ologist has an index of suspicion for LQTS (intermediate or high probability score), or (2) an asymptomatic patient with no family history but an incidental ECG finding with a QTc >480 ms prepuberty and >500 ms postpuberty that is confirmed on repeat ECG testing.1 Genetic testing may be considered for individuals with an incidental QTc finding (repeated) of ≥460 ms prepuberty and ≥480 ms postpuberty.1 Calculating the QTc Whether the ECG was obtained for screening or diagnostic purposes, the heart rate corrected QT interval (QTc) derived by the computer must be confirmed manually because the accuracy of the computer generated QTc is only about 90– 95%. Notably, the computer-derived QTc for the ECG in figure 1 was off by about 400 ms as the true QTc was around 760 ms but was read as 360 ms. Although this is an extreme example of an inaccurate computer QTc calculation, studies have also suggested that the ability of cardiologists and even heart rhythm specialists to accurately measure the QTc is suboptimal.24 However, an accurate assessment of the QTc can be taught and achieved by adhering to the following six principles.25 First, most ECG machines utilise the Bazett’s heart rate correction formula (QTc=QT/√RR; note the RR interval is measured in seconds).26 Figure 1: ECG demonstrating a profoundly prolonged QT interval. The computer derived a QTc measurement of 362 ms which was inaccurate. Manual measurement of the QTc was 760 ms. Although there are many heart rate correction formulas for the QTc, it is recommended to use Bazett’s correction to confirm the computer’s QTc as the population-based QTc distributions most frequently used Bazett-derived QTc values. Second, Bazett’s formula loses accuracy at slow heart rates and can underestimate the individual’s inherent QTc at heart rates <60 bpm, especially at heart rates <50 bpm. Accordingly, if an athlete has a heart rate <50 bpm, repeat the ECG after some mild aerobic activity to get his/her heart rate into a range (60–90 bpm) where the formula is most accurate. Third, if there is beat-to-beat variation in heart rate (sinus arrhythmia) which is common among athletes, do not take the maximum QT interval on the ECG and divide it by the square root of the shortest RR interval,27 which will grossly overestimate the QTc. Instead, it is more accurate to derive an average QT interval and average RR interval. Fourth, to perform a manual confirmation, the critical issue is identifying the end of the Twave since the onset of the QRS is seen easily. The rhythm strip at the bottom of the ECG generally includes leads II, V1 and/or V5, and lead II and V5 usually provide the best delineation of the Twave. Fifth, it is incorrect to include the commonly seen lowamFigure 2: ECG from a 27-year-old man with long QT syndrome (QTc=520 ms). Heart rate is 52 bpm. Bazett’s formula: QTc=QT/√RR. Note the RR interval is measured in seconds. This figure is only reproduced in colour in the online version. Figure 3: ECG from a patient with short QT syndrome. The QT interval is 250 ms and the QTc 270 ms. Note the tall, peaked T waves across the precordial leads characteristic of short QT syndrome. (ECG courtesy of Professor P Mabo). nummer 5 | november 2013 | Sport & Geneeskunde 23 Pagina 22
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