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
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
V
olatile 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.
Supported in part by the Austrian Ministry for Social Security and
Generations.
Accepted for publication May 10, 2002.
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 [email protected].
DOI: 10.1213/01.ANE.0000023281.42095.CA
650
Anesth Analg 2002;95:650–5
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.
(Anesth Analg 2002;95:650 –5)
Methods
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
©2002 by the International Anesthesia Research Society
0003-2999/02
ANESTH ANALG
2002;95:650 –5
TECHNOLOGY, COMPUTING, AND SIMULATION
KNOLLE ET AL.
CO FORMATION AND CO2 ABSORPTION IN ABSORBENTS
651
Table 1. CO2 Absorbing Compounds in the Tested Absorbents
Sample (brand)
Variable
Ca(OH)2 (%)
NaOH (%)
KOH (%)
Ba(OH)2 (%)
C
A
B
(Drägersorb
D
E
F
G
H
(Baralyme) (Drägersorb 800)
800 Plus)
(Intersorb) (Spherasorb) (LoFloSorb) (Superia) (Amsorb)
73
—
5
11
75–81
1–3
2–4
—
78–84
2–4
—
—
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 (COMax) and the amount of CO formation in the soda lime samples as follows:
CO(L)
冘 COconc (ppm) 䡠 ⌬t共min兲
k
⫽ gas flow(L/min) 䡠
i
i⫽1
81
3
—
—
78.5
1.5
—
—
⬎75
—
—
—
78
—
—
—
⬎75
—
—
—
冘 COconc (ppm) 䡠 1(min),
60
⫽ 5 共 L/min兲 䡠
i
(1)
i⫽1
where COconci 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 (TempBaseline), the maximum temperature
(TempMax), and the mean of all the temperature values
(TempMean).
We determined the outlet volume of isoflurane from
the outlet isoflurane concentration values and the gas
flow as
冘 ISOconc 共%兲 䡠 1共min),
60
ISO(L) ⫽ 5 共 L/min兲 䡠
i
i⫽1
(2)
where ISOconci 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
652
TECHNOLOGY, COMPUTING, AND SIMULATION KNOLLE ET AL.
CO FORMATION AND CO2 ABSORPTION IN ABSORBENTS
ANESTH ANALG
2002;95:650 –5
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
(TCO2⬍0.5%) was recorded. The inlet volume of CO2
during the exposure period was calculated as
CO2(inlet)(L)
⫽ 0.25 共 L/min) 䡠 5.1(%) 䡠 TCO2⬍0.5%(min). (3)
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:
CO2(outlet)(L)
冘 CO
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.
k
⫽ 1/3 䡠 0.25 共 L/min兲 䡠
共 % 兲 䡠 ⌬t 共 min), (4)
2conc
i⫽1
where CO2conc 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
(TCO2⬍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, COMean, and TCO2⬍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 TempMax and
TempMean with CO was partialized for TempBaseline.
To correlate the values of TCO2⬍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
TCO2⬍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
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 COMean)
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 (TCOMax) did not correlate
with CO formation. For example, Baralyme, Spherasorb, and Superia showed a similar TCOMax but different values for CO.
Corresponding with the extent of CO formation, the
highest maximum and mean temperatures were recorded in Samples A and B, whereas in Samples C–E
lower values, and in F–H, still lower values (Table 2),
were recorded. The correlations for TempMax and
TECHNOLOGY, COMPUTING, AND SIMULATION
KNOLLE ET AL.
CO FORMATION AND CO2 ABSORPTION IN ABSORBENTS
4⫾4
12 ⫾ 12
25 ⫾ 15
37 ⫾ 13
25.6 ⫾ 0.7
25.8 ⫾ 0.8
25.5 ⫾ 0.8
50 ⫾ 15
Sample (brand)
3⫾2
9⫾5
13 ⫾ 7
25 ⫾ 1
25.8 ⫾ 0.3
26.2 ⫾ 0.5
25.9 ⫾ 0.3
41 ⫾ 2
26 ⫾ 4
1, 1
0
0
—
—
26.2 ⫾ 0.3
26.3 ⫾ 0.3
26.1 ⫾ 0.2
20 ⫾ 4
15 ⫾ 3
1, 1
Variable
CO (mL)
COMean (ppm)
COMax (ppm)
TCOMax (min)
TempBaseline (°C)
TempMax (°C)
TempMean (°C)
ISOLoss (%)
TISO⬍3.2% (min)
F⬘
(LoFloSorb)
G⬘
(Superia)
H⬘
(Amsorb)
21 ⫾ 2
69 ⫾ 7
180 ⫾ 16
10 ⫾ 0
24.6 ⫾ 0.1
25.2 ⫾ 0.1
25.0 ⫾ 0.1
19 ⫾ 2
12 ⫾ 1
4⫾0
14 ⫾ 1
23 ⫾ 2
6⫾0
24.4 ⫾ 0.2
24.5 ⫾ 0.2
24.4 ⫾ 0.2
11 ⫾ 1
7⫾0
0
0
—
—
26.0 ⫾ 0.1
26.1 ⫾ 0.1
26.0 ⫾ 0.0
9⫾1
6⫾1
CO ⫽ carbon monoxide; ISO ⫽ isoflurane.
