Interactions of Inhaled Anesthetics with Absorbents

It is important and desirable to have carbon dioxide absorbents that are neither intrinsically toxic nor toxic when exposed to common anesthetics. Soda lime and Baralyme generally fit this description, but inhaled anesthetics do interact with absorbents to some extent. An uncommon anesthetic, trichloroethylene, reacts with soda lime to produce toxic compounds. In the presence of alkali and heat, trichloroethylene degrades into the cerebral neurotoxin dichloroacetylene, which causes cranial nerve lesions and encephalitis. Phosgene, a potent pulmonary irritant, is also produced, and phosgene can cause adult respiratory distress syndrome (ARDS).^[121]

Sevoflurane produces degradation products on interaction with carbon dioxide absorbents.^[122] The major degradation product produced is an olefin compound known as fluoromethyl-2,2-difluoro-1-(trifluoromethyl)vinyl ether, or compound A. During sevoflurane anesthesia, factors apparently leading to an increase in the concentration of compound A include low-flow or closed-circuit anesthetic techniques, the use of Baralyme rather than soda lime, higher concentrations of sevoflurane in the anesthetic circuit, higher absorbent temperatures, and fresh absorbent.^{[122] [123] [124] [125]} Baralyme dehydration increases the concentration of compound A, and soda lime dehydration decreases the concentration of compound A.^{[126] [127]} The degradation products apparently do not cause toxic effects in humans, even during low-flow anesthesia,^[124] but further studies are needed to verify this.^{[128] [129] [130]}

Desiccated soda lime and Baralyme can degrade contemporary inhaled anesthetics to clinically significant concentrations of carbon monoxide, which can produce carboxyhemoglobin concentrations reaching 30% or more.^[131] Higher levels of carbon monoxide are more likely after prolonged contact between absorbent and anesthetics and after disuse of an absorber for at least 2 days, especially over a weekend. Case reports describing carbon monoxide poisoning have been most common for patients anesthetized on Monday morning, presumably because continuous flow from the anesthesia machine dehydrated the absorbents over the weekend.^{[132] [133]} A fresh gas flow rate of 5 L/min or more through absorbent (without a patient) is sufficient to cause critical drying of the absorbent, particularly if the breathing bag is left off the breathing circuit. Absence of the bag facilitates retrograde flow through the circle system (see Fig. 9-22 ).^[131] Because the inspiratory valve leaflet produces some resistance to flow, the fresh gas flow takes the retrograde path of least resistance through the absorbent and out the 22-mm breathing bag terminal.

Several factors appear to increase the production of carbon monoxide and carboxyhemoglobin:

1. The inhaled anesthetic used (for a given MAC multiple, the magnitude of carbon monoxide production from greatest to least is desflurane ≥ enflurane > isoflurane ≫ halothane = sevoflurane)
2. The absorbent dryness (completely dry absorbent produces more carbon monoxide than hydrated absorbent)
3. The type of absorbent (at a given water content, Baralyme produces more carbon monoxide than does soda lime)
4. The temperature (a higher temperature increases carbon monoxide production)
5. The anesthetic concentration (more carbon monoxide is produced from higher anesthetic concentrations)^[134]
6. Low fresh gas flow rates
7. Reduced animal size^[135] per 100 g of absorbent

Interventions have been suggested to reduce the incidence of carbon monoxide exposure in humans undergoing general anesthesia^[133] :

1. Educating anesthesia personnel regarding the cause of carbon monoxide production
2. Turning off the anesthesia machine at the conclusion of the last case of the day to eliminate fresh gas flow, which dries the absorbent
3. Changing carbon dioxide absorbent if fresh gas was found flowing during the morning machine check
4. Rehydrating desiccated absorbent by adding water to the absorbent^[132]
5. Changing the chemical composition of soda lime (e.g., Dragersorb 800 plus, Sofnolime, Spherasorb) to reduce or eliminate potassium hydroxide
6. Using absorbent materials such as calcium hydroxide lime that are free of sodium and potassium hydroxides

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In late 2003, Abbott Laboratories, North Chicago, IL, makers of sevoflurane, in conjunction with the FDA, published a revised package insert and circulated a letter to anesthesia care providers that explained the potential for fires in the respiratory circuit with the use of sevoflurane. The revised sevoflurane package insert (Abbott Laboratories, reference 58-7208) describes this rare phenomenon occurring when sevoflurane is used in combination with a desiccated carbon dioxide absorbent such as dehydrated Baralyme. A poorly characterized chemical reaction between the sevoflurane and the absorbent reportedly has produced sufficient heat and combustible degradation products to lead to spontaneous generation of fires within the absorber canister subassembly and the breathing circuit. The combination of a rapid color change of the carbon dioxide absorbent (especially Baralyme) and an unusually slow increase in inhaled sevoflurane concentration, compared with the vaporizer concentration control dial setting, may indicate that a decomposition reaction with sevoflurane and the absorbent is occurring. If this happens, the potential for excessive heating or fire may be present within the respiratory circuit. To avoid this, anesthesia care providers should make every effort not to use desiccated carbon dioxide absorbents. Whenever there is a question of whether the absorbent is fresh and adequately hydrated, it should be replaced to avoid the possible occurrence of fire in the respiratory circuit.