The FGF is through the outside tube, and the exhaled gases flow through the inside tube ( Fig. 4.7A ).
Fig. 4.7
(A) The coaxial Lack breathing system. (B) The parallel Lack breathing system. APL, Adjustable pressure limiting.
- 2.
The inside tube is wide in diameter (14 mm) to reduce resistance to expiration. The outer tube’s diameter is 30 mm.
- 3.
The reservoir bag is mounted at the machine end.
- 4.
The APL valve is mounted at the machine end eliminating the drag on the connections at the patient end, which is a problem with the Magill system.
Mechanism of action
- 1.
It has a similar mechanism to the Magill system except the Lack system is a coaxial version. The fresh gas flows through the outside tube whereas the exhaled gases flow through the inside tube.
- 2.
A FGF rate of about 70 mL/kg/min is required in order to prevent rebreathing. This makes it an efficient breathing system for spontaneous ventilation.
- 3.
Since it is based on the Magill system, it is not suitable for controlled ventilation.
Instead of the coaxial design, a parallel tubing version of the system exists ( Fig. 4.7B ). This has separate inspiratory and expiratory tubing, and retains the same flow characteristics as the coaxial version.
- •
Coaxial version of Mapleson A, making it efficient for spontaneous ventilation. FGF rate of about 70 mL/kg/min is required.
- •
FGF is delivered along the outside tube and the exhaled gases flow along the inner tube.
- •
APL valve is at the machine end.
- •
Not suitable for controlled ventilation.
Mapleson B and C systems ( Figs 4.1 and 4.8 )
Components
- 1.
A reservoir bag. In the B system, corrugated tubing is attached to the bag and both act as a reservoir.
- 2.
An APL valve at the patienťs end.
- 3.
FGF is added just proximal to the APL.
Mechanism of action
Both systems are not efficient during spontaneous ventilation. An FGF of more than 1.5–2 times the minute volume is required to prevent rebreathing.
During controlled ventilation, the B system is more efficient due to the corrugated tubing acting as a reservoir. An FGF of more than 50% of the minute ventilation is still required to prevent rebreathing.
Mapleson C (also known as the Waters circuit) is lightweight and compact. It is used in resuscitation situations as an alternative to self-inflating bag. By adjusting the APL valve, it allows positive end-expiratory pressure (PEEP) with a visual and tactile ventilation monitor.
- •
B system has a tubing and bag reservoir.
- •
Both B and C systems are not efficient for spontaneous and controlled ventilation.
- •
B system is more efficient than the A system during controlled ventilation.
- •
C system is lightweight and used in resuscitation situation.
Bain system (Mapleson D)
The Bain system is a coaxial version of the Mapleson D system ( Fig. 4.9 ). It is lightweight and compact at the patient end. It is useful where access to the patient is limited, such as during head and neck surgery.
The Manley ventilator, which has been switched to a spontaneous ventilation mode, is an example of a non-coaxial Mapleson D system.
Components
- 1.
A length of coaxial tubing (tube inside a tube). The usual length is 180 cm, but it can be supplied at 270 cm (for dental or ophthalmic surgery) and 540 cm (for magnetic resonance imaging [MRI] scans where the anaesthetic machine needs to be kept outside the scanner’s magnetic field). Increasing the length of the tubing does not affect the physical properties of the breathing system.
- 2.
The fresh gas flows through the narrow inner tube (6 mm) while the exhaled gases flow through the outside tube (22 mm) ( Fig. 4.10 ). The internal lumen has a swivel mount at the patient end. This ensures that the internal tube cannot kink, thereby ensuring delivery of fresh gas to the patient.
Fig. 4.10
The proximal (machine’s) end of coaxial Bain’s breathing system. The FGF flows through the narrow inner tube.
- 3.
The reservoir bag is mounted at the machine end.
- 4.
The APL valve is mounted at the machine end.
Mechanism of action
- 1.
During spontaneous ventilation, the patienťs exhaled gases are channelled back to the reservoir bag and become mixed with fresh gas ( Fig. 4.11B ). Pressure build-up within the system will open the APL valve allowing the venting of the mixture of the exhaled gases and fresh gas ( Fig. 4.11C ).
