The QT interval is the period from the beginning of the QRS complex through the end of the T wave. The interval is rate-dependent. The lower the heart rate, the longer the QT interval and vice-versa. 

The QTc reports a time-corrected QT interval and is measured by modern ECG machines. It evaluates the QT interval as if the patient’s heart rate is a standard 60 bpm. It can also be calculated manually using the Bazett formula:

• QT = QT interval in seconds
• f = heart rate
• R–R = R–R interval in seconds

The time-corrected QT interval is a more accurate measurement of this phase. If the QT interval is slightly shortened or prolonged, it may still represent a normal ECG finding. 

A shortened QT interval is a rare occurrence that can be present in patients with hypercalcemia. A QTc exceeding 0.46 seconds is considered abnormal.

Causes of a Pathologically Long QT Interval

• Anti-arrhythmic drugs (Class Ia or III) 
• Ischemia
• Romano-Ward syndrome 
• Jervell and Lange-Nielsen syndrome 
• Torsades de pointes: a pronounced prolonged QT or QTU with a polymorphic ventricular tachycardia
• Hypocalcemia  
• Patients with hypokalemia may present with TU fusion that may progress into torsades de pointes