How is prolongation of the QTc interval defined in the workup of long QT syndrome (LQTS)?

Updated: Nov 29, 2017
  • Author: Ali A Sovari, MD, FACP, FACC; Chief Editor: Mikhael F El-Chami, MD  more...
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Prolongation of the QTc interval is defined on the basis of age- and sex-specific criteria (see Table 3, below). QTc is calculated by dividing the measured QT by the square root of the R-R interval, both of which are measured in seconds. QTc prolongation longer than 0.46 seconds indicates an increased likelihood of LQTS. (See the image below.)

Marked prolongation of QT interval in a 15-year-ol Marked prolongation of QT interval in a 15-year-old male adolescent with long QT syndrome (LQTS) (R-R = 1.00 s, QT interval = 0.56 s, QT interval corrected for heart rate [QTc] = 0.56 s). Abnormal morphology of repolarization can be observed in almost every lead (ie, peaked T waves, bowing ST segment). Bradycardia is a common feature in patients with LQTS.

However, approximately 10-15% of gene-positive patients with LQTS present with a QTc duration in the reference range. (See the image below.)

Genetically confirmed long QT syndrome (LQTS) with Genetically confirmed long QT syndrome (LQTS) with borderline values of QT corrected for heart rate (QTc) duration (R-R = 0.68 s, QT interval = 0.36 s, QT interval corrected for heart rate [QTc] = 0.44 s) in a 12-year-old girl. Note the abnormal morphology of the T wave (notches) in leads V2-V4.

Table 3. Definition of QTc Based on Age- and Sex-Specific Criteria (Open Table in a new window)



QTc, sec


QTc, sec

Reference Range, sec 

Children and adolescents (<15 years)












In patients with suspected LQTS with borderline QTc values (or even values in the reference range) on standard ECGs or in patients with a score of 2-3 based on the 1993 Schwartz et al diagnostic criteria, an analysis of the dynamic behavior of QTc duration during exercise ECG or long-term Holter monitoring may reveal maladaptation of the QT interval to a changing heart rate. QTc prolongation may be evident at a fast heart rate. Ventricular arrhythmias are rarely observed during exercise testing or Holter recording in patients with LQTS.

No evidence indicates that invasive electrophysiology with attempts to induce ventricular tachycardia facilitates diagnosis.

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