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Breathing Circuits

The parts of the anesthetic apparatus through which the patient breathes are known as the breathing system or circuit. Basic functions common to all breathing systems are means of supplying fresh gas (usually oxygen or an oxygen/nitrous oxide mixture, and a volatile anesthetic) to the patient and eliminating expired carbon dioxide. Additional functions of the system usually include a method of performing intermittent positive pressure ventilation of the lungs (IPPV).

In the 150 years since the first clinical anesthetics were administered, an enormous number of different breathing systems have been developed, many of which differ from each other only in minor details. Various classification systems have been developed to aid understanding of how the systems operate.

Open, semi-open, semi-closed and closed
This most basic classification of breathing systems divides them into open, semi-open, semi-closed or closed. Unfortunately, in different parts of the world, the same terminology is used for different systems, so this classification is probably best avoided.

Non-rebreathing and rebreathing systems
The classification of non-rebreathing versus rebreathing systems is less open to confusion:

  • Non-rebreathing systems - those in which the exhaled gases containing carbon dioxide are removed from the system.
  • Rebreathing systems - carbon dioxide is removed from the exhaled gases, allowing them to be inhaled again.

Circuit Selection

Although some advocate the use of specific breathing circuits with an almost messianic zeal, the question of which circuit is most suitable for a particular application is essentially a pragmatic one.

Patient Size
Here, the choice is essentially a compromise between controllability of anesthetic depth and economy of anesthetic agent and carrier gas usage. Although it would be perfectly possible to constuct a T-piece system for use in horses, the gas flow required would be in excess of 50 l/min, which is clearly impractical. Equally, it would be possible to construct a circle absorber system for rodents, but the small gas flow rates required for non-rebreathing circuits in very small animals would render this exercise pointless.
It is almost universal to use rebreathing systems, such as the circle absorber, for large animals (over around 100 kg body weight), and variations on the T-piece theme for very small ones (under around 5 kg body weight). For animals in between these weights, the choice often depends on local custom: in Great Britain, the Magill and Bain are commonly used (accepting the increase in anesthetic consumption) whereas in the USA the circle absorber is almost universally employed (accepting the bulk, complexity and dead space of the apparatus, and the reduced controllability of anesthetic depth).

Mode of ventilation
Some circuits (e.g. the Magill) are not efficient during intermittent positive pressure ventilation, so should be avoided if artificial ventilation of the lungs is required.

In many circumstances, convenience may have a bearing on which circuit is selected. For example, it is very much easier to scavenge waste gases with the Lack and the Bain systems than it is with the Magill or the conventional T-piece. For large animal anesthesia in the field, the light weight and compactness of the Water's canister may make it preferable to a circle absorber system.

Non-rebreathing systems  

Comments on this article should be addressed to Dr Guy Watney
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