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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.
Convenience
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 
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