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© 2001 International Anesthesia Research Society
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Abstract |
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When exposed to volatile anesthetics containing a CHF2-group, such as isoflurane, desiccated absorbents produce carbon monoxide (CO). In the anesthesia circuit, the anesthetic flow that passes through the absorber varies with the minute ventilation. We sought to determine CO formation at different levels of test gas flow. Isoflurane 0.5% (series A) or 0.5% isoflurane + 5% CO2 (series B) in pure O2 was passed through dry soda lime samples (600 g, Draegersorb 800®) at flows of 1, 3, 5, 7, and 10 L/min. Each experiment was performed three times. At the outlet, CO concentration, isoflurane concentration and temperature were continuously recorded. In both series, the duration of CO formation was inversely related to the test gas flow. In series B, mean CO concentrations and the calculated amount of CO formation decreased significantly with increasing flow rates, which was not the case in series A. In both series, the higher the flow rate, the higher was the temperature and the shorter the time until the isoflurane concentration increased to the set level. We conclude that anesthetic degradation in dry soda lime is finite and, as long as no CO2 is added, will produce roughly the same amount of CO regardless of inflow rate. The inflow rate influences the duration of CO formation such that at lower minute ventilation longer CO formation can be expected.
IMPLICATIONS: CO formation from isoflurane degradation in dry soda lime was determined at different rates of test gas flow. The duration and, in the presence of CO2, the total amount of CO formation were inversely related to the flow rate.
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Introduction |
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Desiccated soda lime reacts with the volatile anesthetics desflurane, enflurane, and isoflurane to produce carbon monoxide (CO) (1). Pathologically increased concentrations of carboxyhemoglobin may result (2,3). Although the chemical reaction path has not been fully determined (4), CO formation is dependent on the humidity of the soda lime and its chemical composition, as well as on the type of anesthetic and its concentration (1,5). If CO is a product of the reaction of the volatile anesthetic with the soda lime, its formation should depend on both the concentration of the anesthetic and the flow rate through the absorbent in an anesthesia machine, which in turn depends on the patient’s minute ventilation. We hypothesized that the same amount of a volatile anesthetic passing through soda lime at different flow rates leads to a different duration, but the same amount, of CO formation. This might be different when CO2 is added, as CO2 influences CO formation in dry soda lime (6). The aim of this study was to determine the duration and amount of CO formation when dried soda lime is exposed to a constant concentration of isoflurane with and without CO2 at test gas flows ranging from 1 to 10 L/min.
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Methods |
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Soda lime (Draegersorb 800®; Draeger, Luebeck, Germany) was dried with oxygen to a constant weight. The residual water content comprised <0.5% of the fresh soda lime weight. Each of the following experiments was performed with a 600-g sample filled into a CO2 absorbent canister (Dameca, Copenhagen, Denmark).
In the first series of experiments (series A), 0.5% isoflurane was added by a vaporizer to pure O2 and this gas mix was passed through separate samples of dry soda lime at flows of 1, 3, 5, 7, and 10 L/min ± 1%. Each experiment was performed three times (A1-A3). The duration of exposure of the soda lime to isoflurane was 300, 100, 60, 43, and 30 min, respectively, for each flow rate. Total isoflurane load for each flow level was 1.5 L. At the end of the experiments A2 and A3 at each flow rate, we passed pure oxygen through the soda lime at the same rate to determine the amount of CO remaining in the absorbent (washout phase). In the second series of experiments (series B), 5% CO2 was added to 0.5% isoflurane in O2. All other factors remained the same. The total CO2 load for each flow level was 15 L. Because the upper limit of the measuring range of our CO sensor was 1000 ppm, we were not able to perform experiments using either larger isoflurane concentrations or the anesthetics enflurane or desflurane, which react more strongly with dry absorbents (1).
Inlet oxygen and CO2 flow rates were adjusted using two separate mass flow controllers (Mass-Flo® 1259; MKS Instruments, Andover, MA). Total gas flow rates were continuously monitored (AWM 5105 VN; Honeywell, Minneapolis, MN). The humidity of the carrier gas was < 100 ppm (Gas Analyzer 1301; Brüel & Kjær, Nærum, Denmark).
