[Degradation of halothane, enflurane, and isoflurane by dry soda lime to give carbon monoxide].

In the presence of completely dry soda lime volatile anaesthetics will decompose to carbon monoxide (CO). In an in vitro study, the absorbent (soda lime, ICI) was dried with a constant gas flow of 11/min oxygen for 120 h. The weight loss during the drying was 17.1%. Two vol% of halothane, enflurane or isoflurane in oxygen was administered with a constant flow of 0.51/min oxygen through the completely dry absorbent. Concentrations of gases were measured before and after the absorbent using mass spectrometry (MGA 1100, Perkin-Elmer) and an electrochemical NO monitor (Mini PAC CO, Dräger). The temperature inside the soda lime was monitored continuously. Shortly after adding the anaesthetic to the oxygen passing through the absorbent, carbon monoxide appeared in the outlet of the soda lime container. The measured peak concentrations varied around 450 ppm (halothane), 3500 ppm (enflurane) and 3800 ppm (isoflurane). The temperature inside the absorbent rose from the ambient temperature (19.8 degrees C) to a maximum of 52.1 degrees C during CO production and decreased when the CO production lowered after approximately 1 h (all anaesthetics). During CO production no measurable concentration of halothane left the absorber. After passing through the absorbent the concentrations of isoflurane and enflurane were slightly lower than the corresponding concentrations in the fresh gas measured before absorption.