The QTc Interval Calculator allows clinicians to determine the corrected QT interval using three major mathematical models: Bazett, Fridericia, and Framingham.
The Importance of the QT Interval
On an electrocardiogram (ECG), the QT interval measures the total duration of ventricular depolarization (the QRS complex) and repolarization (the ST segment and T wave). A prolonged QT interval indicates a delay in ventricular repolarization, which leaves the heart vulnerable to early afterdepolarizations. This can trigger a dangerous, potentially lethal polymorphic ventricular tachycardia known as Torsades de Pointes.
Heart Rate Dependency
The most critical aspect of evaluating the QT interval is that it is inversely related to heart rate. At a heart rate of 100 bpm, a raw QT of 400 ms is actually dangerously prolonged, whereas at a heart rate of 50 bpm, the same raw QT is perfectly normal. Mathematical formulas correct the QT to what it would be if the heart rate were exactly 60 bpm.
Comparing the Formulas
- Bazett's Formula: The most deeply entrenched in clinical practice and the default on most ECG machines. However, its non-linear relationship causes significant mathematical artifact: it artificially prolongs the QTc during tachycardia and artificially shortens it during bradycardia.
- Fridericia's Formula: Uses a cube-root correction rather than a square root. Studies have shown this provides a much more stable and accurate correction across extreme heart rates.
- Framingham Formula: Derived from the massive Framingham Heart Study, this uses a linear regression model and is highly accurate.
Clinical Thresholds
Generally, a QTc > 450 ms in men and > 460 ms in women is considered prolonged. A QTc > 500 ms in anyone is an absolute warning sign indicating a substantially elevated risk of Torsades de Pointes.