The Anesthesia Gas Machine

Michael P. Dosch CRNA PhD
University of Detroit Mercy Graduate Program in Nurse Anesthesiology
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Revised July 2012


Carbon dioxide absorption

General characteristics

Ohio Model 21 absorber

Function- makes rebreathing possible, thus conserving gases and volatile agents, decreasing OR pollution, and avoiding hazards of carbon dioxide rebreathing. Soda lime- Activator is NaOH or KOH. Silica and kieselguhr added as hardeners. Indicators for SodasorbTM (such as ethyl violet) are colorless when fresh, and purple when exhausted, because of pH changes in the granules.


The strong bases (activators NaOH, KOH) have been convincingly implicated in the carbon monoxide problem with the ethyl-methyl ethers, and the generation of Compound A by sevoflurane. Two approaches to dealing with these problems have surfaced:

  1. Lithium hydroxide lime (LitholymeTM) (Allied Healthcare Products, Inc.) is also an effective carbon dioxide absorbent, and is free of the strong bases (NaOH, KOH).
  2. Eliminating the activators entirely produces an absorbent which has similar physical characteristics (but perhaps less carbon dioxide absorption efficiency), as compared to soda lime. AmsorbTM (Armstrong Medical Ltd., Coleraine Northern Ireland) was planned for introduction to the US market in 2000 by Abbott, but it is not yet widely available.

Medisorb granules on S/5 ADU Medisorb granules on ADU. Click on the thumbnail, or on the underlined text, to see the larger version (33 KB)

New "house brand" absorbents have been created to help deal with the problems of modern volatile anesthetic (desflurane, sevoflurane) breakdown. Dräger makes an absorbent with decreased amounts of NaOH, and no KOH (Drägersorb 800 Plus) and an absorbent which (it is stated) will not generate Compound A under any circumstances (Dragersorb Free). GE makes Medisorb, which also has decreased amounts of strong bases. The canisters which fit on the Aisys and ADU are filled with Medisorb.

BaralymeTM (Allied Healthcare Products Inc, St Louis MO) was withdrawn from the market worldwide in 2005. The - activator was Ba(OH)2 octahydrate; no hardeners, slightly less efficient. Colorless or pink changing to blue-gray with exhaustion.

For all:


Numbers are approximations which may not sum to 100%. Data assembled from Anesth Analg 2001;93:221-5, Anesthesiology 2001;95:1205-12, and Anesth Analg 2000;91:220-4 and Litholyme MSDS.

Component Soda lime Medisorb Dragersorb 800+ Amsorb Litholyme
Ca(OH)2 % 94 70-80 80 83 >75
NaOH % (activator) 2-4 1-2 2 - -
KOH % (activator) 1-3 0.003 2 - -
Lithium chloride % (catalyst) - - - - < 3
CaCl2 % (humectant) - - - 1 -
- - - 1 -
Polyvinylpyrrolidine %
- - - 1 -
Water Content % 14-19 16-20 ~14 14.5 12-19
Size (mesh) 4-8 4-8 4-8 4-8 4-8
Indicator Yes Yes Yes Yes Yes

Chemical reactions

Soda lime

  1. CO2 + H2O --> H2CO3
  2. H2CO3 + 2 NaOH (or KOH) --> Na2CO3 (or K2CO3) + 2 H2O + Energy
  3. Na2CO3 (or K2CO3) + Ca(OH)2 --> CaCO3 + 2 NaOH (or KOH)

#1 is called the first neutralization reaction. In #3 the second neutralization reaction and the regeneration of activator take place. CaCO3 is an insoluble precipitate.

To change canisters

To change canisters in an old dual-canister setup, follow the procedure below. Newer machines allow change of canisters mid-case without leaks or interruption in ventilation.

To change canisters Steps for changing canisters. Click on the thumbnail, or on the underlined text, to see the larger version (23 KB).
  1. Wear gloves
  2. Loosen clamp
  3. Remove & discard top canister
  4. Promote the bottom canister to the top and put the fresh canister on the bottom
  5. Check for circuit leaks
  6. Always remove wrap before inserting canister
  7. Don’t change mid-case; convert to semi-open circuit by increasing FGF to > 5 L/min

Clinical signs of exhaustion

Newer gas machines which measure inspired carbon dioxide (often with alarms) allow us to change canisters when inspired carbon dioxide exceeds a known threshold (2-3 cm H2O).

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