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CO2 absorption after changing the design of the absorber
- Go Hirabayashi (28 August 2007)
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CO2 Absorption after Changing the Absorber design
- Erich Knolle, Hermann Gilly (14 August 2007)
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Go Hirabayashi
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We thank Knolle et al. for their interest and comments regarding our recent report.(1) Knolle et al. stated that we mistook our results as proof of better absorption caused by a cooling effect. We constructed a novel temperature gradient correction (TGC) canister, and investigated the effects of TGC on the water content and longevity (time to exhaustion) of CO2 absorbent. The TGC canister was produced for the purpose of correcting temperature gradients of CO2 absorbent, not for the purpose of cooling. TGC prevented local excessive increase of water content and improved the longevity of CO2 absorbent. Our study indicates a new theory; decreased longevity of CO2 absorbent is caused by heterogeneity of excessive water content due to temperature gradients. Knolle et al. may have misunderstood our study. It is believed that channelling of expired gas along preferential paths of lesser resistance through the CO2 absorbent decreases the efficiency and reliability of CO2 absorption. Usually these preferential paths are along the container walls where granules cannot fit snugly against the plane surfaces (wall effect).(2) This refers to channelling because of heterogeneity of gas flow. The degree of heterogeneity of the void depends on the shape, size and degree of collapse of the granules. The use of hemisphere-shaped CO2 absorbent and careful handling of the CO2 absorbent are thought to improve the wall effect. In this study, using hemisphere-shaped CO2 absorbent such as Drägersorb freeTM and handling the CO2 absorbent with care, it was thought that the preferential path flow caused by heterogeneity of the void would be minimal in both the conventional canister and the TGC canister. The wall effect might not play as important a role in channeling than previously thought. We supposed that the channeling effect should be considered on the basis of both the distribution of reactivity degradation and the pattern of flow. Knolle et al. stated that it is necessary to measure the precise inlet and outlet concentration curves of CO2. We measured EICO2, ETCO2, and PiCO2 of the canister inlet and outlet. However, we decided not to use PiCO2 of the canister inlet and outlet, because of the inconstancy affected by intermittent ventilation. Further, exhaustion of CO2 absorbent is usually determined by increase of EICO2. PiCO2 of the canister outlet is diluted by fresh gas, leading to EICO2. Knolle et al. pointed out that the sample collection time in the TGC experiments was 1-2 hours later than in those with the conventional absorber. The larger the temperature gradient and the longer the duration, the more excessively the water content of CO2 absorbent increases. There was only a slight increase in water content in the TGC canister in spite of the longer duration. Knolle et al. stated that the lower temperature indicates a smaller CO2 absorption. Cooling effect was not examined in this study. However, using the TGC canister cooled by a blower, we investigated the effects of cooling on CO2 absorption in another study (3). Cooling of CO2 absorbent decreased the longevity of CO2 absorption, as stated by Knolle et al. To prove the decreased CO2 absorption, we need to investigate the changes in reactivity of CO2 absorbent in various conditions, water content, temperature, and the type of CO2 absorbent. (1) Hirabayashi G, Uchino H, Sagara T, Kakinuma T, Ogihara Y, Ishii N. Effects of temperature gradient correction of carbon dioxide absorbent on carbon dioxide absorption. Br J Anaesth. 2006; 97: 571-5. (2) Elam J: Channeling and overpacking in carbon dioxide absorbers. Anesthesiology 1958;19: 403-4 (3) Hirabayashi G, Uchino H, Joko T, Kaneko H, Ishii N. Effects of cooling and temperature gradient reduction of carbon dioxide absorbent on water condensation in the anaesthesia circuit. Br J Anaesth. 2007 (in print). Conflict of Interest:None declared |
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Erich Knolle, Department of Special Anaesthesiology and Pain Therapy Medical University of Vienna, Hermann Gilly
