Quick and Dirty QTc
October 1, 1998 2 minutes read
Brief Report
Quick and Dirty QTc
Source: Phoon CKL. Am J Cardiol 1998;82:400-402.
Electrocardiographic qt prolongation in relation to heart rate or QTc is a harbinger of torsade des pointes and can indicate drug toxicity, metabolic derangements, and other problems that need attention. Computer analyzed ECGs report the average QT of all 12 leads, and their heart rate correction techniques are imperfect, leading to frequent errors in QTc determination. Thus, hand measurements of a limb lead with a distinct T wave is the standard. Unfortunately, the application of heart rate correction is cumbersome and often requires a calculator capable of doing square roots. The most popular formula is Bazetts: QTc = QT/ÖRR interval. Phoon mathematically tested the clinical rule of thumb that the QTc is normal if the QT is less than half of the RR interval. He tested this assumption against the Bazett formula and two other correction formulae (Fridericia's cube root prediction and the Framingham Heart Study linear correction). By all three methods, the clinical rule of thumb was accurate at predicting normal QTc as long as the heart rate was more than 70 beats/min or longer. For this analysis, a normal QTc was defined as less than 0.46 seconds, ignoring the small differences between men and women. At slower heart rates, a QT greater than half of the RR interval will not necessarily predict a QTc longer than 0.46 seconds, and a hand calculation of QTc will be necessary. Use of the clinical rule in children younger than 3 years old is also problematic. Thus, this clinical rule of thumb is generally useful for confirming a prolonged QTc. -mhc
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