QT Interval Measurement and Correction

Understanding QT Interval Measurement

The QT interval represents ventricular repolarization time and is measured from the beginning of the QRS complex to the end of the T wave on an ECG, according to the American College of Cardiology 1, 2
QT interval varies with heart rate and must be corrected (QTc) to provide a standardized value, as recommended by the American College of Cardiology 2
For accurate measurement, select a lead with T-wave amplitude of at least 2 mm and well-defined T-wave end, as suggested by the American College of Cardiology 2

Bazett's formula (QTc = QT/√RR) is most commonly used but has significant limitations, according to the American College of Cardiology 3, 2
Linear regression functions are recommended over Bazett's formula for QT-rate correction, as suggested by the American College of Cardiology and the American Heart Association 3, 4
Fridericia's formula (QTc = QT/∛RR) is more accurate, especially at higher heart rates, as recommended by the European Society of Cardiology 2
QT correction should not be attempted when RR interval variability is large (e.g., atrial fibrillation), as advised by the American Heart Association 4

Adult men: normal QTc < 450 ms; prolonged QTc ≥ 450 ms, according to the American College of Cardiology and the American Heart Association 1, 5, 3
Adult women: normal QTc < 460 ms; prolonged QTc ≥ 460 ms, as recommended by the American College of Cardiology and the American Heart Association 1, 5, 3
Short QT: ≤ 390 ms in both adult men and women, according to the American College of Cardiology, the American Heart Association, and the European Society of Cardiology 1, 5, 6

FDA guidelines recommend reporting three severity levels for QTc: > 350 ms, > 480 ms, and > 500 ms, as advised by the American College of Cardiology and the American Heart Association 1, 5
QTc > 500 ms is associated with increased risk of Torsades de Pointes, a potentially life-threatening ventricular arrhythmia, according to the American College of Cardiology 2
An increase in QTc > 60 ms from baseline is considered clinically significant, as suggested by the American College of Cardiology 2

QT interval is longer in young and middle-aged females than males, with differences of 12-15 ms in younger adults and 6-10 ms in older age groups, according to the American College of Cardiology and the American Heart Association 1, 5
Gender differences in QTc become small after age 40 and practically disappear in older adults, as recommended by the American College of Cardiology and the American Heart Association 1, 5
QT interval prolongs in ventricular conduction defects and requires adjustment for QRS duration, according to the American College of Cardiology 1, 7
Athletes may have longer QT intervals due to lower resting heart rates, but their QTc values should be within normal limits, as suggested by the European Society of Cardiology 6

The American Heart Association and American College of Cardiology Foundation define the normal corrected QT (QTc) interval as < 450 ms in adult men and < 460 ms in adult women; a QTc of ≤ 390 ms in either sex is considered a short QT. 8

In younger adults (< 40 years), women’s QTc intervals are 12–15 ms longer than men’s.
In middle‑aged adults (40–69 years), the gender gap narrows to 6–10 ms.
In older adults (> 70 years), the gender difference is essentially absent.
During adolescence, testosterone shortens QTc in males, while females experience little change, explaining the emergence of the gender gap at puberty. 8

Prolonged QTc: ≥ 450 ms in men, ≥ 460 ms in women.
Moderate prolongation (FDA Level 2): > 480 ms.
Severe prolongation (FDA Level 3, highest torsades de pointes risk): > 500 ms.
The FDA recommends that drug‑evaluation studies report all three severity levels when assessing QT‑prolonging effects. 8

Correction formula: Linear regression‑based methods or Fridericia’s formula are preferred; Bazett’s formula tends to over‑correct at heart rates > 80–90 bpm and may generate false‑positive prolongations. 8
Heart‑rate variability: QT correction should not be applied when RR‑interval variability is large (e.g., atrial fibrillation). 8
Ventricular conduction defects (bundle‑branch block): QT interval must be adjusted for QRS duration or the JT interval (QT − QRS) should be used. 8

For children younger than 12 years, the 98th‑percentile QTc limit is approximately 450 ms, with no gender differentiation.
A gender difference of ~8 ms appears in the 12–16 year age group. 8