To the Editor

Recent reports of circuit fires following exposure of sevoflurane to desiccated carbon dioxide absorbent has refocused attention on the risks associated with volatile anesthetics. Volatile anesthetics have been in use for more than 150 years, and it is surprising to many clinicians when “new” complications seem to come to light every few years. In the past 20 years, carbon dioxide absorbents have been implicated in a number of these complications. Although we perform daily machine and circuit checks, and most of us have a good working knowledge of how our machines and ventilators work, the CO₂ absorbent rarely gets much attention, unless of course, its color changes, marking exhaustion of its CO₂ absorbing capability.

One of the job descriptions of an anesthesiologist is that of trend watcher, and a trend involving barium-containing CO₂ absorbers has slowly come to light. Exposure of sevoflurane to CO₂ absorbent results in Compound A formation. Desflurane is associated with carbon monoxide formation when exposed to desiccated CO₂ absorbent, and the recent case reports of extreme heat during sevoflurane use have all been associated with the exposure of agent to CO₂ absorbent. All of these risks are increased when barium is the basis of the CO₂ absorbent:

• Barium-based absorber dehydration increases the concentration of Compound A; whereas soda lime dehydration decreases it.¹
• Desiccated barium-based absorbers are associated with a nearly 7-fold increase in carbon monoxide production (11600 ppm vs. 1800 ppm) versus soda lime when exposed to clinically used concentrations of desflurane.²
• All of the 4 reported cases of circuit fires associated with sevoflurane and dessicated CO₂ absorbent involved barium, and a recent laboratory report described how the CO₂ absorber canister exploded and burst into flames when dehydrated barium-based absorber was exposed to sevoflurane.³

Barium and soda lime neutralize exhaled CO₂ via an exothermic reaction with a strong base: the former via barium hydroxide, and soda lime via sodium and potassium hydroxide. Both have approximately equal absorbing capabilities, and their cost is similar. Amsorb, a newer agent which contains neither potassium, barium, nor sodium has been proposed as a safer alternative, but it has not yet reached wide market acceptance.

The history of anesthesia is littered with products that fell out of practice for reasons of risk to patient safety. We no longer use ether or cyclopropane due to the risk of explosion. We no longer use hanging bellows due to the risks of decreased circuit leak detection. We no longer use droperidol due to the risk of QT prolongation. All, however, fell out of practice only when better substitutes were available. Soda lime is an excellent CO₂ absorbent and is as efficient as barium-containing absorbers; however, soda lime is less likely to be associated with these rare but real risks when used with the newer volatile anesthetics. It is time to reassess our use of this product, and add it to our list of anesthesia museum pieces.

Roy G. Soto, MD Tampa, FL

References

1. Eger EI, Ion P, Laster M, Weiskopf R. Baralyme dehydration increases and soda lime dehydration decreases the concentration of compound A resulting from sevoflurane degradation in a standard anesthetic circuit. Anesth Analg 1997;85:892-8.
2. Fang Z, Eger E, Laster M, 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.
3. Holak E, Mei D, Dunning M, et al. Carbon monoxide production from sevofluration breakdown: modeling of exposures under clinical conditions. Anesth Analg 2003;96:757-64.