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BJA Advance Access originally published online on August 1, 2006
British Journal of Anaesthesia 2006 97(4):571-575; doi:10.1093/bja/ael210
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© The Board of Management and Trustees of the British Journal of Anaesthesia 2006. All rights reserved. For Permissions, please e-mail: journals.permissions@oxfordjournals.org

Effects of temperature gradient correction of carbon dioxide absorbent on carbon dioxide absorption{dagger}

G. Hirabayashi*, H. Uchino, T. Sagara, T. Kakinuma, Y. Ogihara and N. Ishii

Department of Anaesthesiology, Hachioji Medical Center Tokyo Medical University 1163 Tatemachi, Hachioji, Tokyo 193-0998, Japan

*Corresponding author. E-mail: goh{at}tokyo-med.ac.jp

Accepted for publication May 19, 2006.


   Abstract
Top
 Abstract
Introduction
 Materials and methods
 Results
 Discussion
 Conclusions
 References
 
Background. The effects of temperature gradients in CO2 absorbents on water content and CO2 absorption are not clear. We constructed a novel temperature gradient correction (TGC) canister, and investigated the effects of temperature gradient correction on the water content and longevity (time to exhaustion) of CO2 absorbent using a simulated anaesthesia circuit.

Methods. Experiments were divided into two groups according to the type of canister used: the TGC canister (n=6) or the conventional canister (n=6). One kilogram of fresh CO2 absorbent was placed into the canister. The anaesthetic ventilator was connected to a 3 litre bag and 300 ml min–1 of CO2 was introduced. Oxygen (500 ml min–1) was used as fresh gas. The anaesthetic ventilator was set at a ventilatory frequency of 12 bpm, and tidal volume was adjusted to 700 ml.

Results. Before the experiment, the water content of the fresh CO2 absorbent in the conventional canister and TGC canister was 16.1 (0.9)% and 15.7 (1.1)%, respectively. After the experiment, the water content of CO2 absorbent near the upper outer rim of the canister increased to 32.4 (0.7)% in the conventional canister, but increased to only 20.6 (1.3)% in the TGC canister (P<0.01). The longevity of CO2 absorbent in the conventional canister and TGC canister was 434 (9) min and 563 (13) min (P<0.01).

Conclusions. Temperature gradient correction prevented a local excessive increase in water content and improved the longevity of CO2 absorbent.

Keywords: anaesthetic circuits; carbon dioxide, elimination


   Introduction
Top
 Abstract
 Introduction
Materials and methods
 Results
 Discussion
 Conclusions
 References
 
Temperature gradients in CO2 absorbents resulting from local heat production and contact with the canister container, cooled by room air, may locally increase the water content by dew condensation. Previous studies reported that CO2 absorption is decreased by addition of water to the CO2 absorbent.1–3 However, the effects of temperature gradients on water content and CO2 absorption are not clear.

We hypothesized that correction of the temperature gradients will prevent excessive water content preserving the reactivity of the CO2 absorbent, thereby increasing its longevity. We constructed a novel temperature gradient correction (TGC) canister, and investigated the effects of TGC on water content and longevity (time to exhaustion) under low-flow anaesthesia using a simulated anaesthesia circuit.


   Materials and methods
Top
 Abstract
 Introduction
 Materials and methods
Results
 Discussion
 Conclusions
 References
 
TGC canister
The TGC canister (Fig. 1A and B) was produced for the purpose of correcting temperature gradients between the reactive and non-reactive sites of the CO2 absorbent, and between the canister exterior (cooled by room air) and canister interior. It is different from canisters for refrigeration.4–6 To conduct heat easily from the reactive site to the non-reactive site and the canister container, 12 aluminium plates were set into the canister vertically and radially, and the aluminium canister container was connected to these 12 aluminium plates. The aluminium container and aluminium plates were coated with electroless nickel plating to prevent corrosion by the CO2 absorbent. Electroless nickel plating produces nickel deposits, which may contain phosphorus and boron, onto catalytic metallic or catalysed non-metallic substrates by chemical reduction. Unlike electrolytically plated nickel coatings, electroless nickel plating produces very uniform, hard coatings, without an externally applied electric current, and are normally identified according to their phosphorus content. By varying the percentage of phosphorus or boron in the coating, deposits can be produced to exhibit non-magnetic and highly corrosion resistant characteristics or hard deposits with excellent wear resistance. The TGC canister was designed to be mounted on an anaesthesia machine (FabiusTM, Dräger, Lübeck, Germany), the same size as the conventional FabiusTM canister. Thermosensors (Sheath thermocouple, Toho Electronics Inc., Kanagawa, Japan) were installed at eight sites (A, B, C1, D1, E1, C2, D2 and E2) in both the TGC canister and conventional canister to measure the temperature at each site. A and B are points at the canister inlet and outlet, respectively. C1, D1 and E1 are points 5 mm from the internal surface of the outer rim of the canister, with C1 being near the top of the CO2 absorbent, D1 near the middle and E1 near the bottom. C2, D2 and E2 are points 20 mm from the outer rim of the canister, corresponding in height to C1, D1 and E1, respectively.


