Cardiology
Corrected QT (QTc, Bazett)
Heart-rate-corrected QT interval.
Education and reference only. Not a substitute for clinical judgement, local policy or product labelling. Always verify before clinical use. Values are calculated in your browser and never stored.
Background
The corrected QT interval (QTc) adjusts the measured QT interval on the ECG for heart rate, allowing comparison between recordings and assessment of arrhythmia risk. The QT interval shortens as heart rate rises and lengthens as it falls, so an uncorrected value is hard to interpret. Bazett's formula, the oldest and most widely used correction, divides the measured QT (in seconds) by the square root of the RR interval, with the result usually expressed in milliseconds. It dates from Bazett's 1920 analysis of electrocardiographic time relations.
Interpreting the result
A QTc up to about 440 ms is generally normal, 441–470 ms is borderline and above roughly 470 ms is prolonged and carries an increased risk of torsades de pointes. Sex-specific thresholds are often used — broadly above 450 ms in men and above 470 ms in women is considered prolonged. Values approaching or exceeding 500 ms substantially raise arrhythmia risk and usually prompt review of contributing drugs and electrolytes. A markedly short QTc can also be abnormal and warrants specialist assessment.
Worked example
A measured QT of 400 ms at a heart rate of 75/min (RR interval 0.8 s) gives a QTc of 0.40 ÷ √0.8 ≈ 0.447 s, or about 447 ms — a borderline value that should be interpreted with the patient's medications and electrolytes in mind.
Critical actions
Bazett over-corrects at high heart rates and under-corrects at low rates. Thresholds differ by sex (≈>450 ms men, >470 ms women). Interpret with the ECG and medications.
Pearls / pitfalls
- Bazett over-corrects at fast heart rates (overestimating QTc) and under-corrects at slow rates, so prefer Fridericia or another formula at heart-rate extremes.
- Measure the QT in the lead with the clearest T-wave end (often lead II or V5), avoiding U-waves, and use the longest reasonable interval.
- Correct hypokalaemia, hypomagnesaemia and hypocalcaemia, which prolong the QT, before attributing prolongation solely to drugs.
- In atrial fibrillation or marked rate variability, average several beats, as a single RR interval gives an unreliable QTc.
Evidence & validation
Bazett's correction (1920) remains the most commonly reported QTc and is embedded in most automated ECG software, though Fridericia, Framingham and Hodges corrections are preferred at extremes of heart rate; regulatory and cardiology guidance recognise QTc prolongation as a key safety signal.
Frequently asked questions
What QTc is considered prolonged?
Broadly, a QTc above about 450 ms in men and 470 ms in women is prolonged, with values above 500 ms carrying notably higher arrhythmia risk. Interpret thresholds alongside symptoms, drugs and electrolytes.
Why is Bazett's formula sometimes criticised?
It over-corrects at high heart rates and under-corrects at low rates, which can falsely raise or lower the QTc. At rate extremes, Fridericia or Framingham corrections are generally more accurate.
What can prolong the QT interval?
Many drugs, electrolyte disturbances (low potassium, magnesium or calcium), bradycardia and congenital long-QT syndromes can all prolong it. The risk is greatest when several factors combine.
Why correct the QT at all?
Because the raw QT interval changes with heart rate, an uncorrected value cannot be compared between recordings or against thresholds. Correction expresses the QT as it would be at a standard rate.
Does a normal QTc rule out arrhythmia risk?
No — a normal QTc reduces but does not eliminate risk, and dynamic changes or other ECG features may still matter. It should be interpreted with the full clinical and ECG picture.
References
- Bazett HC. An analysis of the time-relations of electrocardiograms. Heart. 1920;7:353–370.
- Drew BJ, Ackerman MJ, Funk M, et al. Prevention of torsade de pointes in hospital settings: AHA/ACCF scientific statement. Circulation. 2010;121(8):1047–1060.
Related calculators
Need a calculator we don't have — or a custom tool?
We build evidence-led clinical calculators, dashboards and reference tools for teams.