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Correcting for repeated outcome measures

Sir—We read with great interest the article by Tsiotou et al regarding the use of dexmedetomidine on pediatric tonsillectomy patients and its effect on emergence delirium (ED). We commend these authors for adding to an area of interest that is particularly important. Although the design of this study is relatively straightforward, there appears to be issues with the statistical analyses. Since the primary outcome was measured at three distinct time points (10, 20, and 30 minutes after postanesthesia care unit [PACU] arrival), the analysis has to account for the within‐subject correlation due to the repeated measurements on the same patients and the significance level needs to be adjusted to correspond to the multiple comparisons. By neglecting these problems, the Type I error becomes inflated beyond what is signified by the stated P‐values. Since the primary outcome is binary, these issues can be resolved by performing a repeated measures logistic regression (generalized estimating equation) with some manner of multiplicity correction if the intervention‐time interaction is significant. There are similar problems with the statistical analysis for ED severity and intraoperative heart rate/blood pressure/BIS, which can be remedied through the use of a two‐way repeated ordinal regression and two‐way repeated measures ANOVA modeling, respectively. Also, it was not explicitly stated whether ED identified at the different time intervals was unique or not. In other words, were the patients identified in one time interval counted again in a later time interval if their ED did not resolve? Including the same patient in more than one time interval does not take into account the possibility of differing ED durations and may create an illusion of an overall increased frequency of ED. Patel et al has shown that dexmedetomidine administration is associated with a shorter duration of severe ED. Therefore, the decrease in ED proportion in the dexmedetomindine group at the 20 and 30 minute post‐PACU intervals may have resulted from a decrease in ED duration rather than ED incidence. A way of rectifying both of these aforementioned issues would be to simply restate some of the study's objectives. Rather than comparing the presence and severity of ED at three different time points, it may be more beneficial to the reader to state whether ED occurred in the PACU (≥1 ED event, regardless of event time) and to present the maximum ED severity during this entire period. From a clinical standpoint, the specific interval (10 vs 20 vs 30 minutes post‐PACU arrival) is unimportant when compared to the overall presence and severity of ED postoperatively. Not only would this study reformulation lead to more easily interpreted results, the statistical analysis for these two outcomes would be less convoluted as well. In addition, the authors failed to mention whether intravenous midazolam was administered to any of the study patients in the PACU due to “persistent ED.” Also, the term “persistent ED” needs to be adequately defined, specifically in relation to whether the ED rescue medication was given during the 30 minute PACU study window. This last point is extremely important since this practice would definitely obscure subsequent ED measurements.