Cardiology
Corrected QT (QTc, Fridericia)
Cube-root heart-rate correction of QT.
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Background
The corrected QT interval (QTc) adjusts the measured QT interval for heart rate, so that the interval can be compared across different rates. The Fridericia correction divides the QT interval by the cube root of the RR interval (in seconds), in contrast to Bazett's formula which uses the square root. Both express the result in milliseconds. Fridericia's cube-root correction was proposed in 1920 and tends to over- and under-correct less than Bazett at the extremes of heart rate.
Interpreting the result
A QTc of 440 ms or below is generally regarded as normal, 441–470 ms as borderline, and above 470 ms as prolonged with an increased risk of ventricular arrhythmia such as torsades de pointes. Thresholds differ slightly by sex and source, with women tolerating marginally higher values, so local reference ranges apply. A markedly prolonged QTc, particularly when driven by drugs or electrolyte disturbance, warrants review of contributing factors. The correction method matters most when the heart rate is well outside the normal range.
Worked example
A patient with a heart rate of 100 bpm (RR interval 0.6 s) has a measured QT of 400 ms. The Fridericia QTc is 400 ÷ (0.6)^(1/3) ≈ 400 ÷ 0.843 ≈ 474 ms, which is in the prolonged range and should prompt review of QT-affecting drugs and electrolytes.
Pearls / pitfalls
- Fridericia is preferred over Bazett at fast or slow heart rates, where Bazett over- or under-corrects respectively.
- No correction is fully accurate during atrial fibrillation or marked rate variability; average several beats and interpret with caution.
- Always look for reversible contributors such as low potassium, magnesium or calcium and QT-prolonging drugs.
- Accurate QT measurement, including correct identification of the T-wave end, is essential, as errors carry straight through to the QTc.
Evidence & validation
Fridericia's correction was described in 1920 and is favoured in many pharmacological QT studies, including regulatory thoroughQT testing, because it correlates less with heart rate than Bazett's formula at high and low rates.
Frequently asked questions
When is the Fridericia correction better than Bazett?
Fridericia uses the cube root of the RR interval and over- or under-corrects less than Bazett at high and low heart rates. Bazett, which uses the square root, tends to overestimate QTc at fast rates and underestimate it at slow rates.
What QTc is considered prolonged?
A QTc above 470 ms is generally regarded as prolonged and associated with increased arrhythmia risk, with 441–470 ms borderline and 440 ms or below normal. Sex-specific and local thresholds vary slightly.
Why does a prolonged QTc matter?
A prolonged QTc increases the risk of the ventricular arrhythmia torsades de pointes, which can cause syncope, cardiac arrest or sudden death. Identifying it allows reversible causes such as drugs and electrolyte disturbances to be addressed.
Can I rely on QTc in atrial fibrillation?
Not reliably. The constantly varying RR interval means any single-beat correction is approximate, so several beats should be averaged and the value interpreted cautiously alongside the clinical context.
What commonly causes a prolonged QTc?
Common contributors include QT-prolonging medicines, low potassium, magnesium or calcium, bradycardia, and inherited long-QT syndromes. These should be sought and corrected whenever a prolonged QTc is found.
References
- Fridericia LS. Die Systolendauer im Elektrokardiogramm bei normalen Menschen und bei Herzkranken. Acta Med Scand. 1920;53:469–486.
- Postema PG, Wilde AAM. The measurement of the QT interval. Curr Cardiol Rev. 2014.
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