Optimized when several simultaneous limb leads are recorded, with the QT interval usually measured in lead II if the T wave is of reasonable amplitude. A consensus does not exist regarding the optimal lead for QT measurement. Since the QT interval is influenced by autonomie tone and heart rate, measurement of the QT interval should be made during a resting state. The QT interval bears an inverse relationship with heart rate, and several formulas are available for correcting the QT interval for heart rate. The most wide¬ ly used algorithm is the Bazett equa¬ tion, in which the corrected QT interval is calculated by dividing the measured QT interval by the square root of the preceding R-R interval in seconds. The upper limit of normal for the corrected QT interval in patients with normal QRS durations is 0.44 s, and values longer than this are prolonged. The criteria for QT-interval prolongation in patients with conduction disturbances of the left or right bundle-branch block are not settled at this time. CLASSIFICATION OF QT PROLONGATION Prolongation of the QT interval may be categorized into a congenital type with or without heritable features and a diverse group of acquired types (Ta¬ ble). The congenital long QT syndrome (LQTS) is usually observed in other¬