49 ⫾ 4
162 ⫾ 13
281 ⫾ 18
23 ⫾ 1
25.4 ⫾ 0.2
26.9 ⫾ 0.2
25.9 ⫾ 0.1
50 ⫾ 2
33 ⫾ 1
1, 2
73 ⫾ 3
239 ⫾ 10
548 ⫾ 23
12 ⫾ 1
25.9 ⫾ 0.3
27.4 ⫾ 0.12
26.6 ⫾ 0.05
31 ⫾ 1
20 ⫾ 0
⬍1, 1
66 ⫾ 3
218 ⫾ 9
620 ⫾ 17
10 ⫾ 1
26.3 ⫾ 0.7
27.8 ⫾ 0.6
27.2 ⫾ 0.5
26 ⫾ 3
16 ⫾ 2
1, 1
CO ⫽ carbon monoxide; ISO ⫽ isoflurane.
181 ⫾ 45
594 ⫾ 146
806 ⫾ 83
21 ⫾ 4
26.1 ⫾ 1.6
29.8 ⫾ 3.6
27.0 ⫾ 1.9
50 ⫾ 15
140 ⫾ 12
458 ⫾ 39
737 ⫾ 49
18 ⫾ 1
27.3 ⫾ 1.2
32.0 ⫾ 0.8
28.7 ⫾ 0.4
36 ⫾ 1
38 ⫾ 2
1, 1
CO (mL)
COMean (ppm)
COMax (ppm)
TCOMax (min)
TempBaseline (°C)
TempMax (°C)
TempMean (°C)
ISOLoss (%)
TISO⬍0.4% (min)
TISO⬍0.4% (min)
(fresh soda
lime)
223 ⫾ 6
730 ⫾ 20
875 ⫾ 34
24 ⫾ 2
24.8 ⫾ 0.5
27.7 ⫾ 4.2
25.4 ⫾ 1.1
63 ⫾ 1
⬎60
⬍1, ⬍1
A–B
B
(Drägersorb 800)
A
(Baralyme)
Variable
653
Table 3. Characteristics of CO Formation During Passage
of 4% Isoflurane
63 ⫾ 11
206 ⫾ 35
483 ⫾ 152
15 ⫾ 7
25.9 ⫾ 0.6
27.4 ⫾ 0.5
26.6 ⫾ 0.6
36 ⫾ 11
8⫾2
26 ⫾ 5
38 ⫾ 8
50 ⫾ 2
24.8 ⫾ 0.2
24.9 ⫾ 0.2
24.5 ⫾ 0.1
89 ⫾ 5
⬎60
2, 2
H
(Amsorb)
G
(Superia)
F
(LoFloSorb)
E
(Spherasorb)
D
(Intersorb)
Group 2
C
(Drägersorb
800 Plus)
Group 1
Table 2. Characteristics of CO Formation During Passage of 0.5% Isoflurane
Sample (brand)
C–E
Group 3
F–H
ANESTH ANALG
2002;95:650 –5
TempMean with CO formation were r ⫽ 0.64 and r ⫽
0.44, respectively.
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 COMean among the three absorbents were
significant (Table 3).
We found a significant difference in TCO2⬍0.5%
among the three absorbent groups, but TCO2⬍0.5% did
not correlate with CO formation (r ⫽ 0.072). Calculated values of CO2 absorption capacity within
TCO2⬍0.5% are shown in Table 4.
Discussion
In this investigation of eight different CO2 absorbents,
desiccated absorbents containing Ca(OH)2 and KOH
showed the largest CO formation. CO formation was
less with absorbents containing Ca(OH)2 and NaOH,
but the three absorbents not containing alkali hydroxides showed the smallest CO formation. CO formation
3:09 ⫾ 0:12
7.8 ⫾ 0.5
5:23 ⫾ 0:06
13.4 ⫾ 0.3
3:00 ⫾ 0:06
7.3 ⫾ 0.3
5:26 ⫾ 0:29
13.5 ⫾ 1.2
5:21 ⫾ 0:19
13.3 ⫾ 0.8
5:04 ⫾ 0:27
12.6 ⫾ 1.1
5:53 ⫾ 0:20
14.6 ⫾ 0.8
4:47 ⫾ 1:13
11.8 ⫾ 3.0
5:53 ⫾ 0:30
14.5 ⫾ 1.3
CO2 ⫽ carbon dioxide.
3:41 ⫾ 0:19
9.1 ⫾ 0.8
TCO2⬍0.5% (h:min)
Absorption capacity
(L/100 g)
F–H
H
(Amsorb)
G
(Superia)
Variable
A
B
(Baralyme) (Drägersorb 800)
A–B
C
(Drägersorb
800 Plus)
D
(Intersorb)
E
(Spherasorb)
C–E
F
(LoFloSorb)
Group 3
Group 2
Group 1
Sample (brand)
3:51 ⫾ 1:08
9.5 ⫾ 2.9
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CO FORMATION AND CO2 ABSORPTION IN ABSORBENTS
Table 4. Characteristics of CO2 Absorption in Untreated Samples
654
ANESTH ANALG
2002;95:650 –5
did not correlate with the duration of CO2 absorption
among the tested absorbents.
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
ANESTH ANALG
2002;95:650 –5
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.
The authors gratefully acknowledge the expert technical assistance
of Martin Zwiefelhofer. We also thank Jane Neuda for editorial
review.
TECHNOLOGY, COMPUTING, AND SIMULATION
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CO FORMATION AND CO2 ABSORPTION IN ABSORBENTS
655
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