Once QTc is identified as prolonged, the next step in a workup is to look for acquired causes. Though Bazett formula seems to be relatively accurate in heart rates between 60 to 100 beats/min, it tends to overcorrect with higher heart rates and undercorrect in lower heart rates. While several equations exist to help correct for variation in heart rate, the most commonly used is Bazett formula ( QTC = QT / √ RR ). A QTc greater than 500 is associated with increased risk of torsade de pointes. This is known as QTc. QTc is prolonged if it is greater than 440 ms in men or greater than 460 ms in women. ĭue to the variation of QT interval with heart rate (higher heart rate has shorter QT interval, lower heart rate has longer QT interval), it is important to correct the QT interval for the heart rate. A helpful tip that helps identify prolonged QT interval on initial examination of the ECG is that a normal QT interval should be less than half the preceding RR interval. On the contrary, if the U wave is small or separate from the T-wave then it should be excluded. The maximum slope-intercept method is used to define the end of the T wave. If a U wave exists and is large (greater than 1 mm), and fused with T-wave, then this should be included in the QT measurement. This should be done on several successive beats, of which the longest interval is chosen. This is often done on lead II or V5-6, whichever is longer. Establishing the diagnosis of prolonged QT starts by measuring the QT interval on ECG.

Normal qt interval