Fig. 4.11
Mechanism of action of the Mapleson D breathing system during spontaneous ventilation. FGF, Fresh gas flow.
- 2.
The FGF required to prevent rebreathing (as seen in Fig. 4.11D ) during spontaneous ventilation is about 1.5–2 times the alveolar minute volume. A flow rate of 150–200 mL/kg/min is required. This makes it an inefficient and uneconomical system for use during spontaneous ventilation.
- 3.
It is a more efficient system for controlled ventilation. A flow of 70–100 mL/kg/min will maintain normocapnia. A flow of 100 mL/kg/min will cause moderate hypocapnia during controlled ventilation.
- 4.
Connection to a ventilator is possible ( Fig. 4.12 ). By removing the reservoir bag, a ventilator such as the Penlon Nuffield 200 can be connected to the bag mount using a 1-m length of corrugated tubing (the volume of tubing must exceed 500 mL if the driving gas from the ventilator is not to enter the breathing system). The APL valve must be fully closed. In this situation it acts a T-piece.
Fig. 4.12
The Bain breathing system connected to a ventilator (e.g. Penlon Nuffield 200) via tubing connected to the bag mount. FGF, Fresh gas flow.
- 5.
A parallel version of the D system is available.
Problems in practice and safety features
- 1.
The internal tube can kink, preventing fresh gas being delivered to the patient.
- 2.
The internal tube can become disconnected at the machine end, causing a large increase in the dead space and resulting in hypoxaemia and hypercapnia. Movement of the reservoir bag during spontaneous ventilation is not therefore an indication that the fresh gas is being delivered to the patient.
- •
Coaxial version of Mapleson D. A parallel version exits.
- •
Fresh gas flows along the inner tube and the exhaled gases flow along the outer tube.
- •
Not efficient for spontaneous ventilation. FGF rate required is 150–200 mL/kg/min.
- •
Efficient during controlled ventilation. FGF rate required is 70–100 mL/kg/min.
Exam tip: A common question is how to safety test the Bain breathing system to ensure that there is no kinking, obstruction or dislodgment of the inner tube delivering the FGF.
T-piece system (Mapleson E and F)
This is a valveless breathing system used in anaesthesia for children up to 25–30 kg body weight ( Fig. 4.13 ). It is suitable for both spontaneous and controlled ventilation.
Components
- 1.
A T-shaped tubing with three open ports ( Fig. 4.14 ).
Fig. 4.14
Mechanism of action of the T-piece breathing system. FGF, Fresh gas flow.
- 2.
Fresh gas from the anaesthetic machine is delivered via a tube to one port.
- 3.
The second port leads to the patienťs mask or tracheal tube. The connection should be as short as possible to reduce dead space.
- 4.
The third port leads to reservoir tubing. Jackson-Rees added a double-ended bag to the end of the reservoir tubing (making it Mapleson F).
- 5.
A modification exists where an APL valve is included before a closed-ended 500 mL reservoir bag. A pressure relief safety mechanism in the APL valve is actuated at a pressure of 30 cm H 2 O ( Fig. 4.15 ). This design allows effective scavenging.
Fig. 4.15
Intersurgical T-piece incorporating an APL valve and closed reservoir bag to enable effective scavenging.
Mechanism of action
- 1.
The system requires an FGF of 2.5–3 times the minute volume to prevent rebreathing with a minimal flow of 4 L/min.
- 2.
The double-ended bag acts as a visual monitor during spontaneous ventilation. In addition, the bag can be used for assisted or controlled ventilation.
- 3.
The bag can provide a degree of continuous positive airway pressure (CPAP) during spontaneous ventilation.
- 4.
Controlled ventilation is performed either by manual squeezing of the double-ended bag (intermittent occlusion of the reservoir tubing in the Mapleson E) or by removing the bag and connecting the reservoir tubing to a ventilator such as the Penlon Nuffield 200.
- 5.
