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Print version ISSN 0034-7094
On-line version ISSN 1806-907X
Rev. Bras. Anestesiol. vol.54 no.3 Campinas May/June 2004
Preventing toxic substances production during carbon dioxide absorption by soda lime with halogenate anesthetics*
Como evitar la formación de substancias tóxicas durante la absorción de dióxido de carbono por la cal sodada con uso de anestésicos halogenados
Renato Ângelo Saraiva, TSA, M.D.
Coordenador de Anestesiologia da Rede SARAH de Hospitais do Aparelho Locomotor
BACKGROUND AND OBJECTIVES: Since the beginning,
soda lime use has presented some complications which resulted in its difficult
application. However major advantages such as decreasing fresh gas flow, anesthetic
consumption and operating room pollution, and improving breathing system and
airway humidity, have pushed research forward to improve it and assure the continuity
of its use. Currently, there are problems with dehydration, increased temperature
and metabolic degradation of halogenate agents, which require special care to
prevent toxic substances formation.
CONTENTS: There is a chain reaction as from dehydrated or dried out soda lime with very low percent volume of water. There is increased temperature, more halogenate anesthetic absorption by lime granules, followed by higher metabolic degradation of these agents' molecules and, as a consequence, the production of toxic substances, such as Compound A by reaction of hydroxides with sevoflurane. There is also carbon monoxide production by reaction of halogenate anesthetics and strong lime bases. Compound A is nephrotoxic and carbon monoxide may lead to hypoxia and severe coagulation problems. In addition to care with soda lime hydration it is possible to use it without strong bases, such as potassium and sodium hydroxides, with just calcium hydroxide to prevent excessive temperature increase and major metabolic degradation of halogenate anesthetics without impairing carbon dioxide absorption.
CONCLUSIONS: Care should be taken to use the newest possible soda lime; and when it is exposed to environment (dry air) for many hours, such as during weekends (more than 48 hours) it is recommended to add distilled water in the ratio of 25 mL to 500 g of soda lime. Industry is currently well aware of lime composition problems so, soda lime containing exclusively calcium hydroxide and totally potassium and sodium hydroxide-free should be preferred
Key Words: EQUIPMENTS: soda lime
JUSTIFICATIVA Y OBJETIVOS: La cal sodada
desde el inicio de su uso siempre presentó algunas complicaciones que resultaron
en dificultad en su aplicabilidad. No entanto, debido a las grandes ventajas
que ofrecía en relación a la reducción del flujo de gases frescos,
despolución de la sala de cirugía y humidificación del sistema
de inhalación y vía aérea, hicieron con que continuasen las pesquisas
para que pudiese ser mejorada y corregida de forma que la continuidad de su
utilización sea asegurada. Actualmente existe el problema de la deshidratación
con elevación de la temperatura y de la degradación metabólica
de los anestésicos halogenados que necesitan de cuidados especiales para
evitar la formación de productos tóxicos.
CONTENIDO: Existe una reacción en cadena a partir de la cal sodada deshidratada o resecada con bajos volúmenes porcentuales de agua. Hay aumento de la temperatura, mayor absorción de anestésico halogenado para el interior del granulo de cal y en seguida mayor degradación metabólica de las moléculas de estos agentes y consecuentemente la producción de substancias tóxicas como el Compuesto A por la reacción de los hidróxidos con el sevoflurano. Hay también formación de monóxido de carbono producido de la misma forma por la reacción entre los halogenados y las bases fuertes de la cal. El compuesto A es nefrotóxico y el monóxido de carbono lleva a la hipóxia y alteraciones graves de la coagulación de la sangre. Además de los cuidados para la hidratación de la cal sodada es posible usar ésta sin contener las bases fuertes como los hidróxidos de potasio y de sodio, conteniendo apenas hidróxido de calcio para evitar excesivo aumento de la temperatura y grande degradación metabólica de los halogenados sin perjudicar la absorción del dióxido de carbono.
