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Rebreathing Circuits

In these 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.

Soda-lime is a mixture of 94% calcium hydroxide, 5% sodium hydroxide and 1% potassium hydroxide. Carbon dioxide absorption occurs by the following chemical reactions:

H2O + CO2    H2CO3    H+ + HCO3-

NaOH + H2CO3    NaHCO3 + H2O

2NaHCO3 + Ca(OH)2    2NaOH + CaCO3 + H2O

The calcium 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 granularity.

There are 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.

Baralyme 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.

The contents 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.

Advantages of rebreathing systems

  • Economy of anesthetic consumption.
  • Warming and humidification of the inspired gases.
  • Reduced atmospheric pollution.

Disadvantages

  • Poor control of the inspired anesthetic concentration, since fresh gas delivered from the anesthetic machine is diluted by the gas already contained within the circuit.

Uses
Rebreathing systems are invariably used in large animals (horses and cattle) where the high gas consumption makes use of non-rebreathing systems uneconomical.

Application
Two examples of rebreathing systems are the Circle absorber and Waters' canister.

Circle absorber   

Comments on this article should be addressed to Dr Guy Watney
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