QTc Interval Calculator (Bazett’s, Fridericia’s, Hodge’s and Framingham Formulas)

QTc Calculator

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.

The QT interval represents the time between the onset of ventricular depolarization and the end of ventricular repolarization. It reflects the total time the ventricles take to de- and repolarize. Abnormalities in the QT interval can cause serious arrhythmias and sudden cardiac death. QT prolongation can precipitate torsades de pointes, a specific form of polymorphic ventricular tachycardia characterized by irregular (polymorphic) QRS complexes that appear to twist around the isoelectric baseline. The risk of ventricular fibrillation is very high during torsade de pointes.

ECG showing an episode of sinus rhythm (with multiple ventricular premature beats) spontaneously converting to Torsade de Pointes ventricular tachycardia. Notice how arterial blood pressure (ABP) drops at the onset of TdP. ECG by Nakstad et al (Scand J Trauma Resusc Emerg Med. 2010; 18: 7)
ECG showing an episode of sinus rhythm (with multiple ventricular premature beats) spontaneously converting to Torsade de Pointes ventricular tachycardia. Notice how arterial blood pressure (ABP) drops at the onset of TdP. ECG by Nakstad et al (Scand J Trauma Resusc Emerg Med. 2010; 18:7).

Short QT interval

A shortened QT interval is also associated with an increased risk of ventricular arrhythmias.

Corrected QT Interval (QTc interval)

The QT interval varies inversely with heart rate; it shortens at higher heart rates and lengthens at lower heart rates. To account for this variability, the QT interval is often corrected for heart rate, resulting in the corrected QT (QTc) interval. Several formulas exist to calculate QTc, each with its own characteristics.

Bazett’s formula

One of the earliest and most widely used correction methods, Bazett’s formula calculates QTc by dividing the QT interval by the square root of the RR interval (the time between two successive R-wave peaks). However, it tends to overcorrect at high heart rates and undercorrect at low heart rates, potentially leading to inaccuracies.

Bazett’s Formula: QTc = QT interval / √(RR interval)

Fridericia’s formula

This formula corrects the QT interval by dividing it by the cube root of the RR interval. Studies suggest that Fridericia’s formula may provide more accurate corrections across a wider range of heart rates compared to Bazett’s formula.

Fridericia’s Formula: QTc = QT interval / (RR interval)1/3

Framingham (Sagie’s) Formula

Derived from the Framingham Heart Study, this linear correction formula adjusts the QT interval based on heart rate without involving square or cube roots. It has been shown to offer better correction over a broad spectrum of heart rates.

Framingham Formula: QTc = QT interval + 154 x (1 – RR interval)

Hodges’ formula

This method corrects the QT interval by adding a factor based on the difference between the heart rate and 60 beats per minute. It is another approach used to account for heart rate variability.

Hodges Formula: QTc = QT interval + 1.75 x [(60 / RR interval) − 60]

For all formulas: RR interval = 60 / HR

Validity of QT correction formulas

The accuracy of QT correction formulas can vary depending on the heart rate and individual patient characteristics.

  • Bazett’s formula has a tendency to overcorrect or undercorrect at extreme heart rates. Thus, it should not be used during bradycardia and tachycardia.
  • Fridericia’s and Framingham formulas are often preferred in scenarios where a broader range of heart rates is encountered, as they may provide more reliable corrections.
  • Hodges’ formula offers an alternative linear correction method.

https://ecgwaves.com/topic/longt-qt-syndrome-interval-lqts-torsades-de-pointes/Related article.

References

Luo, S., Michler, K., Johnston, P., & Macfarlane, P. W. (2004). A comparison of commonly used QT correction formulae: the effect of heart rate on the QTc of normal ECGs. Journal of Electrocardiology, 37, 81-90.

Vandenberk, B., Vandael, E., Robyns, T., Vandenberghe, J., Garweg, C., Foulon, V., Ector, J., & Willems, R. (2016). Which QT correction formulae to use for QT monitoring? Journal of the American Heart Association, 5(6), e003264.

Vandenberk, B., Vandael, E., Robyns, T., Vandenberghe, J., Garweg, C., Foulon, V., Ector, J., & Willems, R. (2016). Which QT Correction Formulae to Use for QT Monitoring? Journal of the American Heart Association, 5(6), e003264.

Vandenberk, B., Vandael, E., Robyns, T., Vandenberghe, J., Garweg, C., Foulon, V., Ector, J., & Willems, R. (2016). Which QT Correction Formulae to Use for QT Monitoring? Journal of the American Heart Association, 5(6), e003264.

Updated on 2025-01-18