Up to 0.42 s (≤ 420 ms) is chosen as normal QTc of QT B and QT F in this diagram. Upper limit of normal QT interval, corrected for heart rate according to Bazett's formula, Fridericia's formula, and subtracting 0.02 s from QT for every 10 bpm increase in heart rate.

Q T c B = Q T R R 1 s Īgain, here QT and QTlc are in milliseconds and RR is measured in seconds.Ī recent retrospective study suggests that Fridericia's method and the Framingham method may produce results most useful for stratifying the 30-day and 1-year risks of mortality. The mathematically correct form of Bazett's formula is: The most commonly used QT correction formula is the Bazett's formula, named after physiologist Henry Cuthbert Bazett (1885–1950), calculating the heart rate-corrected QT interval (QTcB).īazett's formula is based on observations from a study in 1920.īazett's formula is often given in a form that returns QTc in dimensionally suspect units, square root of seconds. To account for this, and thereby improve the reliability of QT measurement, the QT interval can be corrected for heart rate (QTc) using a variety of mathematical formulae, a process often performed automatically by modern ECG recorders. These changes make it harder to compare QT intervals measured at different heart rates. The QT interval changes in response to the heart rate - as heart rate increase the QT interval shortens.

The 12 median beats are superimposed on each other and the QT interval is measured either from the earliest onset of the Q wave to the latest offset of the T wave or from the point of maximum convergence for the Q wave onset to the T wave offset. 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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