How is the QT interval measured in long QT syndrome (LQTS)?

Updated: Nov 29, 2017
  • Author: Ali A Sovari, MD, FACP, FACC; Chief Editor: Mikhael F El-Chami, MD  more...
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The QT interval on the ECG, measured from the beginning of the QRS complex to the end of the T wave, represents the duration of activation and recovery of the ventricular myocardium. A QT interval corrected for heart rate (QTc) that is longer than 0.44 seconds is generally considered to be abnormal, although a normal QTc can be more prolonged in females (up to 0.46sec). The Bazett formula is the formula most commonly used to calculate the QTc, as follows: QTc = QT/square root of the R-R interval (in seconds). (See Workup.)

To measure the QT interval accurately, the relationship of QT to the R-R interval should be reproducible. This issue is especially important when the heart rate is lower than 50 beats per minute (bpm) or over 120 bpm, and when athletes or children have marked beat-to-beat variability of the R-R interval. In such cases, long recordings and several measurements are required. The longest QT interval is usually observed in the right precordial leads. When marked variation is present in the R-R interval (atrial fibrillation, ectopy), correction of the QT interval is difficult to define precisely. (See Workup.)

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