The Anesthesia Gas Machine

Michael P. Dosch CRNA PhD, Darin Tharp CRNA MS
University of Detroit Mercy Graduate Program in Nurse Anesthesiology
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Revised March 2016


Carbon dioxide absorption

General characteristics

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 soda lime (such as ethyl violet) are colorless when fresh, and purple when exhausted, because of pH changes in the granules. Problems:

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

  1. Lithium-containing (Litholyme) or lithium-based (Spiralith) absorbents
  2. Absorbents which lack activators

Lithium hydroxide lime (LitholymeTM) (Allied Healthcare Products, Inc.) is an effective carbon dioxide absorbent, and is free of the strong bases (NaOH, KOH). "Litholyme contains: LiCl as the catalyst to accelerate the formation of CaCO3; ethyl violet as the indicator; and does not contain KOH or NaOH." The CO2 absorbing capacity is similar to Sodasorb- and more than Amsorb (Dahms et al. 2010, Abstract A717). Litholyme does not produce carbon monoxide from breakdown of desflurane or other methyl-ethyl ethers under any circumstances- even when it is dessicated Dahms et al 2010, Abstract A718. The manufacturer shows data here that indicate that Litholyme:

Spiralith has lithium hydroxide (rather than calcium hydroxide) as its main constituent. Spirolith is a unique absorbent which is efficient, and does not degrade volatile agents. It does not desiccate since its water content is chemically bound. Spirolith is a powder enclosed in a polymer sheet, making the absorbent recyclable, and also limiting the danger to eyes and skin posed by dust in traditional absorbents. Since it lacks a color indicator, it may only be used with FICO2 monitoring.

Both Litholyme and Spiralith were found to be quite efficient in Aisys canisters (J Clin Monit Comput. 2016;30:193). Lithium hydroxide is a mature technology with a long history of use in space and submarine environments (though somewhat new in anesthesia applications).

Eliminating the activators entirely produces an absorbent which has similar physical characteristics (but perhaps less carbon dioxide absorption efficiency), as compared to soda lime: Amsorb (Armstrong Medical Ltd., Coleraine Northern Ireland).

Read more:

Dragersorb 800 granules in CLIC absorber on Apollo Drägersorb 800 granules in CLIC absorber on Apollo. Click on the thumbnail, or on the underlined text, to see the larger version.

New "house brand" absorbents have been created to help deal with the problems of modern volatile anesthetic (desflurane, sevoflurane) breakdown. "House brand" absorbents made by GE and Dräger have less activator currently; especially less KOH. 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 Avance are filled with Medisorb.

Baralyme (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 APSF Newsl 2005;2:25; Anesth Analg 2001;93:221; Anesthesiology 2001; 95:1205; Anesth Analg. 2000;91:220; www.spiralith.com; and

Component Soda lime Medisorb Dragersorb 800+ Amsorb Litholyme Spiralith
Ca(OH)2 % 94 70-80 80 83 >75 -
NaOH % (activator) 2-4 1-2 2 - - -
KOH % (activator) 1-3 0.003 2 - - -
LiOH % - - - - - 95
Li2CO3 % - - - - - 3
Lithium chloride % (catalyst) - - - - < 3 -
Water Content % 14-19 16-20 ~14 14.5 12-19 Unknown*
Size (mesh) 4-8 4-8 4-8 4-8 4-8 -
Indicator Yes Yes Yes Yes Yes No
* The water in Spiralith is chemically bound, so it does not dessicate. It does not have a granular size, since absorbent powder is encased in a polymer matrix, and it does not have an indicator.

Chemical reactions

Soda lime

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

#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.

If present, KOH participates in the reactions above in the same manner as NaOH.

To change canisters

While it is possible to change canisters mid-case, it is safer to wait until the end of the case to do so. There are many case reports of inability to ventilate due to leaks after canisters could not be fit back together (Br J Anaesth 2011. Available at: J Clin Monit Comput. 2010;24:143. Anesth Analg 2009;109:1350. Anesthesiology 2013;118:751.)

Canister Change- Aisys Canister Change in Aisys. Click on the thumbnail, or on the underlined text, to see the larger version.

To change the absorbent granule canister on Aisys, press the latch release (arrow at left). The canister will pivot, and may then be slid off its rails (right). A new canister is then slid on, and pivoted upwards to latch.


CLIC canisters on Apollo CLIC canister Change on Apollo. Click on the thumbnail, or on the underlined text, to see the larger version.

The CLIC canister is changed in a similar way on the Apollo.


To change canisters Steps for changing Aestiva canisters. Click on the thumbnail, or on the underlined text, to see the larger version (23 KB).

To change canisters in an Aestiva dual-canister setup, follow the procedure below.

  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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