Downstream CO concentration was measured with a polarographic CO sensor (CO-3E 300; Sensoric, Bonn, Germany; range: 0–1000 ppm) placed in sidestream of the gas outflow tubing 21 cm above the absorbent’s surface. The measurement period corresponded with the duration of the test gas flow of each experiment. In experiments A2 and A3, the CO concentration was also recorded during the washout phase up to the point at which the CO concentration decreased to <10 ppm, a level we considered negligible. Before use, the sensor was calibrated with a certified tank (900 ppm ± 2% CO in N2) (AGA, Schwechat, Austria). In the test for linearity we used the undiluted gas obtained from the certified tank and a 1:1 mixture of the calibration gas with oxygen. The oxygen concentration was measured using a calibrated polarographic sensor (Oxygen Sensor Code No. 6850645; Draeger). The test yielded a linearity of ± 3% for CO concentration measurement.
To measure outlet gas temperature, a thermocouple (7563 digital thermometer; Yokogawa, Tokyo, Japan; accuracy ± 0.1°C) was placed in the effluent stream of gases 27 cm above the absorbent’s surface. Inlet and outlet isoflurane concentration were measured with an anesthetic-specific infrared analyzer (7) (IRINA; Draeger) (resolution 0.01%) in the mainstream. The sensor in the effluent stream was 66 cm from the absorber outlet. Inlet and outlet CO2 concentration were determined by sidestream measurement (M1026A; Hewlett-Packard, Andover, MA) (resolution 0.1%).
CO concentrations (ppm = 10-6), temperature (°C), CO2 concentrations (%), and isoflurane concentrations (%) were recorded digitally at 1-min intervals (LR 8100; Yokogawa). From the recorded CO concentration values, we determined the maximum (COMax), the mean (COMean), the concentration at the end of the exposure period (COEnd), and the point at which CO concentration declined to at least half of COMax (TCOMax/2) for each experiment. (TCOMax/2 does not represent the exact half-time of CO concentration because the decrease in CO concentration over time cannot be precisely described by a single exponential function.) Total CO formation during the test gas flow (COTotal) and CO formation during the washout phase (COWashout) were calculated from gas flow, CO concentration, and duration of exposure, assuming that inlet flow was considered to represent outlet flow, even though, in fact, outlet flow must have been less than inlet flow because of the absorption of isoflurane in the soda lime as previously described (6):
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where conci is the mean concentration during time interval (
t = 1 min) and k is the number of time intervals during the test gas flow and during the washout phase.
We determined the outlet gas temperatures at the beginning of test gas flow (TempBaseline) and the maximum temperatures (TempMax), and we calculated the mean temperatures (TempMean).
From the downstream isoflurane concentration curves, we determined the elapsed time to the increase in isoflurane concentration to 0.4% (T0.4%ISO). This value can be easily identified before the concentration curve begins to flatten toward the 0.5% level. For both experimental series, i.e., with and without CO2, we also determined these time points when the test gas was passed through fresh (moist) soda lime samples (600 g Draegersorb 800®).
Values of the outcome measures are presented as mean (range). The Spearman rank correlation coefficient was used to compute the association of test gas flow and the various outcome measures in the two experimental groups. To compare the corresponding values of COMax, TCOMax/2, COMean, COTotal, TempMax, and TempMean, in series A and series B, we performed two-way analysis of variance with the factors "experimental group" and "test gas flow". For TempMax and TempMean we additionally included baseline temperature as a covariate in an analysis of covariance. Because the distributions were skewed, all outcome measures were log-transformed before the computations were performed. P values<0.05 were considered to indicate statistical significance. The SAS System for Windows 8.1 (SAS Institute, Cary, NC) was used for statistical analysis.
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Results |
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In both series, A (isoflurane in O2), and B (isoflurane + CO2 in O2), the higher the level of test gas flow, the more quickly COMax and TCOMax/2 were reached (Fig. 1). The correlation analysis yielded a significant inverse relationship between flow and TCOMax and also between flow and TCOMax/2. In test series B, COMean and COTotal also showed a significant inverse relationship with the flow rates, which was not the case in series A.
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In series A, CO formation had not yet come to an end when the test gas flow was discontinued (Fig. 1A). The amount of CO that could be washed out (COWashout) in experiments A2 and A3 after isoflurane was discontinued was significantly larger at higher test gas flow rates (Table 1). Because in all cases it was <5% of COTotal, we did not add it into total CO formation. Even had we done so, our results would not have been affected. In series B, CO formation ended before isoflurane was discontinued (Fig. 1B); the concentration of CO was <10 ppm. TempMax and TempMean varied in line with test gas flow rates, i.e., the higher the flow the higher the temperature, in both test series (Tables 1 and 2).
