Breathing systems

Published on 07/02/2015 by admin

Filed under Anesthesiology

Last modified 07/02/2015

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Breathing systems

Breathing systems must fulfil three objectives:

There are several breathing systems 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 systems A, D, E and F 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.

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. Synonymous terms for the APL valve are expiratory valve, spill valve and relief valve.

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 H2O is present in some designs (Fig. 4.3).

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.

Reservoir bag

The reservoir bag is an important component of most breathing systems.

Problems in practice and safety features

Magill system (Mapleson A)

This breathing system is popular and widely used in the UK.

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. 4.5C), the gases coming from the anatomical dead space (i.e. they have not undergone gas exchange so contain no CO2) are exhaled first and enter the tubing and are channelled back towards the reservoir bag which is being filled continuously with FGF.

3. During the expiratory pause, pressure built up within the system allows the FGF to expell the alveolar gases first out through the APL valve (Fig. 4.5D).

4. By that time the patient inspires again (Fig. 4.5B), 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 patient’s alveolar minute volume (about 70 mL/kg/min).

6. The Magill system is not an efficient system for controlled ventilation. A FGF rate of three times the alveolar minute volume is required to prevent rebreathing.

Problems in practice and safety features

It 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 patient’s end, especially if connected to a scavenging system. This places a lot of drag on the connections at the patient end.

Lack system (Mapleson A)

This is a coaxial modification of the Magill Mapleson A system.

Mechanism of action

Instead of the coaxial design, a parallel tubing version of the system exists (Fig. 4.6B). This has separate inspiratory and expiratory tubing, and retains the same flow characteristics as the coaxial version.

Mapleson B and C systems (see Fig. 4.1)

Bain system (Mapleson D)

The Bain system is a coaxial version of the Mapleson D system (Fig. 4.7). 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.

A Manley ventilator which has been switched to 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 inner tube while the exhaled gases flow through the outside tube (Fig. 4.8). The internal lumen has a swivel mount at the patient end. This ensures that the internal tube cannot kink, so ensuring delivery of fresh gas to the patient.

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 patient’s exhaled gases are channelled back to the reservoir bag and become mixed with fresh gas (Fig. 4.9B). 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. 4.9C).

2. The FGF required to prevent rebreathing (as seen in Fig. 4.9D) 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. 4.10). 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.

5. A parallel version of the D system is available.