QTc Calculator (Bazett, Fridericia)

About the QTc Calculator (Bazett, Fridericia)

The QT interval on an ECG — from the start of the QRS complex to the end of the T wave — gets shorter as the heart beats faster, so it is corrected for heart rate before it is compared with a cut-off. The result is the QTc. This calculator takes the measured QT and either the heart rate or the RR interval, and shows four corrections side by side:

• Bazett (QT ÷ √RR), printed by many ECG machines;
• Fridericia (QT ÷ ∛RR), more accurate than Bazett when the heart rate is fast or slow;
• Framingham (QT + 154 × (1 − RR)), a linear correction from the Framingham Heart Study;
• Hodges (QT + 1.75 × (HR − 60)).

Each result is compared with the cut-offs of the AHA/ACCF/HRS ECG recommendations — prolonged from 450 ms in men and 460 ms in women — and a QTc over 500 ms is flagged as the level of particular concern in drug safety. Add an earlier QTc to see the change from baseline. Nothing you enter leaves your browser.

Without a pass, the free preview shows the QTc by the formula you choose, with its cut-off, every warning and the first rows of the formula table; a Pro pass unlocks the rest of the formula table, the note on why the formulas differ, the change from baseline and Copy result.

Common uses

• Checking the QTc an ECG machine printed against the other corrections, especially at a fast heart rate.
• Monitoring the QTc before and during treatment with a medicine that can prolong the QT interval.
• Converting between heart rate and RR interval while correcting the QT.
• Teaching how the four rate corrections behave at fast and slow heart rates.

The formulas

With QT in milliseconds, RR in seconds (RR = 60 ÷ heart rate) and HR in beats per minute:

• Bazett: QTc = QT ÷ √RR
• Fridericia: QTc = QT ÷ ∛RR
• Framingham: QTc = QT + 154 × (1 − RR)
• Hodges: QTc = QT + 1.75 × (HR − 60)

At 60 beats per minute the RR interval is 1 second and every formula gives the measured QT. The further the rate is from 60, the more they disagree.

Why Bazett over-corrects at fast heart rates

Bazett divides the QT by the square root of the RR interval. When the heart beats fast the RR interval is short, and dividing by its square root lengthens the QTc too much; below 60 beats per minute it does too little. The ICH E14 guideline on QT studies says exactly this — Bazett’s correction over-corrects at elevated heart rates and under-corrects below 60 bpm — and that Fridericia’s correction is more accurate in people with such heart rates. In a large hospital study comparing five formulas, Fridericia and Framingham gave the best rate correction and Bazett the worst.

So at a fast rate a QTc by Bazett can look prolonged when Fridericia or Framingham says it is not. Many ECG machines still print Bazett, which is why the calculator shows all four together.

The cut-offs

• Prolonged QTc: 450 ms or more in men and 460 ms or more in women, as recommended by the AHA/ACCF/HRS recommendations for the standardisation and interpretation of the ECG.
• Over 500 ms: in drug development, a QTc above 500 ms during treatment is a threshold of particular concern, and an increase of more than 60 ms over baseline is also commonly used (ICH E14). Both are flagged here.

A QTc on its own does not diagnose long QT syndrome or measure an individual’s risk: that needs the ECG itself, the history and often several recordings.

Measuring the QT

• Measure from the start of the QRS complex to the end of the T wave, in the lead where the end of the T wave is clearest; do not include a separate U wave.
• Use the heart rate or RR interval of the same beats.
• With an irregular rhythm such as atrial fibrillation the RR interval changes from beat to beat; an average over several beats is usually used.
• With a wide QRS (bundle branch block or ventricular pacing) the QT includes the longer depolarisation, so the QTc overstates repolarisation; specialist methods are used then.

Sources

• ICH E14: The clinical evaluation of QT/QTc interval prolongation and proarrhythmic potential for non-antiarrhythmic drugs — sections 2.1.2, 3.1 and 3.2
• Rautaharju PM 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
• Vandenberk B et al. Which QT correction formulae to use for QT monitoring? — J Am Heart Assoc
• Sagie A et al. An improved method for adjusting the QT interval for heart rate (the Framingham Heart Study)90562-D) — Am J Cardiol
• Bazett HC. An analysis of the time-relations of electrocardiograms — Heart; Fridericia LS. Die Systolendauer im Elektrokardiogramm bei normalen Menschen und bei Herzkranken — Acta Medica Scandinavica

Limitations

• For adults. It corrects a QT you have measured; it does not read an ECG.
• QT 200–800 ms and heart rate 20–300 beats per minute (RR 200–3,000 ms). At very fast or very slow rates the formulas disagree most.
• A wide QRS, an irregular rhythm or a difficult T-wave end make every QTc less certain.
• Not a diagnosis of long QT syndrome or of drug-induced QT prolongation: decisions belong to the clinician who reads the ECG.

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