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Department of Anesthesia and Perioperative Medicine; University of California, San Francisco; San Francisco, CA; talkep{at}anesthesia.ucsf.edu
In Response:
We appreciate the interest and careful review by Dr. Sneyd of our article (1). We demonstrated that intraoperative treatment with 3 µg/kg of clonidine significantly reduced postoperative shivering in neurosurgical patients. This was our primary result, and Dr. Sneyd expressed no concern with regards to this finding. Dr. Sneyd did, however, express other concerns.
Dr Sneyd lists five concerns with our study. His first concern is that our study lacks power calculation or primary outcome measure. To address this concern we would like to direct his attention to page 227 (in the "Methods" section), which reads: "Our sample size analysis indicates that 34 subjects (17 in each group) will provide a 80% change (power = 0.8) to identify a 5-min difference in time to follow commands between the two groups at an 
level of 0.05," and to page 231 (in the "Discussion" section), which reads "Our sample size calculation for the second study was based on our primary endpoint (to follow commands)."
His second concern is that our study was underpowered and that we made robust statements regarding recovery from anesthesia endpoints. We would like to direct Dr Sneyd again to page 231, which reads "Our sample size calculation for the second study was based on our primary endpoint (to follow commands). Thus, this study may be underpowered for several other variables...that did not reach statistical significance."
Dr. Sneyd also expressed concern that our sedation data are misrepresented. He implies that 3-min and a 4-min differences in the times to eye opening and extubation of the trachea, respectively, after an operation that lasts longer than 6 h are clinically significant. Unfortunately, we are not clear on what we have supposedly misrepresented. In addition to providing the descriptive statistics, we displayed the data in a Figure 2, from which the reader can determine the recovery values for each individual patient. We feel that the data are clear and not subject to misinterpretation. With reference to the Dr. Sneyd’s assertion that the study had an inadequate sample size, please note that this is clearly acknowledged in the publication on page 231: "this study may be underpowered for several other variables...that did not reach statistical significance."
With regards to survival analysis, Dr. Sneyd expressed concern that survival analysis will show that clonidine delays emergence from anesthesia in some patients. Please note that we did not do survival analysis and hence we cannot make this postulation. We did, however, provide the reader with additional information on the distribution of the data that is not evident from summary statistics (see Figure 2). Moreover, we disagree with Dr. Sneyd’s assumption that the delay in some patients’ emergence is attributable to clonidine. From our data we are unable to conclude the cause for delayed emergence. Similar distribution of recovery data and occasional patients with delayed emergence after resection of brain tumors has been reported in previous studies without clonidine (2).
Finally, we thank Dr. Sneyd for bringing to our attention a mistake in the Methods section, where the visual analog scales should read 0–10. Although it is difficult to determine what magnitude change in the pain visual analog scale scores should be considered clinically significant, we are also in total agreement with Dr. Sneyd who feels that it is safe to assume that for any individual patient, ANY increase in pain is clinically significant. We hope that these explanations satisfy Dr. Sneyd concerns and thank him for his interest in our study.
References
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