Figure 1
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  		Fig 1 Photograph of the TGC canister (A), and schematic longitudinal plane of the TGC canister (B). 1, aluminium canister container with electroless nickel plating; 2, 12 aluminium plates with electroless nickel plating connected to the canister container vertically and radially. Thermosensors were installed at eight sites (A, B, C1, D1, E1, C2, D2 and E2) in both the TGC canister and conventional canister to measure the temperature at each site. A and B are points at the canister inlet and outlet, respectively. C1, D1 and E1 are points 5 mm from the internal surface of the outer rim of the canister, with C1 being near the top of the CO2 absorbent, D1 near the middle and E1 near the bottom. C2, D2 and E2 are points 20 mm from the outer rim of the canister, corresponding in height to C1, D1 and E1, respectively.

 
Experimental protocol
Experiments were divided into two groups according to the type of canister used: the TGC canister (n=6) or conventional canister (n=6). One kilogram of fresh CO2 absorbent (Drägersorb freeTM, Dräger) was placed into the canister for each experiment. The anaesthetic ventilator was connected to a 3 litre bag. Oxygen was used as fresh gas at flow rates of 500 ml min–1. To simulate the oxygen consumption of a patient, sample gas was aspirated at a rate of approximately 200 ml min–1 from the inspiratory limb of the circuit and delivered to a medical gas analyzer (Datex, Helsinki, Finland). CO2 (300 ml min–1) was introduced through a needle situated in the 3 litre bag. During the experiment, the anaesthetic ventilator was set at an inspiratory:expiratory ratio of 1:2, a ventilatory frequency of 12 bpm and tidal volume was adjusted to an expired tidal volume of 700 ml. Room temperature was maintained at approximately 20°C.

Longevity (time to exhaustion) was measured as the time taken for the inspired CO2 tension (Formula) to increase from 0 to 5 mm Hg, and then the experiment was halted. Formula, end tidal CO2 (Formula), temperature of CO2 absorbent at each site of the canister and temperature of the canister inflow gas and outflow gas were continuously monitored during the experiment. Approximately 3 g of fresh CO2 absorbent was saved before the experiment. Immediately after completion of the experiment, approximately 3 g each of CO2 absorbent used was collected from six sites: C1, D1, E1, C2, D2 and E2. The collected CO2 absorbent was weighed, heated for 8 h or longer at 105°C in an oven and weighed again. The water content was determined by the difference between the wet and dry weights.

Statistical analysis
All values are shown as means (SD). Statistical differences were evaluated with the Mann–Whitney U-test. Statistical significance was established at the level of P<0.05.


   Results
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 Abstract
 Introduction
 Materials and methods
 Results
Discussion
 Conclusions
 References
 
Formula was maintained at approximately 30–31 mm Hg from the start of the experiments, and increased to approximately 34–35 mm Hg at the end of the experiments in both canisters.

The temperatures of canister inflow and outflow gas are shown in Figure 2A and B. There were no significant differences in the temperature of the canister inflow gas between the TGC and conventional canisters, and no significant differences in the temperature of the canister outflow gas.


Figure 2
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  		Fig 2 Temperature of canister inflow gas (A) and outflow gas (B) in the conventional (filled diamond) and TGC (open diamond) canisters. All values are shown as means (SD).

