The Anesthesia Gas Machine

Michael P. Dosch CRNA PhD, Darin Tharp CRNA MS
University of Detroit Mercy - Nurse Anesthesia
This site is https://healthprofessions.udmercy.edu/academics/na/agm/index.htm.

Revised Jan 2024

ANESTHESIA GAS MACHINE> COMPONENTS & SYSTEMS> DELIVERY> CARBON DIOXIDE ABSORPTION

• General characteristics and composition
• Chemistry- soda lime
• Safe use
  • Changing canisters
  • Clinical signs of exhaustion

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:

• Sevoflurane is unstable in soda lime, producing Compound A (lethal at 130-340 ppm, or renal injury at 25-50 ppm in rats; but incidence of toxic [hepatic or renal] or lethal effects in millions of humans are very low, and comparable to desflurane). Compound A concentrations of 25-50 ppm are easily achievable in normal clinical practice. Sevoflurane package insert recommends avoiding FGF at 1 L/min for no more than 2 MAC-Hours (2 L/min FGF can be used indefinitely). However, use of absorbents without strong bases (e.g. Amsorb, Litholyme, etc.) mean that prolonged low fresh gas flow with sevoflurane is safe (Anaesth Intensive Care 2019;47:223 at doi 10.1177%2F0310057X19843304)
• Carbon monoxide is produced by (desflurane > enflurane > isoflurane) >> (halothane = sevoflurane). Worse in dry absorbent. So turn oxygen off at end of case, change absorbent regularly, change if FGF left on over the weekend or overnight, and use low flows (this will tend to keep granules moist). Again, carbon monoxide production with desflurane is extremely unlikely if absorbents with strong bases as activators are avoided (Anesth Analg 2021;132:993).

The strong bases (NaOH, KOH which function as activators) have been convincingly implicated in the carbon monoxide problem with the methyl-ethyl ethers (desflurane, sevoflurane, isoflurane), 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 (Litholyme^TM, 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, ASA Abstract A717). Litholyme does not produce Compound A or carbon monoxide from breakdown of any anesthetic agent under any circumstances, even when it is desiccated (Dahms et al 2010, ASA Abstract A718). Data supplied by the manufacturer shows that Litholyme:

• does not produce Compound A, even when the absorbent is fully desiccated
• when exhausted, undergoes a permanent color change which will not revert upon resting the absorbent
• generates less heat than soda lime
• is comparable in price to soda lime

Spiralith has lithium hydroxide (rather than calcium hydroxide) as its main constituent. Spiralith  is a unique absorbent which is efficient, and does not degrade volatile agents. It does not desiccate since its water content is chemically bound. Spiralith 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 capnography monitoring.

Both Litholyme and Spiralith were found to be quite efficient in Aisys canisters (J Clin Monit Comput 2016;30:193 at doi 10.1007/s10877-015-9699-2). 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:

• Acta Medica Marisiensis 2015;61:4 at doi 10.1515/amma-2015-0023.
• Anesthesiology 2001;95:750 at doi 10.1097/00000542-200109000-00030.

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 Free) which will not generate Compound A or carbon monoxide under any circumstances (low flows, or desiccated granules; see Anaesthesia 2004;59:584 at doi 10.1111/j.1365-2044.2004.03704.x. GE makes Medisorb, which also has decreased amounts of strong bases.

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:

• Size a compromise between absorptive capacity and resistance to airflow
• Resistance of full canister is < 1 cm H2O at 60 L/min flow through the canister
• Inhaled dust is caustic and irritant
• May see exhaustion without color change, due to channeling or inactivation of indicator along the canister walls by UV light. So check color at the end of the case, and change on a regular basis (there are few recommended intervals published because the way soda lime is used varies so greatly from room to room). Do not assume that lack of color change means that the granules are intact (see Anesthesiology 2000;92:1196 at doi 10.1097/00000542-200004000-00041.)

Composition

* The water in Spiralith is chemically bound, so it does not desiccate. It does not have a granular size, since absorbent powder is encased in a polymer matrix, and it does not have an indicator.
** Draegersorb Free and Amsorb Plus contain CaCl₂ as a humectant (helps retain moisture).
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 at doi 10.1097/00000542-200111000-00026
• Anesth Analg. 2000;91:220
• spiralithusa.com/product-support/#papers
• Litholyme MSDS
• Anaesthesia 2004;59:584 at doi 10.1111/j.1365-2044.2004.03704.x

Chemical reactions

Soda lime

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. See

• Br J Anaesth 2011;107(e-Letters Supplement 23 December) at doi 10.1093/bja/el_7463; J Clin Monit Comput 2010;24:143 at doi 10.1007/s10877-010-9223-7; Anesth Analg 2009;109:1350; Anesthesiology 2013;118:751 at doi 10.1097/ALN.0b013e318282a32c.

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 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.

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). Using F_ICO₂ as the trigger for absorbent granule change is a more efficient use of granules than relying on color change Anesth Analg 2021;132:993 at doi 10.1213/ANE.0000000000005137.

Signs of elevated inspired CO₂

• Rise (later a fall) in heart rate and blood pressure
• Hyperpnea
• Respiratory acidosis on blood gas analysis
• Dysrhythmia
• Signs of SNS activation
  • Flushed
  • Cardiac irregularities
  • Sweating
• Increased bleeding at surgical site
• Increased end tidal carbon dioxide
• NOT dark or cherry-red blood!