CONCLUSIONES: Se debe tener el cuidado en usar la cal sodada lo antes posible y cuando ella queda expuesta al medio ambiente (aire seco) por muchas horas como por ejemplo en un final de semana (mas de 48 horas) es recomendable colocar agua, de preferencia destilada, en la relación de 25 ml para cada 500 g de cal. Actualmente la industria está bien informada sobre el problema de la composición de la cal, entonces, se debe preferir la cal sodada que tenga solamente el hidróxido de calcio y sea totalmente desprovista de hidróxido de potasio e hidróxido de sodio.
Carbon dioxide absorption in the anesthesia machine allows the use of lower fresh gases flow to decrease anesthetic consumption, maintain body temperature, retain airway humidity and prevent operating room pollution.
Wilson 1 and Waters 2 have reported that carbon dioxide absorption was first used in anesthesia in 1906 by a German surgeon who has used a filter of underground mines life-saving equipment. According to the same studies, pharmacologist Dennis Jackson has developed in 1915 a soda lime filter based on sodium hydroxide, but carbon dioxide absorption would result in major temperature increase. For this reason, Wilson has modified lime composition which became calcium hydroxide-based.
In 1923, Ralph Waters started using carbon dioxide absorption routinely in anesthesia and has published his results with a 500 mL filter placed close to patients' mouth in 1926 3. Fresh gases flow was 0.5 L.min-1 and would enter the upper part of the filter connected to the tracheal tube or facial mask. A respiratory bag was placed in the lower part of the filter. This system was called "to and fro". Currently soda lime composition remains the same: 95% calcium hydroxide, 4.5% sodium hydroxide and 0.2% silica to make lime more consistent and prevent powder formation.
To this mixture 10% to 22% water was added (mean 15%) and then it was formatted into granules 4.
Then, another soda lime blend was devised containing 1% potassium hydroxide, 4% sodium hydroxide, silica (small amounts, approximately 0.2%), 14% to 19% water and enough calcium hydroxide to complete 100% 5.
Barium lime has 20% barium hydroxide, 1% potassium hydroxide, less water than soda lime (approximately 8%) and calcium hydroxide to complete 100% 6.
In an attempt to make granules more resistant to powder transformation, barium lime was introduced to replace part of calcium hydroxide (approximately 20%) by barium hydroxide. The latter, however, produces more heat during carbon dioxide absorption 6.
Small amounts of indicators are used, which change color when pH decreases. The most popular to date is violet ethyl which is transparent but becomes violet when pH is below 10.3 7.
Carbon dioxide contact with soda lime produces the following reactions:
In a first stage, carbon dioxide (CO2) combines with water (H2O) in lime forming carbonic acid (H2CO3).
CO2 + H2O ® H2 CO3
In a second stage, carbonic acid reacts with sodium hydroxide (OH Na) and calcium hydroxide Ca(OH)2 producing sodium carbonate (CO3 Na2) and calcium carbonate (CO3 Ca) and releasing water.
2 Na OH + 2 H2 CO3 + Ca ( OH)2 ® CaCO3 + Na2 Co3 + 4H2O
Carbon dioxide reactions with barium lime are as follows 8,9:
9 CO2 + 9 H2O ® 9H2 CO3
9 H2 CO3 + 9 Ca (OH)2 ® 9 Ca CO3 + 18 H2O
H2 CO3 + 2K OH ® K2 CO3 + 2 H2O
9 H2 CO3 + 9 Ba (OH)2 ® 9 Ba CO3 + 18 H2O
As it can be seen, water is a major component in carbon dioxide absorption reactions. In addition to helping chemical reactions, water decreases direct contact of halogenate inhalational anesthetics with hydroxides and carbon dioxide. When there is direct prolonged contact with the participation of a large number of molecules (high anesthetic concentrations), there is a higher probability for the formation of other compounds, including carbon monoxide 10.
Adults eliminate approximately 15 L of carbon dioxide per hour, which may be absorbed by 70 g lime, so 700 g lime may absorb 10 hours of expired CO2, and 1000 g lime may absorb approximately 14 hours of expired CO2.
