Medical AICorrected QT interval (QTc)
The QT interval shortens as heart rate rises, so it is corrected to a standard rate (QTc) before judging whether repolarisation is prolonged. This tool reports both the Bazett correction (the long-standing clinical default) and the Fridericia correction (more stable at extreme heart rates). Per the AHA/ACCF/HRS statement, a QTc at or above 450 ms in men or 460 ms in women is prolonged, and a QTc at or above 500 ms marks a substantially increased risk of torsades de pointes.
How to measure each input
- QT interval
- Measure from the onset of the QRS complex to the end of the T wave, ideally in lead II or V5 to V6, averaging over several beats. Exclude any U wave.
- Heart rate
- Use the ventricular rate. The tool converts it to the RR interval (RR in ms = 60000 / HR), which both corrections need.
- Which correction to read
- Bazett is the headline value because it is the long-standing clinical default, but it over-corrects at fast rates and under-corrects at slow rates. Fridericia, shown alongside, is more stable across heart rates.
Interpretation
| Band | Meaning |
|---|---|
| QTc < 450 ms, normal | Within normal limits for men. For women the upper limit is about 460 ms, so a QTc of 450 to 460 ms may still be normal in a female patient. No QT-specific concern on this value alone. |
| QTc 450 to 499 ms, prolonged | Prolonged QTc (>= 450 ms in men, >= 460 ms in women per AHA/ACCF/HRS). Review QT-prolonging drugs, correct hypokalaemia, hypomagnesaemia, and hypocalcaemia, and reassess. The risk of torsades rises but is uncommon below 500 ms. |
| QTc >= 500 ms, markedly prolonged | A QTc at or above 500 ms is associated with a substantially increased risk of torsades de pointes. Stop or avoid QT-prolonging agents, aggressively correct electrolytes, monitor on telemetry, and seek cardiology input. |
Pitfalls, exclusions and caveats
- Bazett over-corrects at fast heart rates (overestimating QTc) and under-corrects at slow rates (underestimating QTc); use Fridericia or another correction at the extremes of heart rate.
- QTc is unreliable in atrial fibrillation because the RR interval varies beat to beat; average over several cycles or use a method designed for irregular rhythms.
- Wide-QRS states (bundle branch block, ventricular pacing) inflate the QT because part of the interval reflects slowed depolarisation, not repolarisation; consider a QRS-adjusted or JT-based assessment.
- Manual identification of the end of the T wave is subjective, and a prominent U wave can be mistaken for the T wave, both of which change the result.
- Automated ECG QTc values can be inaccurate, especially with abnormal T-U morphology; confirm a borderline or prolonged machine reading by hand.
RR (sec) = 60 / heart rate (bpm). Bazett QTc = QT / sqrt(RR). Fridericia QTc = QT / RR^(1/3). QT and QTc in ms, RR in seconds.Frequently asked
What QTc is considered prolonged?
Per the AHA/ACCF/HRS statement, a QTc at or above 450 ms in men and at or above 460 ms in women is prolonged. A QTc at or above 500 ms (in either sex) marks a substantially increased risk of torsades de pointes and usually prompts stopping offending drugs, correcting electrolytes, and cardiac monitoring.
Should I use Bazett or Fridericia?
Bazett is the traditional clinical default and is reported as the headline value here, but it over-corrects at fast heart rates and under-corrects at slow rates. Fridericia (shown alongside) is more stable across heart rates and is often preferred when the heart rate is well outside 60 to 100 bpm.
Why is QTc unreliable in atrial fibrillation or bundle branch block?
In atrial fibrillation the RR interval changes beat to beat, so a single correction is unstable; average over several beats. In bundle branch block or ventricular pacing the QRS is wide, so part of the measured QT reflects slow depolarisation rather than repolarisation, which falsely lengthens the QTc.
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