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Department of Anaesthesia; University of California, San Francisco; San Francisco, CA; egere{at}anesthesia.ucsf.edu
In Response:
Dunning and Woehlck have correctly taken us to task for neglecting the effect of desflurane degradation products on the RGM 5250 determination of desflurane (and isoflurane) concentration. Using gas chromatography, we confirmed the correctness of their criticism by measuring the concentrations of desflurane in the presence of desiccated Baralyme®. Duplicate measurements (two runs) supplied the data for desflurane presented in Figure 1. As in the previous study (1), peak temperatures of 100°C were reached 20–30 min after initiating delivery of 9% desflurane. Temperatures declined thereafter, despite appreciable continuing desflurane degradation for up to an hour of desflurane delivery.
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Although the data presented in Figure 1 quantitatively differ greatly from the data presented in a similar figure in our original report (Fig. 5) (1), the qualitative implications are unchanged. Degradation of desflurane, and to a lesser extent isoflurane, is not sustained. In contrast, degradation of sevoflurane continues at appreciable levels for the duration of sevoflurane administration (note that data for the full 2 hours are not presented because fire occurred in one run, and inclusion of those data would skew the results). The data for desflurane and isoflurane are consistent with an action of potassium hydroxide (KOH) on these anesthetics. The limited presence of KOH (5% of base), and its consequent rapid depletion, restricts the degradation of desflurane and isoflurane. Degradation of desflurane decreases faster than degradation of isoflurane because the concentration of desflurane is three times greater, thus exhausting the stores of KOH sooner. The continuing degradation of sevoflurane suggests that barium hydroxide and/or calcium hydroxide, as well as KOH, degrade sevoflurane.
The important message of our previous report (1) remains unchanged by the present correction. Sevoflurane degradation by desiccated Baralyme® can lead to temperatures exceeding 150°C and to fire in the anesthetic circuit. In contrast, degradation of desflurane and isoflurane by desiccated Baralyme® leads to lower temperatures (peaks of 100°C), and these lower temperatures do not lead to fire. As an aside, we note that a concern that fires from sevoflurane degradation may be more likely with Baralyme® than soda lime has resulted in the withdrawal of desiccated Baralyme® from commercial use. Whether this problem extends to soda lime remains to be determined.
[Dr. Eger is a paid consultant to Baxter Healthcare, the manufacturer of desflurane.]
Reference
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