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LETTERS TO THE EDITOR

Airway Responses to Sevoflurane vs. Desflurane: Comparing Apples to Oranges?

Department of Anesthesiology, Medical College of Wisconsin, Milwaukee, WI

To the Editor:

Eshima et al. (1) suggest there are no differences in the incidences of coughing and breath holding between sevoflurane and desflurane for maintenance of anesthesia with a laryngeal mask airway, after providing analgesic adjuvants (fentanyl and nitrous oxide). However, they also report a nearly twofold higher percentage of patients received over 1 MAC of sevoflurane compared with desflurane. Although not stated, the percentage of patients receiving less than 1 MAC of anesthesia was presumably larger in the desflurane group. This difference invalidates all comparative findings in this report. It is the higher concentrations of a pungent anesthetic, desflurane, where undesirable airway responses would likely occur. In fact, an unpaired t-test of the average MAC fraction of sevoflurane and desflurane suggests statistical significance (P < 0.05) although the authors report nonsignificance (Table 1 in the article). The authors then inappropriately report statistically significant differences in secondary end-points of their research that include time to response to command, orientation, and minutes to modified Aldrete score of 13. The authors discuss the possibility that differences in secondary outcome end-points might be related to the imbalance in the anesthetic exposures. It seems that a similar argument should be made about the absence of differences in the airway responses between desflurane and sevoflurane.

Reference

1. Eshima RW, Maurer A, King T, et al. A comparison of airway responses during desflurane and sevoflurane administration via a laryngeal mask airway for maintenance of anesthesia. Anesth Analg 2003; 96: 701–5.[Abstract/Free Full Text]

Response

Department of Anesthesia and Perioperative Care, University of California San Francisco, San Francisco, CA

In Response:

We welcome the opportunity to respond to Dr. Ebert’s concerns. Dr. Ebert did not appreciate that the published values were rounded to give an average difference of 0.1 MAC. In fact, the difference was 0.0914 MAC, and the resulting t-test value was 1.92, just missing the P of 0.05 chosen a priori. Similarly, the MAC-hours did not differ significantly, nor was the median MAC lower for desflurane.

But suppose that a difference did exist. Dr. Ebert notes the greater proportion of patients anesthetized with sevoflurane who received maintenance concentrations in excess of 1 MAC (48% vs 27%), concluding that ‘it is the higher concentrations of a pungent anesthetic, desflurane, where undesirable airway responses would likely occur.‘ That conclusion fails because the 17 patients anesthetized with desflurane at concentrations exceeding 1 MAC did not have a greater incidence of coughing (5.9% vs 3.2%) or laryngospasm (0.0% vs 0.0%) than the 31 patients anesthetized with sevoflurane at concentrations exceeding 1 MAC. Perhaps had we examined more patients, the difference for coughing would have become significant, but we estimate that this might require study of approximately 1,000 patients to reveal what was a minor respiratory event. None of these observations invalidates our conclusion that "the respiratory complications that arise during maintenance delivery of anesthesia through an LMA are minor, their incidence is small, and the incidence does not differ for desflurane versus sevoflurane."

Returning to the issue of rapidity of awakening, again assume that the administered concentrations differed modestly. When the results for patients are divided into those receiving more than 1 MAC and those receiving less than 1 MAC, significant differences continue to favor a more rapid awakening with desflurane. For example, patients given more than 1 MAC desflurane had an Aldrete score of 13 at 22.0 ± 9.2 min (mean ± SD) after anesthesia, but patients given more than 1 MAC sevoflurane reached that Aldrete score 31.2 ± 18.1 minutes after anesthesia (P < 0.006). Thus, we believe we appropriately reported statistically significant differences in secondary end-points, indicating a more rapid awakening with desflurane than with sevoflurane.

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