Prolonged QT interval

The QT interval represents the time between the beginning of the Q wave until the end of the T wave. This interval is best measured in lead II and represents both the depolarization and repolarization phases of the ventricles. It is significantly influenced by many factors including heart rate, various medications (especially quinidine, procainaminde and disopyramide), hypokalemia, hypomagnesemia and athletic training. Therefore, tables or formulas are often needed to calculate the corrected QT interval (ATc) to determine if the QT interval on a particular electrocardiogram is appropriate for its demonstrated heart rate. One accepted calculation in determining this QTc is a modified version of Bazett's formula. This formula states that the QTc = QT + 1.75(ventricular rate - 60). Normal values for this corrected QT interval are found to approximate 0.41 seconds, although this value is slightly longer in females and in patients of increasing age. If this calculation is applied to the ECG demonstrated below, the QTc is measured as 0.52 seconds [QTc = 0.52 + 1.75 (60 -60)]