Carbon monoxide formation in dry soda lime is prolonged at low gas flow

When exposed to volatile anesthetics containing a CHF2-group, such as isoflurane, desiccated absorbents produce carbon monoxide (CO). In the anesthesia circuit, the anesthetic flow that passes through the absorber varies with the minute ventilation. We sought to determine CO formation at different levels of test gas flow. Isoflurane 0.5% (series A) or 0.5% isoflurane + 5% CO2 (series B) in pure O-2 was passed through dry soda lime samples (600 g, Draegersorb 800(R)) at flows of 1, 3, 5, 7, and 10 L/min. Each experiment was performed three times. At the outlet, CO concentration, isoflurane concentration and temperature were continuously recorded. In both series, the duration of CO formation was inversely related to the test gas flow. In series B, mean CO concentrations and the calculated amount of CO formation decreased significantly with increasing flow rates, which was not the case in series A. In both series, the higher the flow rate, the higher was the temperature and the shorter the time until the isoflurane concentration increased to the set level. We conclude that anesthetic degradation in dry soda lime is finite and, as long as no CO2 is added, will produce roughly the same amount of CO regardless of inflow rate. The inflow rate influences the duration of CO formation such that at lower minute ventilation longer CO formation can be expected.