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TECHNOLOGY, COMPUTING, AND SIMULATION

Small Carbon Monoxide Formation in Absorbents Does Not Correlate with Small Carbon Dioxide Absorption

Erich Knolle, MD^*, Georg Heinze, PhD, and Hermann Gilly, PhD^*

Departments of *Anesthesiology and General Intensive Care (B) and Medical Computer Sciences, University of Vienna; and Ludwig Boltzmann Institute for Experimental Anesthesiology and Research in Intensive Care Medicine, Vienna, Austria

Address correspondence and reprint requests to Erich Knolle, MD, Department of Anesthesiology and General Intensive Care (B), University of Vienna, Waehringer Guertel 18-20, A-1090 Vienna, Austria. Address e-mail to erich.knolle{at}univie.ac.at

	Abstract

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In this study we sought to determine whether an absorbent in which little carbon monoxide (CO) forms has a correspondingly small capacity to absorb carbon dioxide (CO₂). Completely dried samples (600 g) of Baralyme (A), Drägersorb 800 (B), Drägersorb 800 Plus (C), Intersorb (D), Spherasorb (E), LoFloSorb (F), Superia (G), and Amsorb (H) were exposed to a flow of 0.5% (A–H; n = 4–5) and 4% isoflurane (F–H; n = 3) in pure oxygen at 5 L/min for 60 min. Downstream CO concentration, temperature, and isoflurane concentration were recorded every 60 s to calculate CO formation and isoflurane loss. The CO₂ absorption capacity of each brand was determined by passing 5.1% CO₂ in oxygen (flow, 250 mL/min) through untreated samples (30 g; n = 5) until the outlet CO₂ concentration reached 0.5%. CO formation was largest in absorbents containing potassium hydroxide (A and B) and negligible in absorbents not containing any alkali hydroxide (F–H). The outlet temperature correlated with CO formation, but the isoflurane loss did not. The duration of CO₂ absorption also did not correlate with CO formation. We conclude that absorbents that allow only very little CO formation are not necessarily poor CO₂ absorbents.

IMPLICATIONS: In an in vitrostudy, carbon dioxide (CO₂) absorption capacity and possible carbon monoxide (CO) formation were tested in different absorbent brands. Absorbents with very small CO formation are not necessarily poor CO₂ absorbents.

	Introduction

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Volatile anesthetics, such as isoflurane, that contain a CHF₂ group degrade in dry carbon dioxide (CO₂) absorbents, resulting in a marked loss of anesthetic and in carbon monoxide (CO) formation (1) that can be associated with CO poisoning (2–7). Because the CO formation seems to depend on the presence of alkali hydroxides in soda lime (8), new absorbents have been developed combining Ca(OH)₂ with NaOH but not with KOH, or only Ca(OH)₂ with no alkali hydroxides at all (9). However, some recent investigations of alkali-reduced or alkali-free absorbents point to reduced CO₂ absorption capacity in these absorbents (10–13). In this study, we wanted to determine whether commercially available absorbents with different chemical compositions differ with regard to CO formation and to their capacity to absorb CO₂. We also wanted to investigate whether smaller CO formation implies decreased CO₂ absorption capacity of the absorbent.

	Methods

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Each absorbent was dried at room temperature with anhydrous oxygen until its weight remained stable. The residual water content of the absorbents was <0.5% of the fresh soda lime weight. After drying, samples, each weighing 600 ± 0.05 g, were put into a glass cylinder (90-mm diameter, 200-mm length) with a sintered glass filter at the bottom. The different absorbents (Samples A–H), whose chemical composition is set out in Table 1, were

A. Baralyme (Allied Healthcare Products, St. Louis, MO).

B. Drägersorb 800 (Dräger, Lübeck, Germany).

C. Drägersorb 800 Plus (Dräger).

D. Intersorb (Intersurgical, Wokingham, UK).

E. Spherasorb (Intersurgical).

F. LoFloSorb (Intersurgical).

G. Superia (Molecular Products, Essex, UK).

H. Amsorb (Armstrong, Coleraine, Northern Ireland).

The absorbents were subdivided into three groups with different alkali hydroxide content: Group 1 (A and B), containing KOH and NaOH; Group 2 (C–E), containing only NaOH; and Group 3 (F–H), with no alkali hydroxide.

