A prolonged QT interval has a variety of causes and can be a very serious ECG finding, as it may lead to the potentially fatal arrhythmia polymorphic ventricular tachycardia (Torsades de Pointes).

Lower heart rates also result in a longer QT interval. A quick way to distinguish a prolonged QT interval is to examine if the T wave ends beyond the halfway point between the RR interval.

History of QT prolongation. Congenital long QT syndrome. Recent acute MI. Uncompensated heart failure. Concomitant drugs that cause QT prolongation, including dofetilide, sotalol, quinidine ...