Long QT syndrome and ECG I:
In situations in which the end of the T wave may be difficult to determine (e.g. biphasic or notched T waves, T waves with superimposed U waves), the end of the T wave can be determined by drawing the tangent from the peak of the T wave following the steepest T wave downslope. The intersection of this line with
the isoelectric baseline is considered the end of the T wave.
The QT interval is usually corrected for heart rate because the QT interval shortens at fast heart rates and prolongs at slow heart rates. The Bazett formula (QTc = QT/√RR, with all intervals in seconds) remains the standard for clinical use, despite some limitations at particularly fast or slow heart rates.
Normal QTc values are <440 for age 1–15 years, <430 for adult males, and <450 ms for adult females.