BJSM Figure 4: This figure illustrates the ‘Teach-the-Tangent’ or ‘Avoid-the-Tail’ method for manual measurement of the QT interval. A straight line is drawn on the downslope of the T wave to the point of intersection with the isoelectric line. The U wave is not included in the measurement. This figure is only reproduced in colour in the online version. plitude U wave in the QT calculation. Such U wave inclusion will inflate greatly the QTc. Instead, follow the ‘Teach-theTangent’ or ‘Avoid-the-Tail’ method as shown in figure 4.25 Sixth, the morphology of the T waves, not just the length of the QT interval, can suggest the presence of a QT syndrome.28 As shown in figure 5, a notched Twave in the lateral precordial leads may be a tip off to LQTS even in the absence of overt QT prolongation. With this framework, the easiest and most efficient way to confirm the computer-derived QTc is to examine lead II and/or V5 and determine if the manually measured QT interval matches the computer’s QT measurement. If there is concordance within about 10 ms of each other, one can trust that the computer can derive accurately an average RR interval and complete Bazett’s calculation. As such, the computer generated QTc has been confirmed manually. If, however, the manually measured QT interval is >10 ms different than the computer’s QT measurement, calculate an average RR interval and recalculate the QTc using Bazett’s formula. QTc cut-offs: how long is too long? How short is too short? Figure 6 shows the overlap between the distribution of QTc values in population-derived cohorts of healthy individuals compared to patients with genetically confirmed LQTS.29–31 Considering that 25–40% of genotype positive individuals (mostly relatives of the index cases/probands) have normal QT interval/concealed LQTS, it must be acknowledged that no screening programme will identify all persons with either LQTS or SQTS.32 Instead, the QTc cut-off values, where Figure 5: ECG of a patient with long QT syndrome. Note the notched T waves (arrows) in the lateral precordial leads (V4–V5) that are typical of long QT type-2. This figure is only reproduced in colour in the online version. Figure 6: Distribution and overlap of QTc values for healthy individuals compared to patients with genetically confirmed LQTS. 24 Sport & Geneeskunde | november 2013 | nummer 5 the QTc measurement requires further evaluation, must be chosen carefully to balance the frequency of abnormal results and the positive predictive value that an SQTS or LQTS host has been identified. Published definitions of a ‘prolonged QTc’ requiring further evaluation have varied. Guidelines from the European Society of Cardiology for ECG interpretation in athletes define a QTc value of >440 ms in men and >460 ms in women (but <500 ms) as a ‘grey zone’ requiring further evaluation, and a QTc ≥500 ms, otherwise unexplained and regardless of family history and symptoms, as indicative of unequivocal LQTS.6 In the USA, the AHA/ACC/HRS guideline has dropped the term ‘borderline’ QT prolongation and instead now annotates a QTc ≥450 ms in men and ≥460 ms in women as ‘prolonged QTc’.33 Concern has been raised that these QTc cut-offs will produce a high number of false-positive test results if followed Pagina 23
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