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Breathing systems


Breathing systems must fulfil three objectives:

  • 1.

    delivery of oxygen,

  • 2.

    removal of carbon dioxide (CO 2 ) from the patient and

  • 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. 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 in post-anaesthesia recovery units 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 and is effective in eliminating CO 2 .

  • 8.

    Has a low resistance and minimal dead space.

  • 9.

    Ability to conserve heat and moisture.

  • 10.

    Easy to maintain with minimal running costs.

Components of the breathing systems

Adjustable Pressure Limiting Valve

The adjustable pressure limiting (APL) valve is a valve that allows the exhaled gases and excess FGF to leave the breathing system ( Fig. 4.1 ). 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 .

Fig. 4.1, Diagram of an adjustable pressure limiting valve.

Components

  • 1.

    The APL valve has three ports: the inlet, the patient and the exhaust. The latter can be opened 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, is 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. 4.2 ), even when the cap is screwed down and the valve is fully closed.

    Fig. 4.2, Intersurgical adjustable pressure limiting 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 from condensing on the disc.

  • 4.

    The valve can add bulk to the breathing system.

Adjustable pressure limiting valve

  • 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 (0.1 kPa) is needed to actuate it.

  • A pressure relief safety mechanism is actuated at 60 cm H 2 O (6 kPa) 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 plastic (latex-free) or antistatic rubber. Designs tend to be ellipsoidal in shape.

  • 2.

    The standard adult size is 2 L ( Fig. 4.3 ). 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. 4.3, 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. 4.4 ).

    Fig. 4.4, 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.

Reservoir bag

  • 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 ( Fig. 4.5 ). 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 widespread use of the circle breathing system.

Fig. 4.5, Mapleson classification of anaesthetic breathing systems. The arrow indicates entry of fresh gas to the system. See text for details.

Magill system (Mapleson A)

This breathing system was popular and widely used in the United Kingdom.

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’s end.

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