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Editor — We read with interest Hirabayashi and colleagues’ (1) paper concerning an absorber, which differs from a "conventional" absorber by separating the absorbent into 6 chambers in order to improve CO2 absorption by a cooling effect of the chamber walls. By testing this new absorber in an anaesthesia machine with a 3l-bag connected as an artificial lung the authors found a longer CO2 absorption time and a lower absorber temperature in comparison to a "conventional" absorber. Obviously the authors mistake their results as a proof of better absorption caused by a cooling effect. CO2-absorption is a chemical reaction between CO2 and the absorbent, which lasts until the absorbent is exhausted. The better the absorption capacity (amount of CO2 which can be absorbed in 100 g of absorbent) the longer CO2-absorption will hold on. However, CO2-absorption will also be extended, when (some) CO2 passes the absorber without being absorbed (so called channelling). This can happen presumably at the absorber wall because of a lesser resistance to airflow in this area (2). By increasing the wall area - as it happened with the “new absorber” - the authors had possibly induced CO2 channelling accompanied by a reduced CO2 absorption per time unit. This reduction was accompanied by a smaller chemical reaction temperature and longer time period until the absorbent was exhausted. Hence the lower temperature indicates a smaller CO2 absorption and not – as speculated by the authors – a cooling effect of the absorber. To rule out channelling as the cause of the longer absorption time it would have been necessary to perform suitable tests for instance as described by Shaw and Scott (3). Alternatively it would have been necessary to compare the amounts of absorbed CO2 as the difference of inlet and outlet amount of CO2. However, the authors failed to determine the necessary precise inlet and outlet concentration curves of CO2 for the complete absorption process. Moreover, by ignoring PiCO2-concentrations lower than 5 mm Hg behind the absorber the passage of unabsorbed CO2 due to channelling at the walls of the new absorber remained undetected. Looking at the lower water content in the samples tested in the new absorber the authors ignore that the experiments with the new absorber were stopped 1-2 hours later than those experiments with the conventional absorber. Thus the absorbent samples in the new absorber were dried for 1- 2 hours longer by the ongoing gas flow, which explains their smaller water content. The authors’ interpretation, that the smaller water content is induced by the absorber construction is without any evidence. Obviously the authors deduce their idea of a better CO2 absorption (by cooling the absorber) from the well known fact that cooling an absorber reduces the formation of Compound A. This explains why they used almost the same absorber, which they had developed originally for reducing the formation of Compound A (4). However, the authors’ presumption that cooling an absorber additionally increases CO2 absorption is wrong and not at all proven by the present study. Moreover, for underlying their hypothesis the authors refer to inappropriate references (2-3) as in these papers the anaesthetic degradation in dry absorbents, but not CO2 absorption was investigated (5-6). In summary the authors unfortunately did not show that their canister construction might result in an overall improvement of CO2 absorption. (1) Hirabayashi G, Uchino H, Sagara T, Kakinuma T, Ogihara Y, Ishii N. Effects of temperature gradient correction of carbon dioxide absorbent on carbon dioxide absorption. Br J Anaesth. 2006 Oct;97(4):571-5. (2) Elam JO. The design of circle absorbers. Anesthesiology 1958; 19: 99-100 (3) Shaw M, Scott DHT. Performance characteristics of a “to and fro” disposable soda lime canister. Anaesthesia 1998; 53: 454-460 (4) Hirabayashi G, Mitsui T, Kakinuma T, Ogihara Y, Matsumoto S, Isshiki A, Yasuo W: Novel radiator for carbon dioxide absorbents in low- flow anesthesia. Ann Clin Lab Sci 2003;33(3):313-9 (5) Bito H, Ikeuchi Y, Ikeda K. Effects of the water content of soda lime on compound A concentration in the anesthesia circuit in sevoflurane anesthesia. Anesthesiology 1998; 88: 66–71 (6) Moriwaki G, Bito H, Ikeda K. Partly exhausted soda lime or soda lime with water added, inhibits the increase in compound A concentration in the circle system during low-flow sevoflurane anaesthesia. Br J Anaesth 1997; 79: 782–6 Conflict of Interest:None declared |
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