QTc (Bazett / Fridericia)
Heart-rate-corrected QT interval with Bazett or Fridericia
- ESC 2022 ventricular arrhythmia guideline (LQTS threshold)
Sources last checked: 2026-10-13
QTc (Bazett / Fridericia)
Heart-rate-corrected QT interval with Bazett or Fridericia
Bands are not sex-adjusted automatically: normal limits are < 450 ms in men and < 460 ms in women. Bazett over-corrects at high heart rates and under-corrects at low rates; Fridericia is preferred above about 90/min and is the standard in drug-safety monitoring. QT should be measured manually in the lead with the longest QT, excluding U waves.
Interpretation
| < 450 ms — normal | Below the upper limit of normal for both sexes (AHA/ACCF/HRS 2009: prolonged ≥ 450 ms in men, ≥ 460 ms in women). |
|---|---|
| 450–459 ms | Prolonged in men; still within normal limits in women. |
| 460–479 ms — prolonged | Prolonged in both sexes. Common causes include QT-prolonging drugs, hypokalaemia, hypomagnesaemia, hypocalcaemia and structural heart disease. |
| 480–500 ms | Reaches the ESC 2022 threshold for diagnosing long-QT syndrome (QTc ≥ 480 ms on repeated 12-lead ECGs, after excluding secondary causes). |
| > 500 ms — high risk | Markedly prolonged with a substantially higher risk of torsade de pointes. ESC 2022 describes stopping QT-prolonging drugs at this level (drug table). A rise ≥ 60 ms from baseline on a QT-prolonging drug is also a warning sign. |
What is the QTc (Bazett / Fridericia)?
The QT interval shortens as heart rate rises, so it is corrected to a reference heart rate of 60/min (RR = 1 s) before it is judged. Bazett (1920) divided QT by the square root of RR, Fridericia (1920) by the cube root. A prolonged QTc marks delayed ventricular repolarisation and an increased risk of torsade de pointes, whether congenital (long-QT syndrome) or acquired (drugs, electrolytes).
When to use it
- Before and after starting QT-prolonging drugs (e.g. some antiarrhythmics, antipsychotics, macrolides, methadone, ondansetron).
- Evaluating syncope, palpitations or a family history of sudden cardiac death.
- Monitoring patients with hypokalaemia, hypomagnesaemia or hypocalcaemia.
- Screening for long-QT syndrome against the ESC 2022 threshold of 480 ms.
When not to use it
- Wide QRS (bundle-branch block, ventricular pacing) — the QT is lengthened by depolarisation; use a JT-based or QRS-adjusted approach.
- Highly irregular rhythms without averaging — a single RR interval gives an unreliable correction.
- As a substitute for cardiology review when a congenital long-QT syndrome is suspected.
How it is calculated
RR (s) = 60 / heart rate Bazett: QTc = QT / √RR Fridericia: QTc = QT / ∛RR
Clinical pearls and limitations
- Bands are not sex-adjusted automatically: normal limits are < 450 ms in men and < 460 ms in women. Bazett over-corrects at high heart rates and under-corrects at low rates; Fridericia is preferred above about 90/min and is the standard in drug-safety monitoring. QT should be measured manually in the lead with the longest QT, excluding U waves.
- Bazett exaggerates QTc at fast heart rates — a QT of 400 ms at 100/min gives 516 ms with Bazett but 474 ms with Fridericia.
- Measure from QRS onset to T-wave end using the tangent method, usually in lead II or V5, excluding U waves.
- Sex matters: women have longer normal QTc (upper limit ≈ 460 ms vs 450 ms in men).
- QTc > 500 ms or a rise ≥ 60 ms from baseline are the usual alarm thresholds for drug-induced torsade risk.
Frequently asked questions
Should I use Bazett or Fridericia?
Fridericia is more accurate at heart rates outside 60–90/min and is used in drug-safety monitoring. Bazett remains common in automated ECG reports, so know which one the machine uses.
What is a normal QTc?
Under about 450 ms in men and 460 ms in women. Values ≥ 480 ms on repeated ECGs meet the ESC 2022 threshold for long-QT syndrome once secondary causes are excluded.
At what QTc should a QT-prolonging drug be stopped?
QTc above 500 ms, or an increase of at least 60 ms from baseline, are widely used thresholds for stopping or switching the drug and correcting electrolytes.
How is RR calculated from heart rate?
RR in seconds = 60 divided by the heart rate per minute; at 75/min RR is 0.8 s.
Evidence and guidelines
- Primary source: Fridericia LS. Die Systolendauer im Elektrokardiogramm bei normalen Menschen und bei Herzkranken. Acta Med Scand 1920;53:469-486; Bazett HC. An analysis of the time-relations of electrocardiograms. Heart 1920;7:353-370 (reprinted Ann Noninvasive Electrocardiol 1997;2:177-194).
- Zeppenfeld K et al. 2022 ESC Guidelines for the management of patients with ventricular arrhythmias and the prevention of sudden cardiac death. Eur Heart J 2022;43:3997-4126.
- 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 2009;119:e241-e250.
- Drew BJ et al. Prevention of torsade de pointes in hospital settings: AHA/ACCF scientific statement. Circulation 2010;121:1047-1060.
- Bazett HC. An analysis of the time-relations of electrocardiograms (reprint). Ann Noninvasive Electrocardiol 1997;2:177-194.
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