Baralyme[registered sign] Dehydration Increases and Soda Lime Dehydration Decreases the Concentration of Compound A Resulting from Sevoflurane Degradation in a Standard Anesthetic Circuit

Abstract

Soda lime and Baralyme[registered sign] brand carbon dioxide absorbents degrade sevoflurane to CF2=C(CF3)OCH2 F, a potentially nephrotoxic vinyl ether called Compound A.Dehydration of these absorbents increases both the degradation of sevoflurane to Compound A and the degradation of Compound A. The balance between sevoflurane degradation and Compound A degradation determines the concentration of Compound A issuing from the absorbent (the net production of Compound A). We studied the effect of dehydration on the net production of Compound A in a simulated anesthetic circuit. Mimicking continuing oxygen delivery for 1, 2, or 3 days after completion of an anesthetic, we directed a "conditioning" fresh gas flow of 5 L/min or 10L/min retrograde through fresh absorbent in situ in a standard absorbent system for 16, 40, and/or 64 h. The conditioned absorbent was subsequently used (without mixing of the granules) in a standard anesthetic circuit in which a 3-L rebreathing bag substituted for the lung. Metabolism was mimicked by introducing 250 mL/min carbon dioxide into the "lung," and the lung was ventilated with a minute ventilation of 10 L/min. At the same time, we introduced sevoflurane in a fresh gas inflow of 2 L/min at a concentration sufficient to produce an inspired concentration of 3.2%. Because of increased sevoflurane destruction by the absorbent, progressively longer periods of conditioning (dehydration) and/or higher inflow rates increased the delivered (vaporizer) concentration of sevoflurane required to sustain a 3.2% concentration. Dehydration of Baralyme[registered sign] increased the inspired concentration of Compound A by up to sevenfold, whereas dehydration of soda lime markedly decreased the inspired concentration of Compound A. Implications: Economical delivery of modern inhaled anesthetics requires rebreathing of exhaled gases after removal of carbon dioxide. However, carbon dioxide absorbents (Baralyme[registered sign] / soda lime) may degrade anesthetics to toxic substances. Baralyme[registered sign] dehydration increases, and soda lime dehydration decreases, degradation of the inhaled anesthetic sevoflurane to the toxic substance, Compound A. (Anesth Analg 1997;85:892-8)