QTc calculator (corrected QT interval)
Correct a measured QT interval for heart rate with the Bazett, Fridericia, Framingham and Hodges formulas at once, and see whether the QTc is prolonged by the AHA/ACCF/HRS cut-offs for men and women.
How this QTc calculator works
This QTc calculator adjusts a measured QT interval to the value expected at a heart rate of 60 beats per minute, an RR interval of 1 second, so QT intervals recorded at different heart rates can be compared. It runs four published correction formulas on the same QT and heart rate and shows all four at once; you choose which one is shown large and graded. RR in seconds is 60 divided by the heart rate, and an RR interval entered in ms is divided by 1000.
Bazett: QTc = QT / √RR
Fridericia: QTc = QT / ∛RR
Framingham: QTc = QT + 154 × (1 − RR)
Hodges: QTc = QT + 1.75 × (HR − 60)
QT and QTc in ms. RR = RR interval in seconds. HR = heart rate in beats per minute.
Bazett and Fridericia both published their formulas in 1920. Bazett's square-root formula was derived from only 39 young subjects. The Framingham formula is a linear regression from the baseline ECGs of 5,018 Framingham Heart Study participants (2,239 men and 2,779 women, ages 28 to 62, without coronary heart disease), and its authors found it applies to men and women alike. The Hodges formula is linear in heart rate rather than RR interval. At exactly 60 bpm all four formulas return the measured QT.
QTc categories (AHA/ACCF/HRS)
The QTc calculator grades the result with the cut-offs in the 2009 AHA/ACCF/HRS recommendations for the standardization and interpretation of the ECG: a QTc of 450 ms or more in men, or 460 ms or more in women, is prolonged. A QTc over 500 ms is shown as its own category because the 2010 AHA/ACCF scientific statement on preventing torsade de pointes in hospital settings calls it highly abnormal in both sexes, and notes that in congenital long QT syndrome studies it is linked to a 2- to 3-fold higher risk of torsade de pointes.
| Category | Men (QTc, ms) | Women (QTc, ms) |
|---|---|---|
| Normal | Under 450 | Under 460 |
| Prolonged | 450–500 | 460–500 |
| Markedly prolonged | Over 500 | Over 500 |
The 2010 statement also gives approximate 99th percentile upper limits of 470 ms for males and 480 ms for females in otherwise healthy people after puberty. These cut-offs are adult values and are applied the same way to every formula here.
Choosing a formula, and when QTc is less accurate
The QTc calculator shows four formulas because they disagree most at fast and slow heart rates. Bazett overcorrects the QT at fast heart rates and undercorrects it at slow ones, and the 2010 AHA/ACCF statement notes that it gives overlong QTc values particularly above 85 bpm. In 6,609 adults in sinus rhythm at University Hospitals Leuven, Fridericia and Framingham gave the best rate correction and predicted 30-day and 1-year mortality better than Bazett, which performed worst. In 10,303 normal ECGs, Luo and colleagues found the Hodges QTc the least dependent on heart rate.
Any QTc is only as good as the QT and RR it is built from. Take both from the same ECG and, when the rhythm is irregular, as in atrial fibrillation, keep in mind that the RR interval changes from beat to beat. A wide QRS, from bundle branch block or ventricular pacing, lengthens the QT without a matching change in repolarization, so the QTc reads longer. The formulas are least reliable at very fast or very slow heart rates, where the four results spread furthest apart.
Questions
What is a normal QTc?
By the 2009 AHA/ACCF/HRS recommendations, a QTc under 450 ms in men and under 460 ms in women is not prolonged. A QTc over 500 ms is markedly prolonged in both sexes.
Which QTc formula is the most accurate?
No single formula is best at every heart rate. In large comparisons, Fridericia and Framingham corrected for heart rate better than Bazett, and Hodges was the least dependent on heart rate in normal ECGs. Bazett is still the most widely used, and a QTc reported without a formula name usually means Bazett, so the calculator shows all four side by side.
Why is the Bazett QTc higher at fast heart rates?
Dividing by the square root of RR overcorrects when RR is short, so at fast rates Bazett gives a longer QTc than the other formulas, and at slow rates a shorter one. At 60 bpm all four formulas give the measured QT.
Can I enter the RR interval instead of heart rate?
Yes. Pick RR interval (ms) under the rate box. The calculator converts the value you already typed and works out heart rate as 60,000 divided by RR in ms.
Is my data sent anywhere?
No. The calculation runs in your browser, and the values you type are not sent or stored.
Sources
- Bazett HC. An analysis of the time-relations of electrocardiograms. Heart. 1920;7:353–370. Reprinted in Ann Noninvasive Electrocardiol. 1997;2(2):177–194. doi:10.1111/j.1542-474X.1997.tb00325.x
- Fridericia LS. Die Systolendauer im Elektrokardiogramm bei normalen Menschen und bei Herzkranken. Acta Med Scand. 1920;53:469–486. doi:10.1111/j.0954-6820.1920.tb18266.x
- Sagie A, Larson MG, Goldberg RJ, Bengtson JR, Levy D. An improved method for adjusting the QT interval for heart rate (the Framingham Heart Study). Am J Cardiol. 1992;70(7):797–801. doi:10.1016/0002-9149(92)90562-D
- Hodges M, Salerno D, Erlien D. Bazett's QT correction reviewed: evidence that a linear QT correction for heart rate is better. J Am Coll Cardiol. 1983;1:694 (abstract).
- Luo S, Michler K, Johnston P, Macfarlane PW. A comparison of commonly used QT correction formulae: the effect of heart rate on the QTc of normal ECGs. J Electrocardiol. 2004;37 Suppl:81–90. doi:10.1016/j.jelectrocard.2004.08.030
- Rautaharju PM, Surawicz B, Gettes LS, et al. AHA/ACCF/HRS recommendations for the standardization and interpretation of the electrocardiogram: part IV: the ST segment, T and U waves, and the QT interval. Circulation. 2009;119(10):e241–e250. doi:10.1161/CIRCULATIONAHA.108.191096
- Drew BJ, Ackerman MJ, Funk M, et al. Prevention of torsade de pointes in hospital settings: a scientific statement from the American Heart Association and the American College of Cardiology Foundation. Circulation. 2010;121(8):1047–1060. doi:10.1161/CIRCULATIONAHA.109.192704
- Vandenberk B, Vandael E, Robyns T, et al. Which QT correction formulae to use for QT monitoring? J Am Heart Assoc. 2016;5(6):e003264. doi:10.1161/JAHA.116.003264