 
The temperature of the CO2 absorbent at each site is shown in Figure 3A and B. In the conventional canister, from the start to around 3–4 h, the area of maximum absorbent temperature (reactive site) was E2, reaching approximately 50°C. Over time, the area of maximum temperature shifted from the bottom to the top of the canister. There were temperature gradients vertically between the local reactive site and the non-reactive site. In addition, the absorbent temperature on the canister exterior was lower than that in the canister interior. On the other hand, in the TGC canister, the temperature gradients between the reactive site and non-reactive site and between the canister exterior and canister interior were reduced.


Figure 3
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  		Fig 3 Temperature of CO2 absorbent at C1 (filled circle), D1 (filled square), E1 (filled diamond), C2 (open circle), D2 (open square) and E2 (open diamond) in the conventional (A) and TGC (B) canisters. Values are shown as means (SD).

 
The temperature gradients between the reactive site (maximum temperature area) and C1 are shown in Figure 4, These were reduced in the TGC canister throughout the experiment, compared with the conventional canister (P<0.01).


Figure 4
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  		Fig 4 Temperature gradient between the reactive site (maximum temperature area) and C1 in the conventional (filled diamond) and TGC (open diamond) canisters. All values are shown as means (SD). *P<0.01 significant difference vs the conventional canister.

 
The absorbent water content is shown in Table 1. Before the experiment, the water content was 16.1 (0.9)% in the conventional canister and 15.7 (1.1)% in the TGC canister. At the end of the experiment, the absorbent in the middle or bottom layer of the canister had dried in both canisters. The water content at C1 increased excessively to 32.4 (0.7)% in the conventional canister, but increased to only 20.6 (1.3)% in the TGC canister (P<0.01). The longevity of CO2 absorbent in the conventional canister and TGC canister was 434 (9) min and 563 (13) min, respectively (P<0.01), i.e. approximately 30% increase.


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  		Table 1 Water content of CO2 absorbent. Fresh CO2 absorbent reserved before the experiment, and after completion of the experiment at C1, D1, E1, C2, D2 and E2. Values are shown as means (SD). P<0.05 significant difference vs the conventional canister

 

   Discussion
Top
 Abstract
 Introduction
 Materials and methods
 Results
 Discussion
Conclusions
 References
 
The water content of the CO2 absorbent increased excessively at C1 in the conventional canister. Temperature gradient correction prevented the local excessive water content increases in the TGC canister. The TGC canister increased the longevity of the CO2 absorbent by approximately 30%. These results indicate that channelling of CO2 absorbent is most likely because of local excessive water content in the CO2 absorbent.

One hundred grams of soda lime can absorb about 26 litre of CO2 according to the following chemical processes. During this exothermic reaction 1 mol of water contained in soda lime is consumed, and 13.7 kcal and 2 mol of water are formed by the absorption of 1 mol of CO2.

Formula

The FabiusTM canister used in the present study has an inflow passage pipe in the centre of the canister. The reactive site shifts from the bottom of the canister to the top with time. The CO2 absorbent, which reacts with CO2 locally at the reactive site, generates heat and water, resulting in drying of the absorbent. The CO2 absorbent in the middle and bottom layer of the canister is dry as a result of chemical reaction.

In the conventional canister, there were large temperature gradients vertically between the local reactive site and the non-reactive downstream site. In addition, the temperature of the CO2 absorbent on the canister exterior cooled by room air was lower than that in the canister interior. These large temperature gradients between the reactive site and lower temperature sites (especially C1) might cause excessive water content at C1 by dew condensation.

In the TGC canister, the temperature gradients were reduced throughout the experiment, compared with the conventional canister. This temperature gradient correction prevented local excessive increases in water content.

It is reasonable to suggest that the expired gas was bypassed through the top of the exterior of the canister, so that a large mass of unused CO2 absorbent inside the canister was wasted, leading to decreased longevity of CO2 absorbent in the conventional canister, because the water content increased excessively and the reactivity decreased locally at C1. Therefore, correction of temperature gradients prevents development of local excessive water content and preserves the reactivity of the CO2 absorbent, leading to improved longevity.

This hypothesis implies channelling because of heterogeneity of reactivity degradation, and warrants an investigation about degradation of reactivity, distribution of reactivity degradation, and the pattern and direction of flow.