In the first experimental series, isoflurane 0.5% (Forane^®; Abbott, Queenborough, UK), added by a vaporizer (Vapor 19.3; Dräger) to a pure oxygen flow of 5 ± 0.1 L/min, was directed through the samples from below for 60 min. The inlet isoflurane concentration was checked with an anesthetic monitor in mainstream (IRINA, Dräger). The gas flow was adjusted with a mass flow controller (Mass-Flo^® 1259; MKS Instruments, Andover, MA) and checked intermittently with a flowmeter (Model 4140A; TSI Inc., Minneapolis, MN). The humidity of the carrier gas was <100 ppm (Gas Analyzer 1301; Brüel & Kjær, Nærum, Denmark). The experiments were performed at room temperature; to simulate clinical conditions, the absorbent samples were not kept at a constant temperature. The absorbents of Group 1 and 2 were tested five times. Those of Group 3 were tested only four times, because we expected minimal to no CO production in this group, which contained no alkali hydroxides. Four experiments were considered sufficient to demonstrate a clear difference in CO production between Groups 1 and 2 and Group 3. Additionally, the experiments were performed three times with samples of Group 3 with the isoflurane concentration adjusted to 4%.

Downstream CO concentration was measured continuously with a polarographic CO sensor (CO3E-1000; Sensoric, Bonn, Germany; range, 0–1000 ppm) placed in a sidestream of the gas outflow tubing. The sensor was calibrated with a certified tank (900 ppm CO in nitrogen; AGA, Schwechat, Austria) and checked for linearity (±3%). The downstream isoflurane concentration in the outlet gas stream was measured continuously with the IRINA monitor. A thermocouple (7563 digital thermometer; Yokogawa, Tokyo, Japan; accuracy ±0.1°C) was placed in the effluent gas stream to measure outlet temperature continuously.

CO concentration (ppm = 10^-6), isoflurane concentration (%), and temperature (°C) at the outlet were recorded digitally at 1-min intervals (LR 8100; Yokogawa). From CO concentration values we determined the CO maximum values (CO_Max) and the amount of CO formation in the soda lime samples as follows:

equation

where COconc_i is the mean CO concentration during time interval t = 1 min and k is number of time intervals t during exposure.

From the outlet temperature measurements, we determined the initial value at the beginning of the experiment (Temp_Baseline), the maximum temperature (Temp_Max), and the mean of all the temperature values (Temp_Mean).

We determined the outlet volume of isoflurane from the outlet isoflurane concentration values and the gas flow as

equation

where ISOconc_i is the mean isoflurane concentration during time interval t = 1 min.

The loss of isoflurane in soda lime was calculated from the difference between the inlet amount (1.5 L) and outlet amount of the gas and is presented as a percentage of the inlet amount. Additionally, we determined the elapsed time to the increase in isoflurane concentration to 0.4%. We chose this value because it can be easily discerned before the concentration curve begins to flatten toward the 0.5% level. In the experiments with the isoflurane concentration adjusted to 4%, we determined the elapsed time to the increase in isoflurane concentration to 3.2%. To determine the elapsed time when fresh absorbents were used, all the previous experiments were repeated twice with fresh samples of each absorbent; the outlet isoflurane concentration was recorded at 20-s intervals.

In the second series of experiments, a glass cylinder (30-mm diameter, 350-mm length) with a sintered glass filter at the bottom was filled in turn with 30 g of five untreated samples each of the absorbents A–H. By using two mass flow controllers, a gas mixture of 5.1% ± 0.1% CO₂ in oxygen was generated and directed through the samples at a flow of 250 mL/min (±0.1%). The inlet CO₂ concentration was measured with a sidestream analyzer (M1026A; Hewlett-Packard, Andover, MA; resolution, 0.1%). Outlet CO₂ concentration was determined every 20 s with the same monitoring equipment as used for inlet CO₂ concentration. When outlet CO₂ concentration reached 0.5%, the experiments were stopped, and the exposure time (T_CO2<0.5%) was recorded. The inlet volume of CO₂ during the exposure period was calculated as

equation

The outlet volume of CO₂ during the exposure period was calculated in the same way as the outlet volume of isoflurane, by using the CO₂ outlet concentration values recorded at intervals of 20 s and at a gas flow of 0.25 L/min:

equation

where CO_2conc is the mean outlet CO₂ concentration during time interval t = 1/3 min and k is the number of time intervals t during the exposure period (T_CO2<0.5%).

The CO₂ absorption in each sample was calculated as the difference between inlet and outlet amounts of the gas. The CO₂ absorption capacity was defined as CO₂ absorption per 100 g of absorbent (L/100 g).