Since CO2 reactions with lime are reversible, considerable part of it may be regenerated. The period in which lime is depleted in a 1000 g filter is at least 14 hours, considering a close system with fresh gases flow equal to oxygen consumption. When fresh gases flow is equal to or above inspiratory minute volume, absorption is almost null, in intermediary flows the absorption is also intermediary and lime depletion period may be twice or even higher, depending on the flow.
The indicator shows lime depletion turning from white to violet when violet ethyl is used. With reversion reaction, lime returns to white and may be used for "some" time before another color change. This time is not precise. Color change should be observed in addition to heating which is an sign of exothermal reaction between CO2 and lime.
CO2 reaction with a strong base such as sodium or potassium hydroxide releases more heat than the reaction of carbonic acid (H2CO3), which is a weak acid, and strong bases. So it is easy to understand the importance of a certain amount of water to complete the first reaction with CO2 and form carbonic acid to continue with the second reaction with calcium and sodium hydroxides, eventually barium and potassium, and form carbonates.
The presence of water in lime is also important to prevent halogenate anesthetic absorption by granules which could delay anesthetic induction and promote further metabolic degradation of those agents.
The contact of halogenate agents with lime may result in their metabolism. There are reports of this event with all anesthetics currently used: halothane, isoflurane, desflurane and sevoflurane. Sevoflurane has the highest metabolic rate and its major degradation product is Compound A 11,12.
Some factors interfere with absorption and degradation of those agents, especially sevoflurane: low fresh gases flow, barium lime, high agent concentration, high lime temperature (< 70 ºC) and lime dehydration 13,14.
HALOGENATE ANESTHETICS DEGRADATION BY SODA LIME
Soda lime degrades inhalational halogenate anesthetics through exothermal reactions. Compound A, formed by sevoflurane metabolism is also halogenate. A different reaction degrades sevoflurane transforming it into formaldehyde hydrofluoric acid with methanol formation as from formaldehyde. Methanol combined with compound A forms compound B, which combined with other metabolites and being defluorinated forms compound C 15-17.
There are reports that halogenate anesthetics absorption and metabolism by soda lime increase when water content decreases to below 10%. Similarly, these agents' degradation by lime is linearly increased with increased temperature. This is clearly seen with sevoflurane 17,18.
The first reaction when CO2 is absorbed by lime is processed between this gas and water generating carbonic acid which then reacts with calcium, sodium, potassium and barium hydroxides. This is an exothermal reaction but there is more heat release when the reaction is between CO2 and hydroxides.
Increased temperature increases halogenate metabolism and such metabolism further increases this metabolic degradation.
In addition to producing toxic substances, the contact of halogenate agents with soda lime may result in carbon monoxide (CO). This is more frequent when some of the following factors are associated: barium lime, high lime temperature, dehydrated lime, high anesthetic concentrations and prolonged absorption time 19,20.
Higher CO levels have been described with desflurane in the previous-mentioned conditions. These conditions have been deliberately prepared in experimental studies. There are no clinical cases in the literature about this event.
It should be observed that formation of toxic substances such as compound A may promote severe damage to kidney, liver and brain. Carbon monoxide may combine with hemoglobin, preventing its binding to oxygen and severely impairing its transportation, leading patients to hypoxia which may be severe. The magnitude of this clinical event will depend on the amount of carbon monoxide formed in one minute as compared to oxygen consumption.
High carbon monoxide volumes of 715 mL in two hours, correspond to 6 mL.min-1, which is close to half oxygen consumption of a 3-kg neonate, which is approximately 12 mL.min-1. This amount of CO by contact of anesthetics with lime is only experimentally obtained.
It should be noted that CO intoxication risk with halogenate and soda lime is always higher in children or low weight adults with low oxygen consumption. Wissing 21 has reported two cases in Germany with sevoflurane 22. Coincidentally, this is the agent less forming CO in contact with soda lime, as described in an experimental study with all currently used halogenates 21.
