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

• General characteristics
• Chemistry- soda lime, barium hydroxide lime, Amsorb, and other new formulations
• 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 Sodasorb^TM (such as ethyl violet) are colorless when fresh, and purple when exhausted, because of pH changes in the granules.

Soda lime is absolutely incompatible with trichloroethylene (causes production of dichloroacetylene, a cranial neurotoxin and phosgene, a potent pulmonary irritant). 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 comparable to desflurane). Compound A concentrations of 25-50 ppm are easily achievable in normal clinical practice. Sevoflurane is not recommended at total fresh gas flows less than 1 L/min for more than 2 MAC-Hours. Carbon monoxide is produced by (desflurane >= enflurane > isoflurane) >> (halothane = sevoflurane). Worse in dry absorbent, or with baralyme as compared to soda lime. 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).

Amsorb 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. Eliminating the activators produces an absorbent which has similar physical characteristics and carbon dioxide absorption efficiency (perhaps- this is controversial), as compared to soda lime. Amsorb (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. Lithium hydroxide is also an effective carbon dioxide absorbent.

Read more:
• Anesthesiology 1999 Nov;91:1342-8
• Anesth Analg 2001;93:221-5
• Anesthesiology 2002;96:173-82
• Anesthesiology 2001;95:1205-12

Medisorb granules on S/5 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. North American Dräger makes an absorbent with decreased amounts of NaOH, and no KOH - Drägersorb 800 Plus. Datex-Ohmeda makes Medisorb, which has similar composition. The canisters which fit on the S/5 ADU are filled with Medisorb.

Baralyme- activator 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 Apr;92:1196-8.)

Composition

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.

Chemical reactions

Soda lime

Barium hydroxide lime (Baralyme^TM)

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

• Rise (later a fall) in heart rate and blood pressure
• Hyperpnea
• Respiratory acidosis
• 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!