[Causes for the reaction between dry soda lime and halogenated inhalation anesthetics]

All volatile anesthetics undergo chemical breakdown to multiple, partly identified degradation products in the presence of dry soda lime. These chemical reactions are highly exothermic, ranging from 100 degrees C for halothane to 120 degrees C for sevoflurane. The increase in temperature correlates with the moisture content of the soda lime, being maximal below 5%. Sevoflurane and isoflurane were exposed to dry soda lime in a circle system. The anaesthetic gas was condensed in a series of cold temperature traps and the degradation products of the volatile anesthetics were analysed using GC/MS. Surprisingly, neither sevoflurane nor its degradation products could be measured in the gas-flow emerging from the soda lime during the first 15-20 min of exposure. After 20 minutes, larger quantities of methanol, compounds C and D as well as compounds A and B were detected. After 40-60 min of exposure, sevoflurane's degradation markedly decreased and unaltered sevoflurane emerged from the soda lime canister. Additionally, using isoflurane in the same experimental set-up resulted in various degradation products due to its reaction with dry soda lime. Obviously, all volatile anesthetics are prone to such a reaction. In conclusion, sevoflurane and isoflurane react with dry soda lime. These reactions are caused by the presence of two components of soda lime, sodium hydroxide and potassium hydroxide. A modification of soda lime to prevent its reaction with volatile anaesthetics is discussed.