Function of soda lime
Soda lime is used in breathing systems to absorb expired CO2 during anaesthesia. It can be incorporated in a Mapleson C system or a circle system.
1) H2O + CO2 ====> H2CO3
2) H2CO3 + 2 NaOH ====> Na2CO3 + 2H2O
3) Na2CO3 + Ca(OH)2====> CaCO3 + 2 NaOH
4) H2CO3 + Ca(OH)2 ====> CaCO3 + 2H2O
Note: reaction 4 occurs only at a very slow rate in these absorbents.
CO2 in solution reacts with sodium hydroxide to form the respective carbonates, which then react with calcium hydroxide to produce calcium carbonate, replenishing sodium hydroxide. Heat and water are produced during the reaction. Exhaustion of its activity is indicated by dyes; the most common one changes from pink to white.
Size of granules
The size of the soda lime granules is 4-8 mesh (i.e. will pass through a mesh of 4-8 strands per inch in each axis or 2.36–4.75 mm).
Key learning points:
-Exhaled gases are circled back to the canister, where CO2 absorption takes place and water and heat are produced. The warmed and humidified gas joins the fresh gas flow to be delivered to the patient.
-Compound A is a pentafluoroisoproprenyl fluoromethyl ether (an olefin). It is no longer considered to be a clinical hazard. Compound A is only produced with sevoflurane. Canada and Australia maintain minimum flow rates with use of sevoflurane, this restriction does not apply in the UK.
-Carbon monoxide production has occurred when volatile agents containing the CHF2 moiety (enflurane/isoflurane/desflurane) are passed over soda lime that has become desiccated or dried out.
[i] Carbon monoxide production from desflurane, enflurane, halothane, isoflurane, and sevoflurane with dry soda lime. Wissing H, Kuhn I, Warnken U, Dudziak R. Anesthesiology 2001; 95(5): 1205-12
[ii] Renal toxicity with sevoflurane: a storm in a teacup? Gentz BA, Malan TP Jr. Drugs 2001; 61(15): 2155-62
[iii] Compound A: does it matter? Eger EI 2nd. Can J Anaesth 2001; 48(5): 427-30