Values of the outcome measures are presented as the mean ± SD for each absorbent. To compare the corresponding values of CO, CO_Mean, and T_CO2<0.5% among the three groups of absorbents with different contents of hydroxides (Group 1, A and B; Group 2, C–E; Group 3, F–H), we performed an analysis of variance including hydroxide group (three levels) and absorbent sample within each hydroxide group (two, three, and three levels, respectively) as factors. To compare absorbent samples within the hydroxide groups, we performed separate analyses of variance with the outcome variables of the absorbent samples, taking data from the first experimental series for hydroxide Groups 1 and 2 and from the experimental series with the isoflurane concentration adjusted to 4% for hydroxide Group 3. Multiplicity was corrected for by applying Tukey’s Studentized range test for pairwise comparisons. Pearson’s linear correlation coefficient r was used to assess the association of the outcome variables. The correlation of Temp_Max and Temp_Mean with CO was partialized for Temp_Baseline. To correlate the values of T_CO2<0.5% with CO, which result from different experimental series, we generated 1000 matched data sets by randomly assigning values of CO from the first series to values of T_CO2<0.5% from the second series for each absorbent and calculated r for each sample. The final correlation coefficient was calculated as the mean r of the 1000 samples. Unlike the simple correlation of the means for each absorbent, this more elaborate procedure accounts for interabsorbent variability. P values of <0.05 were considered to indicate statistical significance. SAS Version 8.1 (SAS Institute, Inc., Cary, NC) was used for statistical analysis.

	Results

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In the first experimental series, no CO formation was measured in the Amsorb (Sample H) when 0.5% isoflurane was directed through them. For all the other tested absorbents (A–G), there were reproducible CO concentration curves (Fig. 1). The corresponding calculated values of CO formation (CO and CO_Mean) differed significantly among the absorbents (Table 2): the absorbent samples containing KOH (A and B) produced the largest mean CO concentration and CO formation. Lower values were determined when the absorbent samples contained Ca(OH)₂ and NaOH (C–E). The lowest mean CO concentration levels and CO formation were recorded in the experiments with absorbents containing no alkali hydroxides (F–H). The time to maximum CO levels (T_COMax) did not correlate with CO formation. For example, Baralyme, Spherasorb, and Superia showed a similar T_COMax but different values for CO.

View larger version (37K): [in this window] [in a new window]   Figure 1. Time course of carbon monoxide (CO) concentrations measured at the outlet of desiccated samples (600 g) of different absorbent brands during passage of 0.5% isoflurane in oxygen. Individual data of all experiments were recorded at 1-min intervals. Note the negligible small CO concentration values recorded downstream of the alkali hydroxide-free absorbents LoFloSorb, Superia, and Amsorb.

Isoflurane loss did not vary with the content of alkali hydroxides in the absorbents, nor did it correlate with CO formation (r = 0.15): A and B and F–H showed the same isoflurane loss (50% ± 15%), whereas CO formation was largest in A and B and smallest or not detectable in F–H. The largest isoflurane loss (89% ± 5%) took place in LoFloSorb, but the level of CO formation in this absorbent was among the smallest. The elapsed time to the increase in outlet isoflurane concentration to 0.4% did not correlate with the absorbent’s alkali hydroxide content. With Amsorb (containing no alkali hydroxide), the elapsed time of 15 ± 3 min was the shortest, whereas with LoFloSorb (also with no alkali hydroxide) and Baralyme (containing KOH), the outlet isoflurane concentration had not reached 0.4% by the end of the experimental period of 60 min.

When the inlet isoflurane concentration was increased to 4% from 0.5%, the mean CO formation with LoFloSorb was approximately twofold larger, but with Superia, CO formation was approximately the same. Amsorb produced no CO. The differences in CO formation and CO_Mean among the three absorbents were significant (Table 3).

	Discussion

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The smaller CO formation of the three absorbents containing Ca(OH)₂ and NaOH compared with the absorbents containing Ca(OH)₂ and KOH in this study (Table 2) accords with the findings of Neumann et al. (8), namely, that KOH has a greater capacity than NaOH to increase CO formation. In contrast, Stabernak et al. (11) determined CO formation to be comparable in absorbents containing NaOH and KOH to those containing NaOH without KOH. Their unexpected result can be partly explained by the erroneous classification of Drägersorb 800 as an absorbent that does not contain KOH. In fact, Drägersorb 800 contains NaOH and KOH (Table 1).

