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Duration of carbon dioxide absorption by soda lime at low rates of fresh gas flow
M Ohrn1, N Gravenstein, M L Good
1Department of Anesthesiology, University of Florida College of Medicine, Gainesville.
Study Objective:
To determine the impact of a low fresh gas flow rate on the duration of carbon dioxide (CO2) absorption by soda lime.
Design:
Nonclinical, experimental.
Setting:
Experimental laboratory.
Methods:
In vitro test with Sodasorb and a semiclosed breathing circle ventilating a test lung with a CO2 inflow of 250 ml per minute. Fresh gas flow rates of 0.25, 0.5, 1, 2, and 4 L/min were studied.
Measurements And Main Results:
CO2 was measured at the breathing circuit test lung interface with a mainstream capnometer. Duration of CO2 absorption was determined as the time for the inspired CO2 tension (PICO2) 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 less than 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 CO2 monitoring, the CO2 absorbent must be replaced more frequently.