Degradation of reactivity
The reactivity of CO2 absorbent changes depending on its water content. Water is present as a thin film on the granule surface.7 8 Water is essential because the reactions take place between ions that exist only in the presence of water. Absorbents with low water content exhaust rapidly. On the other hand, those with high water content have a slower rate of absorption, stickiness and increased resistance.1 Previous studies reported that CO2 absorption is decreased by addition of moisture to the CO2 absorbent.2 Moisture is necessary, but reactivity decreases when the CO2 absorbent is excessively moist.

Changes in reactivity of CO2 absorbent were not measured in the present study. However, in the conventional canister, the reactivity of CO2 absorbent at C1 is thought to be decreased largely because its water content exceeds 30%. The expired gas should be bypassed without absorption of CO2 through the low reactivity area, leading to channelling.

Distribution of degraded reactivity
Distribution of degraded reactivity becomes massive if exposed to large temperature gradients for long times leading to enhanced channelling effects. Distribution of degraded reactivity along the direction of the gas flow enhances the channelling effect, while distribution of degraded reactivity perpendicular to the direction of the gas flow does not. In the present study, the water content of the CO2 absorbent was excessive only at C1 in the conventional canister, while not in the TGC canister. However, distribution details are not clear.

Pattern and direction of flow
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).9 This refers to channelling because of heterogeneity of gas flow.

However, the channelling effect should be considered on the basis of both the distribution of degraded reactivity and the pattern of flow. In the case of strongly decreased reactivity, a wide distribution of decreased reactivity and preferential gas flow in the decreased reactivity area, the channelling effect would be enhanced. Adversely, dew condensation might occupy the void space (between the granules) along the container walls, leading to increased resistance to gas flow. Decreased gas flow at the decreased reactivity site would not enhance the channelling effect.

In the present study, the pattern and direction of flow were not measured, and the effect of 12 aluminium plates set into the TGC canister on the pattern and direction of flow is not clear.

However, in the conventional canister, the channelling effect might be synergistically enhanced because of heterogeneity of gas flow and degraded reactivity. On the other hand, in the TGC canister the channelling effect might be improved mainly because of improved degradation of reactivity.


   Conclusions
Top
 Abstract
 Introduction
 Materials and methods
 Results
 Discussion
 Conclusions
References
 
We constructed a novel TGC canister. Temperature gradient correction prevented a local excessive increase in water content and improved the longevity of CO2 absorbent. The TGC canister is a useful and simple device to improve the CO2 absorbent longevity in low-flow anaesthesia.


   Acknowledgments
 
Grant-in-Aid for Young Scientific Research (B) (15790834) from The Ministry of Education, Science and Culture (G.H.). Research Promotion Fund from Maruishi Pharmaceutical Co., Ltd (G.H.).


   Footnotes
 
{dagger}Declaration of interest. All experiments were performed in the Department of Anaesthesiology, Hachioji Medical Center Tokyo Medical University. Back


   References
Top
 Abstract
 Introduction
 Materials and methods
 Results
 Discussion
 Conclusions
 References
 
1 Brown ES, Bakamjian V, Seniff AM. Performance of absorbents: effect of moisture. Anesthesiology 1959; 20:613–17[Web of Science][Medline]

2 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[CrossRef][Web of Science][Medline]

3 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[Abstract/Free Full Text]

4 Hirabayashi G, Mitsui T, Kakinuma T, et al. Novel radiator for carbon dioxide absorbents in low-flow anesthesia. Ann Clin Lab Sci 2003; 33:313–19[Abstract/Free Full Text]

5 Osawa M and Shinomura T. Compound A concentration is decreased by cooling anaesthetic circuit during low-flow sevoflurane anaesthesia. Can J Anaesth 1998; 45:1215–18[Web of Science][Medline]

6 Ruzicka JA, Hidalgo JC, Tinker JH, Baker MT. Inhibition of volatile sevoflurane degradation product formation in an anesthesia circuit by a reduction in soda lime temperature. Anesthesiology 1994; 81:238–44[Web of Science][Medline]

7 Adriani J. Disposal of carbon dioxide from devices used for inhalational anesthesia. Anesthesiology 1960; 21:742–58[Web of Science][Medline]

8 Brown ES. Voids, pores and total air space of carbon dioxide absorbents. Anesthesiology 1958; 19:1–6[Web of Science][Medline]

9 Elam J. Channeling and overpacking in carbon dioxide absorbers. Anesthesiology 1958; 19:403–4[Web of Science][Medline]


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