IJCS | Volume 33, Nº3, May / June 2020

269 Figure 5 - QTc evolution in the true negative group from rest (baseline QTc) to effort (peak QTc) conditions. QTc Baseline QTc Peak QTc reproducibility was weak. Our impression is that, even with the measurements being performed with more modern software, the problem involving the reproducibility of QT intervals measurements remains a considerable one. With the results of catheterization, QT dispersion, QTc and stress test, we built ROC curves in order to find the possible cutoff values of QTd and and stress- rest QTc, so as to subsequently calculate the sensitivity and specificity of QTd for the diagnosis of chronic CAD. We did not achieve a minimaly satisfactory ROC curve for QTd and QTc at rest. For QT and QTc dispersion of effort, we obtained ROC curves somewhat better than those obtained at rest, and we found cutoff values for QTd (46 ms) and QTc (57 ms). As for the sensitivity and specificity of stress QTd and QTc it is possible to say that they were comparable to the traditional stress test ischemic criteria. However, what really called attention, was the low specificity of the classical diagnostic criteria of exercise induced ischemia in our sample (32%). On the other hand, the sensitivity was 72%. The high false- positive rate was quite high. We observed that most of the false-positive results were found in the presence of segment depression without a concomitant stenosis of at least 70%. The most plausible explanation found lies in the arbitrariness of considering as the possible cause of ischemia only stenosis with obstruction greater than 70%. Smaller plaques, but under the effect of vasoconstriction substances, can cause ischemia. Another factor to be considered is the possibility of microcirculation disease in patients without significant disease of large coronary arteries. Finally, we must consider that the degree quantification of coronary obstructionmade by the doctor responsible for the catheterization is performed by visual method only and, hence, it is observer-dependent. When we analyzed the data from the three groups formed: true positive (TP), false positive (FP) and true negative (TN), heart rate (HR) and systolic pressure (SP) at stress peak were not statistically different between the three groups (Tabble 2). The incidente of typical chest pain and ST depression at peak stress were higher among the false-positive and true-positive groups, and was not present in the TN group, as we already expected. In relation to the QT dispersion and QTc dispersion at rest, there was no significant statistical difference between the three groups. In contrast, stress QTc was significantly higher among the TP and FP groups, compared with the TN group. The TP and FP groups behaved so similarly that made us wonder about the real importance of considering “people with significant coronary artery disease” only those patients with stenosis of at least 70% in epicardial arteries, or 50% or more in the left coronary trunk. We can speculate that if myocardial perfusion scintigraphy had been performed, instead of cardiac catheterization, as gold standard for significant CAD, it is possible that our results would have been similar to those Barcelos et al. QT interval dispersion in exercise test Int J Cardiovasc Sci. 2020; 33(3):263-271 Original Article