Measurement of Isoflurane and Desflurane Degradation

To the Editor: Laster et al. (1) suggest that only small amounts of isoflurane and desflurane are degraded in their studies of anesthetic absorbent fires. The monitoring device used for anesthetic concentrations was a monochromatic infrared monitor (RGM 5250, Ohmeda, Louisville, CO), which is prone to interference because the infrared absorbance is measured at a single wavelength. The reported anesthetic concentration is based on a user-entered calibration factor for each anesthetic. Because trifluoromethane (CHF3) is produced during the breakdown of isoflurane and desflurane (2), and interferes with anesthetic drug monitoring (3), CHF3 may cause falsely elevated reported concentrations of the selected anesthetic. Calculations (not shown) based on our study (3) demonstrate that the RGM 5250 monitor reports approximately 0.9% isoflurane or 1.2% desflurane for each 1% CHF3. The breakdown of 1 mole isoflurane or desflurane produces 0.23 and 0.84 moles of CHF3, respectively. Therefore, the reported concentration of isoflurane is falsely elevated by about 21% and desflurane by about 98% of the degraded amount due to the interference of CHF3. Thus, little if any change in reported desflurane concentration can result. Although the ratio of the amount of CHF3 produced for each mole of anesthetic degraded depends somewhat on reaction temperature (4), it is probable that little, if any, decrease in the concentration of desflurane can be detected by the RGM 5250 monitor because of the interference of CHF3. Figure 5 in Laster et al.’s article (1) may indicate little to no desflurane degradation as a result, and the discrepancy at the initial measurement may be due to temperature dependence, since the absorbent is under 25°C at that time. Marshall B. Dunning III, PhD, MS Division of Pulmonary/Critical Care Medicine; [email protected] Harvey J. Woehlck, MD Department of Anesthesiology; Medical College of Wisconsin; Milwaukee, WI