Carbon monoxide intoxication goes easily unnoticeable for being very fast. CO has approximately 200 times more affinity to hemoglobin as compared to oxygen. This combination (HbCO) forms a pink pigment similar to oxyhemoglobin (HbO2), shifts saturation curve to the left and promotes clinical repercussions such as tachycardia and increased coagulation time. These changes may be seen with low HbCO saturation rates, approximately 4% to 8% 21,23.
Since the beginning, soda lime has presented some complications which have resulted in its difficult use, however major advantages, such of decreasing fresh gases flow and operating room pollution, and improved inhalation system humidity, have pushed research forward to improve it and assure the continuity of its use.
Lack of calcium hydroxide or major predominance of sodium hydroxide would produce excessive heat being necessary an adjustment on its composition which started to have more calcium hydroxide and much less sodium and potassium hydroxide.
Another difficulty was the accurate addition of water until the optimal percentage between 14% and 21% was established.
There are currently two basic concerns with lime composition to prevent toxic substances formation when it is used with halogenate anesthetics.
First one should think about the correct way to use sufficiently hydrated lime.
Adequate amount of water on lime, that is, in the recommended amounts, impairs anesthetic absorption by lime granules. If this occurs there is decreasing anesthetic inspired concentration when it should be high (induction) and increasing it when it should be low (emergence). In addition, it increases contact of halogenates with lime, favoring their metabolism and the formation of toxic substances.
Another important aspect of lime hydration is that during carbon dioxide absorption, the first reaction is between this gas and water forming carbonic acid which is a weak acid. The reaction of this acid with strong base hydroxides is less exothermal as compared to the direct reaction of CO2 with such strong bases.
Lack or significant water decrease reaching critical levels below 4.5% may even contribute in a major and determinant way to high lime temperature increase resulting in more halogenate anesthetic metabolism and surely the production of potentially or knowingly toxic substances 18.
There is a chain reaction as from dehydrated or dried out soda lime. There is temperature increase, more anesthetic absorption by lime granules and more degradation of these agents' molecules with the production of compound A, CH2 F - O - C (CF3) = CF2, by reaction of hydroxides with sevoflurane. Carbon monoxide is also produced by the reaction of halogenates and lime bases 18. These products are knowingly noxious for humans. Compound A is nephrotoxic and may produce irreversible renal injury, and carbon monoxide leads to severe hypoxia with blood coagulation changes 24.
PREVENTING TOXIC SUBSTANCES PRODUCTION
In addition to care with soda lime hydration, it is possible to use lime without sodium and potassium hydroxides which are strong bases and produce exothermal reaction with major temperature increase 25.
An experimental study has shown that with dehydrated soda lime without sodium and potassium hydroxide, just with calcium hydroxide, carbon monoxide production is much lower in the presence of desflurane, and compound A production is lower in the presence of sevoflurane. With this same lime, however adequately hydrated, the production of those undesirable substances is even lower 26.
Care should be taken to use the newest possible soda lime and when it is exposed to environment for many hours, such as during weekends (more than 48 hours) it is recommended to add water (preferably distilled water) in the ratio of 25 mL for each 500 g of lime.
Sodium and potassium hydroxide-free soda lime, with calcium hydroxide only, should be preferred.
Soda lime in the anesthesia machine inhalation system is still desirable for its major advantages, such as decreasing fresh gases flow, anesthetic consumption and operating room pollution and maintaining airway humidity. However it is necessary that anesthesiologists take some care with lime composition, preferring those sufficiently hydrated (15%) and with just calcium hydroxide as base to absorb carbon dioxide.
In case of doubt or suspicion of toxic substance formation fresh gases flow (oxygen) should be increased to a minimum of 1 respiratory minute/volume and distilled water should be added to soda lime, approximately 25 mL for each 500 g of lime.
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Submitted for publication July 15, 2003
Accepted for publication September 23, 2003
* Received from Hospital SARAH, Brasília, DF