QT interval is the length between the beginning of the QRS complex (ventricular depolarization) and end of the T wave (ventricular re-polarization).
QTc is prolonged if > 440ms in men or > 460ms in women
QTc >500ms is associated with increased risk of torsades de pointes
QTc is abnormally short if < 350ms
A useful rule of thumb is that a normal QT is less than half the preceding RR interval
QTc prolongation can be caused by the following:
Goldstein, J. N., Dudzinski, D. M., Erickson, T. B., & Linder, G. (2018). Case 12-2018: A 30-Year-Old Woman with Cardiac Arrest. New England Journal of Medicine,378(16), 1538-1549. doi:10.1056/nejmcpc1800322
QT Interval. Edward Burns, Last updated November 21, 2017.https://lifeinthefastlane.com/ecg-library/basics/qt_interval/