ABC | Volume 114, Nº2, February 2020

Short Editorial Abreu Clinical significance of statistical diferences Arq Bras Cardiol. 2020; 114(2):281-283 interval ≥ 440 ms have a higher risk of total or cardiovascular mortality than those with lower values may reflect the role of QTc as a marker of still subclinical cardiovascular disease. In this analysis, none of the seven studies was designed to test the hypothesis that QTc interval is associated with total or cardiovascular mortality. Thus, uncontrolled confounders arising from uncollected or unknown variables may have influenced the results in either direction. 10 QT dispersion (QTd) is defined as the difference between the longest and shortest QT intervals in any of the measured leads. The use of low number of leads was certainly the main cause of the lack of reproducibility repeatedly demonstrated in several studies that analyzed this parameter, making it difficult to determine its clinical significance especially in patients without known heart disease. 11 Making use of technologies that allow for a larger number of derivations and recordings, with extended time, allowing the analysis of several records at different times is a trend in studies that explore the depolarization / repolarization periods. A study using the body surface mapping system, QT interval was measured in 80 unipolar chest leads, showing QTc 415.2+/-4.1 ms in DM 1 and 401.4+/-6.6 ms in controls (NS). 12 The method allows the analysis of the electrocardiogram, the vetocardiogram and the mapping of depolarization / repolarization in more detail. 13 The advantage of this procedure is its improved spatial sampling, allowing abnormalities that may be difficult to detect and measure using the 12-lead approach be better defined with the additional electrodes. In the 1990s, at our Cardiology Service of the Hospital Federal dos Servidores do Estado, we had the opportunity to use the body surface mapping system in a sample of our population and the results were presented in a scientific conference held in Karlovy Vary, Czech Republic, where the method is quite used at Charles University, Prague. From this center there is an important review published in 2015 analyzing electrocardiographic changes in diabetes, revealing tachycardia, QRS and QT shortening, increased QT dispersion, reduced depolarization wave amplitude, reduced ventricular myocardial activation time, and T wave flattening confirmed by the lowest maximum and minimum value in the isopotential repolarization body surface maps. Most of these changes are even more pronounced in patients with cardiac autonomic neuropathy. Comparison with electrocardiographic changes in other diseases suggests that those present in patients with diabetes are not specific and are caused by an increase in sympathetic nervous system tone, which was indirectly confirmed by findings of heart rate variability in these patients. 14 As we can see, obtaining measures of the depolarization/ repolarization period has a long history and constant challenges. Some of the disparities in interpretation of findings arise from inconsistencies in the measurements obtained. The numerous factors involved in the inaccuracy of QT measurements and their repercussions as predictors in the clinical context make it difficult to assess the significance of minor QT changes even when they are statistically significant. In summary, based on these observations, we cannot affirm that changes in depolarization / repolarization estimated by electrocardiography can be used as predictors of mortality and cardiovascular outcomes in the general population and apparently healthy diabetics. Further research is needed, which should include new technologies for serial measurements of these intervals and their measurement by a central electrocardiography laboratory. Such studies should be specifically designed to test the hypothesis that these depolarization / repolarization changes are associated with cardiovascular mortality. 1. 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