Duration of carbon dioxide absorption by soda lime at low rates of fresh gas flow

Abstract

Study Objective: To determine the impact of a low fresh gas flow rate on the duration o f carbon dioxide (CO 2) absorption by soda lime. Design: Nonchnical, experimental. Setting: Experimental laboratory. Methods: In vitro test with Sodasorb and a semiclosed breathing circle ventilating a test lung with a CO 2 inflow of 250 ml per minute. Fresh gas, flow rates o f 0.25, 0.5, 1, 2, and 4 L/min were studied. Measurements and Main Results: CO 2 was measured at the breathing circuit test lung interface with a mainstream capnometer. Duration of CO 2 absorption was determined as the time for the inspired CO 2 tension (PICO 2) to increase from 0 mm to 7 mm of mercury. The times of this interval were recorded four times for each fresh gas flow rate and compared by analysis of variance; p < 0.05 was considered significant. Time to soda lime failure was significantly longer at 2 L/min than at 1 L/min fresh gas flow and at 1 L/min than at 0.25 L/min fresh gas flow. Conclusion: Because soda lime color indicators are unreliable, when a semiclosed breathing circle is used at a low rate of fresh gas flow without CO 2 monitoring, the CO 2 absorbent must be replaced more frequently.