systems, the expired carbon dioxide is not removed by voiding
the exhaled gases from the system, but by absorbing it using
an absorbing compound (Soda-lime or Baralyme) contained in a
canister. The remaining expired gas is then free to be inspired
again without accumulation of carbon dioxide occurring.
is a mixture of 94% calcium hydroxide, 5% sodium hydroxide and
1% potassium hydroxide. Carbon dioxide absorption occurs by
the following chemical reactions:
+ CO2 H2CO3 H+
+ H2CO3 NaHCO3
+ Ca(OH)2 2NaOH
+ CaCO3 + H2O
hydroxide provides the main capacity for carbon dioxide of soda
lime, the sodium hydroxide and potassium hydroxide being added
to accelerate the rate of absorption. Soda lime also contains
water (since the carbon dioxide must be dissolved before it
can react), a pH sensitive dye which indicates when exhaustion
of the soda lime is taking place, and silica to preserve its
two types of soda lime in use, one of which turns from white
to purple when it is exhausted, the other changing from pink
to white upon exhaustion, which may result in confusion. Since
the color change disappears when soda lime is left to stand,
it should be changed immediately exhaustion occurs.
may be used as an alternative to soda-lime, containing 20% barium
hydroxide and little alkali. It is less dusty and the dust is
less alkaline than that of soda-lime.
of a typical soda lime canister will provide around 8 hours
of use in small animals and it should then be changed whether
or not it has changed color.
of non-rebreathing systems
of anesthetic consumption.
- Warming and
humidification of the inspired gases.
Reduced atmospheric pollution.
control of the inspired anesthetic concentration, since fresh
gas delivered from the anesthetic machine is diluted by the
gas already contained within the circuit.
systems are invariably used in large animals (horses and cattle)
where the high gas consumption makes use of non-rebreathing
Two examples of rebreathing systems are the Circle
absorber and Waters' canister.