Revised July 2012
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
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.
- Soda lime is absolutely incompatible with trichloroethylene (an older volatile agent; 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 (2 L/min fresh gas flow can be used indefinitely).
- Carbon monoxide is produced by (desflurane > enflurane > isoflurane) >> (halothane = sevoflurane). Worse in dry absorbent, or with BaralymeTM 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).
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:
- Lithium hydroxide lime (LitholymeTM) (Allied Healthcare Products, Inc.) is also an effective carbon dioxide absorbent, and is free of the strong bases (NaOH, KOH).
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.
- "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 that indicate 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
- See here for data
- 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 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.
- 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.)
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.
|NaOH % (activator)
|KOH % (activator)
|Lithium chloride % (catalyst)
|Water Content %
- CO2 + H2O --> H2CO3
- H2CO3 + 2 NaOH (or KOH) --> Na2CO3 (or K2CO3) + 2 H2O + Energy
- 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
||Steps for changing canisters. Click on the thumbnail, or on the underlined text, to see the larger version (23 KB).
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.
- Wear gloves
- Loosen clamp
- Remove & discard top canister
- Promote the bottom canister to the top and put the fresh canister on the bottom
- Check for circuit leaks
- Always remove wrap before inserting canister
- 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).
- Rise (later a fall) in heart rate and blood pressure
- Respiratory acidosis
- Signs of SNS activation
- Cardiac irregularities
- Increased bleeding at surgical site
- Increased end tidal carbon dioxide
- NOT dark or cherry-red blood!