Long QT syndrome (LQTS) is a congenital disorder characterized by a prolongation of the QT interval on ECG and a propensity to ventricular tachyarrhythmias, which may lead to syncope, cardiac arrest, or sudden death.
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. QT intervals corrected for heart rate (QTc) longer than 0.44 seconds are generally considered abnormal, though a normal QTc can be more prolonged in females (up to 0.46 sec). 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).
To measure 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 < 50 bpm or >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.
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. QT intervals corrected for heart rate (QTc) longer than 0.44 seconds are generally considered abnormal, though a normal QTc can be more prolonged in females (up to 0.46 sec). 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).
To measure 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 < 50 bpm or >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.
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