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T0.4% ISO, the elapsed time to the increase in downstream isoflurane concentration to 0.4%, was significantly lower at higher test gas flows in both series. With fresh soda lime, the isoflurane concentration increased to 0.4% much more rapidly than with dried soda lime at each level of flow, taking only a few minutes regardless of the flow rate (Tables 1 and 2).
CO2 End increased with increasing test gas flow but was always <1% (Table 2).
Comparing the corresponding results of series A with those of series B (analysis of variance) showed that in series B there were significantly higher values for COMax, TempMax, and TempMean, and significantly lower values for TCOMax/2, COMean, and COTotal.
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Discussion |
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We found that the duration of CO formation is inversely related to the flow rate of the test gas through the absorbent; thus, the higher the flow rate, the shorter the duration of CO formation. When CO2 is absorbed in dried soda lime (as in our series B experiments), CO formation is reduced. Additionally, the higher the inflow rate, not only the shorter the duration of CO formation but also the smaller the absolute amount of CO formation and mean CO concentration. Series A yielded different results; in the absence of CO2, CO formation was not lower at higher flow rates (Table 1).
It is striking that the lower the flow, the smaller the difference between series A and B in CO formation. At a flow of 5–10 L/min, the amount of CO formed in series B was 60% of that formed in series A, and at 1 L/min the amounts of CO formed were nearly identical in the two series. Apparently the degree of reduction of CO formation resulting from simultaneous CO2 absorption (6) is dependent on the flow rate. As a flow increase from 1 L/min to 5 L/min is a fivefold increase in the flow rate whereas a flow increase from 5 L/min to 10 L/min is only a doubling of the flow rate, the largest decrease in CO formation (34 mL) occurred during the increase in flow from 1 L/min to 5 L/min, and there was little further decrease (5 mL) thereafter. Accordingly, the increase from 1 to 3 L/min corresponded to nearly the same difference in CO formation (19 mL) as the increase from 3 to 10 L/min (20 mL).
From our findings, we conclude that reducing minute ventilation and the anesthetic gas flow through the soda lime leads to prolonged CO formation. In clinical practice this could lead to prolonged CO concentration levels in pediatric anesthesia with its small minute volumes. Using an animal model (pigs) Bonome et al. (8) investigated the formation of CO in dried soda lime from the degradation of isoflurane (1.5%) and desflurane (7%) during normoventilation at defined minute ventilations, appropriate to the body weight of the animals (80, 38, and 18 kg). The fresh gas flow in their experiment was low in relation to the minute ventilation (1/10 of the minute ventilation). Thus, probably a large part of the exhaled gases (containing 5% CO2 at normoventilation) passed through the soda lime. When minute volume was small, CO concentrations and carboxyhemoglobin levels were high. We propose that this relationship can be partly explained by our observations in the series B experiments, namely, that CO formation is prolonged and increases in the presence of CO2 when gas flow is decreased.
All the degradation reactions of volatile anesthetics in dry soda lime described to date have been exothermic (9); thus, the temperature increases in soda lime and in the outflowing gas. Our observation of increased maximal and mean temperatures when flow was increased from 1 to 10 L/min in both experimental series is in agreement with Wissing et al. (10). They determined an increase in maximum temperature in soda lime from 56°C to 71°C when the test gas (2% isoflurane in O2) flow rate was doubled from 0.5 L/min to 1 L/min. The fact that the temperatures were lower (under 50°C) in our experiments than they were in those of Wissing et al. (10) can be explained by two factors. First, we used isoflurane at a smaller concentration (0.5%) and, second, our temperature readings were taken in the outflowing gas rather than in the soda lime itself. Nonetheless, our results indicate that the characteristic temperature increases during CO formation in dry soda lime become less pronounced as the gas flow decreases (Fig. 2). As CO2 absorption, too, produces heat (11), the monitoring of soda lime temperature in clinical anesthesia is not a reliable method of detecting CO formation in soda lime. In contrast, the time needed for the isoflurane concentration in the gas mixture downstream of the dried soda lime to return to the set concentration may be well suited as an indirect CO indicator at any flow rate. At a gas flow of 1 L/min and from an outset concentration of 0.5%, it took four hours in series A and three hours in series B for the isoflurane concentration to increase to 0.4% in the gas outflow. In moist soda lime we recorded this concentration at otherwise identical experimental conditions in no more than six minutes (Tables 1 and 2). Thus, comparing the concentration of the volatile anesthetic before and after its passage through an absorbent should provide early warning of possible CO formation even at a small concentration and at any flow. However, it is important to note that monochromatic anesthesia gas monitors—in contrast to multiwavelength (agent-specific) infrared analyzers—are not suitable for indirect CO detection because they show false large isoflurane or desflurane concentrations as a result of interference from trifluoromethane, which is formed in addition to CO in dry soda lime when isoflurane or desflurane degrade (12).
