Clinical Manuals

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

QT duration and corrected QT (QTc) duration

ECG interpretation always includes an assessment of the QT interval (QT duration). The QT duration represents the total time for depolarization 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. QT prolongation may be congenital (due to genetic mutations) or acquired (secondary to medications, electrolyte disorders). QT duration is inversely related to heart rate; QT duration increases at slow heart rates and vice versa. Hence, the QT duration must be adjusted for heart rate, which yields the corrected QT duration (QTc).

There are several equations for calculating the QTc duration. Bazett’s formula is used widely, despite several shortcomings. The formula is as follows:

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

QT correction according to Bazzet, Fridericia, Framingham and Hodges follow in the calculator below.

QTc interval
QTc interval according to Bazzetms
QTc = QT interval / √ (RR interval)
QTc interval according to Fridericiams
QTc = QT interval / (RR interval)1/3
QTc interval according to Framinghamms
QTc = QT interval + 154 x (1 – RR interval)
QTc interval according to Hodgesms
QTc = QT interval + 1.75 x [(60 / RR interval) − 60]

Normal values for QTc interval

1 – 15 years, male and femaleAdult, maleAdult, female
Normal<440 ms<430 ms<450 ms
Upper limit440–460 ms430–450 ms450–470 ms
Prolonged>460 ms>450 ms>470 ms
Table 1. Reference intervals for QTc duration according to Bazett’s formula. Note that a QTc interval >480 is always considered pathological. For patients with bundle branch blocks, the corresponding figure is >500 ms.

Bazett’s formula is several decades old and has been questioned because it performs poorly at low (<50) and high (>100) heart rates. Newer formulas (which are incorporated in modern ECG machines) are to be preferred. QTc duration is calculated automatically in all modern ECG machines. Most machines present the longest QTc interval, which typically occurs in leads V2–V3.

How to measure the QT interval

Measurement of the QT interval may be difficult due to T wave changes.

Causes of prolonged QTc duration

  • Antiarrhythmics (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.

Corrected QT intervals (QTc) can be calculated below.

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 a higher incidence of malignant ventricular arrhythmias. It has been suggested that the high risk of ventricular arrhythmias is due to the vulnerability caused by marked local differences in repolarization.

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

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