QT duration & corrected QT (QTc) duration: ECG features & implications

QT duration and corrected QT (QTc) duration

ECG interpretation always includes assessment of the QT (QTc) duration. The QT duration represents the total time for de- and repolarization. It is measured from the beginning of the QRS-complex to the end of the T-wave. Prolonged QT duration predisposes to life-threatening ventricular arrhythmias and therefore QT duration must always be assessed. Prolonged QT duration may either be congenital (genetic mutations, so called long QT syndrome) or acquired (medications, electrolyte disorders). QT duration is inversely related to heart rate; QT duration increases at low heart rate and vice versa. Therefore one must adjust the QT duration for the heart rate, which yield corrected QT duration (Qtc). Bazett’s formula has traditionally been used to calculate the corrected QT duration. The formula follows (all variables in seconds):

Bazett's formula for calculating corrected QT duration (QTc).
Bazett’s formula for calculating corrected QT duration (QTc).

Normal values for QTc interval

  • Men: <0,450 seconds
  • Women: <0,460 seconds

However, Bazett’s formula is several decades old and has been questioned because it performs poorly at very low and very high heart rates. Newer formulas (which are incorporated in modern ECG machines) are to be preferred over Bazett’s formula. QTc duration is calculated automatically in all modern ECG machines. The result is based on the lead with the longest QTc duration (typically leads V2–V3).

Causes of prolonged QTc duration

  • Antiarrhythmica (procainamide, disopyramide, amiodarone, sotalol)
  • Psychiatric medications (tricyclic antidepressants, SSRI, lithium etc)
  • Antibiotics (macrolides, kinolones, atovaquone, klorokine, amantadine, foscarnet, atazanavir)
  • Hypokalemia
  • Hypocalcemia
  • Hypomagnesemia
  • Cerebrovascular insult (bleeding)
  • Myocardial ischemia
  • Cardiomyopathy
  • Bradycardia
  • Hypothyroidism
  • Hypothermia

A separate chapter discusses Long QT syndrome (LQTS).

A complete list of drugs causing QT prolongation can be found here.

Short (QT) QTc syndrome

Short QT syndrome (QTc <0,390 seconds) is uncommon and can be seen in hypocalcemia and during digoxin treatment. It is very rare, but may cause malignant arrhythmias.

QT dispersion

The QT interval varies somewhat in the different leads. The difference between the shortest and the longest QT interval is the QT dispersion. Increased QT dispersion is associated with increased morbidity and mortality. This is presumably explained by higher incidence of malignant ventricular arrhythmias. It has been suggested that the high risk of ventricular arrhythmias is due to vulnerability caused by marked local differences in the repolarization.

This article is part of the comprehensive chapter: How to read and interpret the normal ECG

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