Quick evaluation of the QT-interval:
T-waves should end before the midpoint of the R-R interval. Beware of a prolonged QT when the T-waves end after ½ the R-R wave distance.
Consider the following potential causes of long QT-intervals:
Electrolytes
Hypokalemia
Hypomagnesemia
Hypocalcemia
Hypothermia
Hypothyroidism
Cardiac Ischemia/ACS
Elevated intracranial pressures
Medications
Congenital
Most causes of QT-intervals prolongation are secondary to abnormal T-waves. If the QT interval is long specifically due to a prolonged ST-segments, consider the following 2 causes:
Hypocalcemia
Hypothermia