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From our findings, we conclude that anesthetic degradation in dry soda lime is finite and, as long as no CO2 is added, will produce roughly the same amount of CO regardless of inflow rate. The inflow rate influences the duration of CO formation such that at lower minute ventilation longer CO formation can be expected.
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Acknowledgments |
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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 Mrs. Jane Neuda for critical review of the manuscript.
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References |
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- Fang ZX, Eger EI, Laster MJ, et al. Carbon monoxide production from degradation of desflurane, enflurane, isoflurane, halothane, and sevoflurane by soda lime and Baralyme. Anesth Analg 1995; 80: 1187–93.[Abstract]
- Lentz RE. Carbon monoxide poisoning during anesthesia poses puzzles. J Clin Monit 1995; 11: 66–7.[Web of Science][Medline]
- Berry PD, Sessler DI, Larson MD. Severe carbon monoxide poisoning during desflurane anesthesia. Anesthesiology 1999; 90: 613–6.[Web of Science][Medline]
- Baxter PJ, Garton K, Kharasch ED. Mechanistic aspects of carbon monoxide formation from volatile anesthetics. Anesthesiology 1998; 89: 929–41.[Web of Science][Medline]
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Neumann MA, Laster MJ, Weiskopf RB, et al. The elimination of sodium and potassium hydroxides from dessicated soda lime diminishes degradation of desflurane to carbon monoxide and sevoflurane to Compound A but does not compromise carbon dioxide absorption. Anesth Analg 1999; 89: 768–73.
[Abstract/Free Full Text] -
Knolle E, Gilly H. Absorption of carbon dioxide by dry soda lime decreases carbon monoxide formation from isoflurane degradation. Anesth Analg 2000; 91: 446–51.
[Abstract/Free Full Text] - Eisenkraft JB, Raemer DB. Monitoring gases in the anesthesia delivery system. In: Ehrenwerth J, Eisenkraft JB, eds. Anesthesia equipment: principles and applications. St. Louis: Mosby, 1993: 210–6.
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Bonome C, Belda J, Alvarez RF, et al. Low-flow anesthesia and reduced animal size increase carboxyhemoglobin levels in swine during desflurane and isoflurane breakdown in dried soda lime. Anesth Analg 1999; 89: 909–16.
[Abstract/Free Full Text] - Strum DP, Eger EI. The degradation, absorption, and solubility of volatile anesthetics in soda lime depend on water content. Anesth Analg 1994; 78: 340–8.[Medline]
- Wissing H, Kuhn I, Dudziak R. Heat production from reaction of volatile anesthetics with dried soda lime. Anaesthesist 1997; 46: 1064–70.[Web of Science][Medline]
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Tsuchiya M, Ueda W. Heat generation as an index of exhaustion of soda lime. Anesth Analg 1989; 68: 783–7.
[Abstract/Free Full Text] - Woehlck HJ, Dunning MB, Kulier AH, et al. The response of anesthetic agent monitors to trifluoromethane warns of the presence of carbon monoxide from anesthetic breakdown. J Clin Monit 1997; 13: 149–55.[Web of Science][Medline]
This article has been cited by other articles:
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  M. J. Laster and E. I. Eger II Temperatures in Soda Lime During Degradation of Desflurane, Isoflurane, and Sevoflurane by Desiccated Soda Lime Anesth. Analg., September 1, 2005; 101(3): 753 - 757. [Abstract] [Full Text] [PDF]
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