G Govender, Y Tang, MJ Turner, AB Baker
Department of Anaesthesia, University of Sydney, RPA Hospital, Sydney, Australia
Introduction: Desiccated CO2 absorbents can degrade volatile anaesthetics into carbon monoxide within anaesthetic breathing circuits1, which may lead to severe carbon monoxide toxicity intraoperatively2. Water is also essential for CO2 absorption. We evaluated the capacity of fully and partially desiccated soda lime to absorb CO2.
Methods: The soda lime was heated in a flask and weighed for water loss. Three mass flow controllers were used to deliver an accurate mixture of 5% CO2, 35% oxygen and 60% nitrogen at 3 l/min. The mixture was passed through 100g of soda lime in a canister within a water bath at 37oC until the outlet CO2 content reached approximately 5%. Outlet gas composition was monitored by an O2 and CO2 analyser (HP M1025B, Denmark) and recorded on a PC at a frequency of 20Hz. Five groups of soda lime: normal (No WL), 5% (5% WL), 10% (10% WL), 15% (15% WL), and maximum weight loss (Max WL) were tested 5 times each. An inlet gas mixture saturated with water vapour at 37oC was also used in the maximum water loss group (100% WV).
Results: The CO2 concentration profiles are plotted in Fig 1. Maximum water content loss was 16.9±0.1g. The outlet CO2 concentration reached 4% after 9.10±0.8, 16.4±2.1, 85.5±32.8, 114.4±10.8, 132.4±3.0 min for the maximum water loss group, 15%, 10%, 5%, and no water loss groups respectively; and the CO2 absorption capacities were 1.3±0.1, 2.2±0.3, 10.7±0.5, 14.9±1.4 and 17.6±0.1 litres respectively. With saturated water vapour added to the gas mixture, the CO2 absorption capacity of maximally dried soda lime was 1.36±0.05 litres and the outlet CO2 concentration reached 4% at 10.4 ± 0.3 min. The normalised CO2 absorption capacities are plotted in Figure 2. Using ANOVA, all groups were statistically significantly different (at P<0.001) except there was no significant difference between 15% WL, Max WL and 100% WV groups.
Conclusions: Our results show that soda lime CO2 absorption capacity depends strongly on its water content. Desiccated CO2 absorbent has significantly reduced CO2 absorption capacity. Gas mixtures with saturated water vapour do not increase CO2 absorption with desiccated soda lime. Taking carbon monoxide generation into consideration3, water loss should be less than 5% to prevent carbon monoxide production and to maintain adequate CO2 absorption capacity.
References: 1. Anesthesia and Analgesia. 144-146, 81, 1995
2. Anesthesiology. 613-616, 90, 2000
3. Anesthesia and Analgesia. 1187-1193, 80, 1995
Copy from:http://www.anesthesia.org.cn/2005china/eng/eng12.doc
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