Carbon Monoxide Formation from Volatile Anesthetics Measured during Simulated Clinical Conditions. Wissing et al. (page 1205)

Recent clinical reports of increased carbon monoxide (CO) hemoglobin concentrations in children anesthetized with sevoflurane that had passed through dry soda lime seem to contradict the laboratory experience with this anesthetic. To examine discrepancies between laboratory investigations and clinical experiences, Wissing et al.  designed an experiment to measure CO formation from five different volatile anesthetics passed through an absorber system that permitted temperature changes.

Experiments were conducted in triplicate. Either 2.5% or 5% of five inhalational anesthetics (desflurane, enflurane, isoflurane, halothane, and sevoflurane) were passed for 2 h through an absorber canister filled with dried soda lime. Baseline CO production was first determined using dry soda lime and a flow of 2 l/min O2with no volatile anesthetic and using fresh wet soda lime and a flow of 2 l/min with 5% anesthetic. CO concentrations were continuously measured at the absorber outlet. Additional experiments were conducted to confirm the sevoflurane results because the magnitude of CO production was unexpected and because of the potential of various breakdown products. CO was detected with all anesthetics passed through dry soda lime, but the time course and rate of CO production and the time course of temperature changes differed between the agents. Measurable amounts of CO were found immediately after desflurane, enflurane, or isoflurane came in contact with the soda lime; with sevoflurane, there was a time delay between contact and CO production. CO production peaked initially and was highest with desflurane, followed by enflurane, isoflurane, sevoflurane, and halothane. The temperature of the absorbent increased with all anesthetics, but was highest for sevoflurane. As a result of these experiments, the researchers caution that although CO production is clearly higher than with other agents, some CO—in possibly relevant amounts—is produced by sevoflurane. Adequate precautions should be taken to ensure that soda lime in absorbers does not become desiccated.