Cardiology
Corrected QT interval (QTc)
QTc adjusts the measured QT interval for heart rate so it can be compared against a threshold. A prolonged QTc raises the risk of torsades de pointes, which is why it is checked before and during QT-prolonging drugs.
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The prolongation threshold differs: 450 ms in men, 460 ms in women.
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This calculator is provided for reference and education. It does not replace clinical assessment, examination, current guidelines, local protocol, or professional judgment. Verify every result before acting on it. Nirogix accepts no liability for clinical decisions made using this tool.
The formula
- RR (seconds) = 60 ÷ heart rate
- Bazett: QTc = QT ÷ √RR
- Fridericia: QTc = QT ÷ RR^(1/3)
- Framingham: QTc = QT + 154 × (1 − RR)
Worked example
QT 400 ms at a rate of 60 (RR = 1.0 s), Bazett → 400 ÷ 1 = 400 ms
When to use it
- Before starting a QT-prolonging drug, and after a dose change
- Syncope, palpitations, or a family history of sudden death
And when not to
- Bazett over-corrects at fast rates and under-corrects at slow ones. At the extremes, Fridericia is the better choice.
- QTc is unreliable in bundle branch block, ventricular pacing and atrial fibrillation.
- Measure the QT in the lead where it is longest, and average several beats in atrial fibrillation.
References
- An analysis of the time-relations of electrocardiograms — Bazett HC, Heart, 1920
- Die Systolendauer im Elektrokardiogramm — Fridericia LS, Acta Medica Scandinavica, 1920
- An improved method for adjusting the QT interval for heart rate — Sagie A et al., American Journal of Cardiology (Framingham), 1992
Last reviewed 07/09/2026.
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