![]() ![]() In the superimposed median beat method, a median ECG complex is constructed for each of the 12 leads. With the increased availability of digital ECGs with simultaneous 12-channel recording, QT measurement may also be done by the 'superimposed median beat' method. QT interval in an ECG complex can be measured manually by different methods, such as the threshold method, in which the end of the T wave is determined by the point at which the component of the T wave merges with the isoelectric baseline, or the tangent method, in which the end of the T wave is determined by the intersection of a tangent line extrapolated from the T wave at the point of maximum downslope to the isoelectric baseline. The accurate measurement of the QT interval is subjective because the end of the T wave is not always clearly defined and usually merges gradually with the baseline. Leads III, aVL and V1 are generally avoided for measurement of QT interval. The QT interval is most commonly measured in lead II for evaluation of serial ECGs, with leads I and V5 being comparable alternatives to lead II. ![]() Illustrations of the tangent and threshold methods of measuring the QT interval ![]()
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