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Calculate the Corrected QT (QTc) interval from an ECG to identify patients at risk of dangerous arrhythmias. Compare Bazett, Fridericia, Framingham, and Hodges formulas instantly.
Last updated: March 3, 2026
If the patient's heart rate is greater than 100 bpm, remember that Bazett's formula tends to artificially over-correct the QT, creating false positives. In these cases, Fridericia or Framingham is preferred by many cardiologists.
QTc (corrected QT interval) adjusts the raw QT interval for heart rate so clinicians can compare repolarization risk on a consistent scale. Because QT shortens during tachycardia and lengthens during bradycardia, uncorrected QT can be misleading.
QTc matters in emergency medicine, cardiology, psychiatry, and pharmacy monitoring. It is commonly used before and after QT-prolonging medications, electrolyte correction, and risk triage in symptomatic patients.
A high QTc can indicate elevated risk of malignant arrhythmias, especially Torsades de Pointes. That is why accurate correction formula choice is clinically important rather than purely mathematical.
Bazett: QTc = QT / sqrt(RR)
Fridericia: QTc = QT / cbrt(RR)
Framingham: QTc = QT + 0.154 x (1 - RR)
Hodges: QTc = QT + 1.75 x (HR - 60)
In these formulas, QT is measured in milliseconds, RR is the R-R interval in seconds, and HR is heart rate in bpm. Different formulas may diverge when heart rate is far from 60 bpm.
QT: 360 ms, HR: 120 bpm
Bazett may overestimate prolongation at high HR.
Fridericia often provides a steadier estimate.
QT: 410 ms, HR: 72 bpm
Formulas usually remain close near normal HR.
Good context for routine medication checks.
QT: 480 ms, HR: 48 bpm
Bazett may under-correct when HR is low.
Cross-check with linear formulas and clinical context.
Compare practical formula behavior and common interpretation ranges used in clinical settings.
| Category | QTc Range (ms) | Typical Clinical Meaning | Suggested Action |
|---|---|---|---|
| Normal (male) | <= 450 | Generally acceptable repolarization time | Routine monitoring as indicated |
| Normal (female) | <= 470 | Typically within accepted range | Trend with serial ECGs if needed |
| Borderline prolonged | 451-500 (male), 471-500 (female) | Potential increased arrhythmic risk | Review meds, electrolytes, repeat ECG |
| High risk prolonged | > 500 | Higher Torsades de Pointes concern | Urgent clinical evaluation |
Because the raw QT interval changes with heart rate, mathematicians have developed several methods to "correct" it. Here is how they differ.
The oldest and most common formula baked into most ECG machines. It uses a square root transformation. However, it is flawed: it over-corrects at fast heart rates (making normal QT intervals look prolonged) and under-corrects at slow heart rates.
Uses a cube root instead of a square root. This formula provides a much more robust and accurate correction at temperature extremes, specifically at faster heart rates (> 100 bpm) and is strongly favored in many modern clinical trials.
Derived from the massive Framingham Heart Study, this is a linear regression formula (`QT + 154 * (1 - RR)`). It is highly accurate and provides a more consistent risk prediction across all heart rates than Bazett.
Another linear equation (`QT + 1.75 * (HR - 60)`). Like Framingham and Fridericia, it tends to outperform Bazett at non-standard heart rates, avoiding the artificial over-correction seen during tachycardia.
A fast, reliable tool for interpreting ECGs on the wards.