The volume of the reservoir tubing determines the degree of rebreathing (too large a tube) or entrainment of ambient air (too small a tube). The volume of the reservoir tubing should approximate to the patienťs tidal volume.
Problems in practice and safety features
- 1.
Since there is no APL valve used in this breathing system, scavenging is a problem.
- 2.
Patients under 6 years of age have a low functional residual capacity (FRC). Mapleson E was designed before the advantages of CPAP were recognized for increasing the FRC. This problem can be partially overcome in the Mapleson F with the addition of the double-ended bag.
- •
Used in paediatric practice up to 25–30 kg body weight.
- •
Requires a high FGF during spontaneous ventilation.
- •
Offers minimal resistance to expiration.
- •
Valveless breathing system.
- •
Scavenging is difficult.
- •
A modified design with an APL valve and a closed-ended reservoir allows effective scavenging.
Exam tip: Make sure you can talk about and reassemble the separate components of a breathing system (tubing, reservoir, APL valve and face mask).
The Humphrey ADE breathing system
This is a very versatile breathing system which combines the advantages of the Mapleson A, D and E systems. It can therefore be used efficiently for spontaneous and controlled ventilation in both adults and children. The mode of use is determined by the position of one lever which is mounted on the Humphrey block ( Fig. 4.16 ). Both parallel and coaxial versions exist with similar efficiency. The parallel version will be considered here.
Components
- 1.
Two lengths of 15-mm smooth-bore tubing (corrugated tubing is not recommended). One delivers the fresh gas and the other carries away the exhaled gas. Distally they are connected to a Y-connection leading to the patient. Proximally they are connected to the Humphrey block.
- 2.
The Humphrey block is at the machine end and consists of
- a.
an APL valve featuring a visible indicator of valve performance
- b.
a 2-L reservoir bag
- c.
a lever to select either spontaneous or controlled ventilation
- d.
a port to which a ventilator can be connected, e.g. Penlon Nuffield 200
- e.
a safety pressure relief valve which opens at pressure in excess of 60 cm H 2 O
- f.
a modified design incorporating a soda lime canister.
- a.
Mechanism of action
- 1.
With the lever up ( Fig. 4.17A ) in the spontaneous mode, the reservoir bag and APL valve are connected to the breathing system as in the Magill system.
Fig. 4.17
Mechanism of action of the parallel Humphrey ADE breathing system. With the lever up (A), the system functions in its Mapleson A mode for spontaneous ventilation. For mechanical ventilation, the lever is down (B) and the system functions in its Mapleson E mode. FGF, Fresh gas flow.
(Courtesy Dr D. Humphrey.)
- 2.
With the lever down ( Fig. 4.17B ) in the ventilator mode, the reservoir bag and the APL valve are isolated from the breathing system as in the Mapleson E system. The expiratory tubing channels the exhaled gas via the ventilator port. Scavenging occurs at the ventilator’s expiratory valve.
- 3.
The system is suitable for paediatric and adult use. The tubing is rather narrow, with a low internal volume. Because of its smooth bore, there is no significant increase in resistance to flow compared to the 22-mm corrugated tubing used in other systems. Small tidal volumes are possible during controlled ventilation and less energy is needed to overcome the inertia of gases during spontaneous ventilation.
- 4.
The presence of an APL valve in the breathing system offers a physiological advantage during paediatric anaesthesia, since it is designed to offer a small amount of PEEP (1 cm H 2 O).
- 5.
During spontaneous ventilation
- a.
an FGF of about 50–60 mL/kg/min is needed in adults
- b.
the recommended initial FGF for children weighing less than 25 kg body weight is 3 L/min. This offers a considerable margin for safety.
- a.
- 6.
During controlled ventilation
- a.
an FGF of 70 mL/kg is needed in adults
- b.
the recommended initial FGF for children weighing less than 25 kg body weight is 3 L/min. However, adjustment may be necessary to maintain normocarbia.
- a.
- •
Can be used efficiently for spontaneous and controlled ventilation.
- •
Can be used in both adult and paediatric anaesthetic practice.
- •
Both parallel and coaxial versions exist.
- •
A ventilator can be connected.
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