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.

The reaction:

1) H₂O + CO₂ ====> H₂CO₃
                     high pH

2) H₂CO₃ + 2 NaOH ====> Na₂CO₃ + 2H₂O
                             high pH

3) Na₂CO₃ + Ca(OH)₂====> CaCO₃ + 2 NaOH
                                high pH 
                                also

4) H₂CO₃ + Ca(OH)₂ ====> CaCO₃ + 2H₂O
                                high pH

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:

-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.

References

[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