The (small) CO formation in LoFloSorb and Superia detected in these investigations (Tables 2 and 3) contradicts the hypothesis of Murray et al. (14) that Ca(OH)₂ alone would not initiate the reaction responsible for CO production. The study of Neumann et al. (8) also reported CO formation from anesthetic (desflurane) breakdown in an absorbent consisting of Ca(OH)₂ with no other bases. We conclude that in dry absorbents, the base-catalyzed initial step in CO formation (15) is triggered not exclusively by the strong bases KOH and NaOH, but also by the main basic component Ca(OH)₂.

Tang et al. (16) recorded inspiratory mean CO concentrations of up to 115 ppm during clinical anesthesia procedures with fresh absorbents, depending on the patient’s smoking status, level of immediately preoperative smoking, and body weight. This means that all of the tested alkali hydroxide-free absorbents—even when completely dry and exposed to 4% isoflurane (Table 3)—did not generate mean CO concentrations larger than those seen in anesthetized smokers when fresh absorbents were used. In contrast, in the experiments with the alkali hydroxide-containing absorbents (A–E), we determined peak CO concentration values of up to 900 ppm with only 0.5% isoflurane. Because CO formation in absorbents increases with increasing anesthetic concentration (1,3), CO concentration values of more than 1000 ppm can easily be expected in these absorbents when 4% isoflurane is used. We did not perform such experiments because the measuring range of our CO sensor had a maximum value of 1000 ppm.

The increasing maximum and mean temperatures that accompanied CO formation in our study reflect the fact that anesthetic degradation in absorbents is an exothermic process (1). However, in our experiments, the anesthetic degradation was probably boosted by the temperature increase because, to simulate clinical conditions, the absorbents’ temperature was not maintained at a constant value (17).

Unexpectedly, isoflurane loss did not correlate with CO formation, and there was a relatively large isoflurane loss in LoFloSorb (89% of the inlet isoflurane) even though CO formation was small (Table 2). There was no CO formation with Amsorb, but there was an isoflurane loss of approximately 20% of the inlet amount. The isoflurane loss in the alkali hydroxide-free absorbents was probably caused not by degradation but by adsorption (18), because the temperature did not increase. In the fresh samples of all the tested absorbents, isoflurane loss was negligible (Table 2). The clinical implication is that isoflurane loss in an absorbent indicates absorbent dryness but does not necessarily indicate that CO is being formed. However, it may cause diminished delivery of the anesthetic, and this may go unnoticed.

A quick reading of our results suggested that the time to exhaustion of CO₂ absorption decreased with decreasing CO formation, but in fact the correlation was weak (r = 0.072); this is due mainly to the absorbent properties of Baralyme and Superia. This indicates to us that those absorbents that react strongly with isoflurane cannot be regarded automatically as good CO₂ absorbents. Similarly, alkali hydroxide-free absorbents that react weakly with isoflurane are not, in principle, poor CO₂ absorbents. The weakness of the correlation accounts for the different results of recent investigations on the CO₂ absorption capacity of alkali hydroxide-free absorbents compared with absorbents containing alkali hydroxides: some studies yielded nearly identical values for the CO₂ absorption capacity of the two types of absorbents (8,14,19). Other studies showed that the tested alkali hydroxide-free absorbents had distinctly smaller CO₂ absorption capacities (10–13). The value we calculated for the CO₂ absorption capacity of Amsorb (7.8 L/100 g) is in agreement with that reported by Higuchi et al. (13). In contrast, Murray et al. (14) had previously determined a much larger CO₂ absorption capacity for Amsorb (10.2 L/100 g). This discrepancy remains unresolved. It is interesting to note that LiOH has proved to have the best CO₂ absorption properties of all absorbents despite its very small tendency to degrade anesthetics (11). However, this compound is not used in clinical anesthesia because it is expensive and corrosive, and its dust is irritating to the respiratory tract (20).

In summary, alkali hydroxide-free absorbents have little ability to degrade anesthetics to generate clinically relevant CO concentrations. An isoflurane loss in dry alkali hydroxide-free absorbents does not necessarily imply that CO is being formed. Small CO formation in absorbents is not necessarily connected with small CO₂ absorption capacity.

	Acknowledgments

 
Supported in part by the Austrian Ministry for Social Security and Generations.

The authors gratefully acknowledge the expert technical assistance of Martin Zwiefelhofer. We also thank Jane Neuda for editorial review.

	References

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