Absorption of carbon dioxide by dry soda lime decreases carbon monoxide formation from isoflurane degradation

This study was performed to determine whether the absorption of carbon dioxide (CO2) influences the formation of carbon monoxide (CO) from degradation of isoflurane in dry soda lime. Isoflurane (0.5%), CO2 (5%), a combination of the two in oxygen and pure oxygen were separately passed through samples of 600 g of completely dried soda lime (duration of exposure, 60 min; flow rate, 5 L/min). Downstream of the soda lime, we measured concentrations of CO, isoflurane, and CO2 as well as the gas temperature. CO2 increased the peaks of CO concentration (842 +/- 81 vs 738 +/- 28 ppm) and shortened the rise time of CO to maximum values (12 +/- 2 vs 19 +/- 4 min). However, CO2 inhibited total CO formation(99 +/- 10 vs 145 +/- 6 mL). At the same time, CO2 absorption by the soda lime decreased in the presence of CO formation (from 21.4 +/- 0.8 to 19.4 +/- 0.9 g). The temperature of the gases increased during the passage of both isoflurane and CO2 (to 32.6 +/- 2.0 degrees C and 39.4 +/- 4.0 degrees C, respectively), but the largest increase (to 41.5 +/- 2.1 degrees C) was recorded when isoflurane and CO2 simultaneously passed through the dry soda lime. We assume that the simultaneous reduction in CO formation and CO2 absorption is caused by the competition for the alkali hydroxides present in most of soda lime brands.