Soda lime is a mixture of chemicals, used in granular form in closed breathing environments, such as general anaesthesia, submarines, rebreathers and recompression chambers, to remove carbon dioxide from breathing gases to prevent CO2 retention and carbon dioxide poisoning.
The main components of soda lime are
- Calcium hydroxide, Ca(OH)2 (about 75%)
- Water, H2O (about 20%)
- Sodium hydroxide, NaOH (about 3%)
- Potassium hydroxide, KOH (about 1%).
During the administration of general anaesthesia, a patient's expired gases, which contain carbon dioxide, are passed through an anaesthetic machine breathing circuit filled with soda lime granules. Medical-grade soda lime includes an indicating dye that changes color when the soda lime loses its carbon-dioxide-absorbing capacity.
To ensure that a soda lime canister (CO2 absorber) is functioning properly, it must be replaced regularly and should not be used if the indicating dye is activated.
Older anaesthesia machines came equipped will large soda lime canisters within the circle system. They could be loaded with up to 2 kg of soda lime granules.
But, with the growing use of expensive inhalation agents in anaesthesia like Desflurane & sevoflurane, and also looking to their greenhouse gas potential, low-flow general anaesthesia is gaining popularity, which necessitates a small soda lime canister usually of 0.9 kg.
Fresh soda lime and end-tidal carbon dioxide monitoring (EtCO2) are a must for low-flow general anaesthesia or else, life-threatening hypercarbia can develop in the anaesthesia circuit.
There are now newer generation CO2 absorbents which may pose less of a risk of toxic by-product as a result of interaction between the absorbent and inhaled anesthetics. Absorbents made from lithium hydroxide (LiOH) have been on the market for a decade or more.
Exhaled gas must be passed through a "carbon dioxide scrubber" where the carbon dioxide is absorbed before the gas is made available to be breathed again. In rebreathers the scrubber is a part of the breathing loop. Color indicating dye was removed from US Navy fleet use in 1996 when it was suspected of releasing chemicals into the circuit. In larger environments, such as recompression chambers or submarines, a fan is used to maintain the flow of gas through the scrubbing canister.
The overall reaction is:
CO2 + Ca(OH)2 → CaCO3 + H2O + heat (in the presence of water)
The reaction can be considered as a strong-base-catalysed, water-facilitated reaction.
1) CO2 → CO2 (aq) (CO2 dissolves in water - slow and rate-determining)
2) CO2 (aq) + NaOH → NaHCO3 (bicarbonate formation at high pH)
3) NaHCO3 + Ca(OH)2 → CaCO3 + H2O + NaOH ((NaOH recycled to step 2) - hence a catalyst)
Each mole of CO2 (44 g) reacted produces one mole of water (18 g).
- J. Jeff Andrews (1 September 2005). "Anesthesia Systems". In Paul G. Barash, Bruce F. Cullen and Robert K. Stoelting. Clinical Anesthesia (5th ed.). United States: Lippincott Williams & Wilkins. p. 1584. ISBN 0-7817-5745-2. Retrieved 1 July 2010.
- Brubakk, Alf O.; Tom S. Neuman (2003). Bennett and Elliott's physiology and medicine of diving, 5th Rev ed. United States: Saunders Ltd. p. 800. ISBN 0-7020-2571-2.
- Richardson, Drew; Menduno, Michael; Shreeves, Karl (eds). (1996). "Proceedings of Rebreather Forum 2.0.". Diving Science and Technology Workshop. (Diving Science and Technology): 286. Retrieved 2009-03-18.
- Lillo RS, Ruby A, Gummin DD, Porter WR, Caldwell JM (March 1996). "Chemical safety of U.S. Navy Fleet soda lime". Undersea Hyperb Med 23 (1): 43–53. PMID 8653065. Retrieved 2009-03-18.