How it's calculated
QTc corrects the measured QT interval for heart rate, since QT naturally shortens as heart rate rises. Bazett's formula (QTc = QT / √RR) is the most widely used but over-corrects at high heart rates; Fridericia's formula (QTc = QT / ∛RR) is generally more accurate outside a normal heart-rate range.
General QTc thresholds
| QTc | Interpretation |
|---|---|
| ≤440 ms (men) / ≤460 ms (women) | Normal |
| 440–460 ms (men) / 460–480 ms (women) | Borderline |
| >460 ms (men) / >480 ms (women) | Prolonged |
| >500 ms | Markedly prolonged — high risk of torsades de pointes |
Clinical use
- QTc prolongation is a marker of increased risk for torsades de pointes, a potentially fatal polymorphic ventricular tachycardia — many drugs are avoided or dose-adjusted specifically because of this risk.
- Bazett's formula is by far the most commonly reported QTc in clinical practice and on ECG machines, but it systematically over-corrects at heart rates above about 100 bpm and under-corrects at very low heart rates.
- Fridericia's correction is often preferred in research and pharmaceutical QT studies for its more consistent accuracy across a wider heart-rate range.
- A single very prolonged QTc (>500 ms, or an increase of >60 ms from baseline) is a stronger and more specific warning sign than a borderline value using either formula.
Frequently asked questions
What is QTc and why does it matter?
QTc is the QT interval corrected for heart rate — it matters because a prolonged QTc increases the risk of torsades de pointes, a dangerous heart rhythm that can be fatal, which is why many medications are avoided or monitored closely in patients with a prolonged QTc.
Why do QT intervals need to be corrected for heart rate?
The QT interval naturally shortens as heart rate increases and lengthens as it slows, so comparing raw QT values across different heart rates would be misleading — the correction formulas adjust for this relationship.
Which formula should I use, Bazett or Fridericia?
Bazett is the most commonly used and reported in routine clinical practice, but it over-corrects at high heart rates; Fridericia tends to be more accurate outside a normal heart-rate range and is often preferred in research settings — checking both, as this calculator does, is a reasonable approach.
What QTc is considered dangerous?
A QTc above roughly 500 ms, or an increase of more than 60 ms from a patient's own baseline, is generally considered a high-risk finding for torsades de pointes — but always interpret against your ECG report's specific reference range and the full clinical picture.
References
- Bazett HC. An analysis of the time-relations of electrocardiograms. Heart. 1920;7:353-370.
- Fridericia LS. Die Systolendauer im Elektrokardiogramm bei normalen Menschen und bei Herzkranken. Acta Med Scand. 1920;53:469-486.
- Al-Khatib SM, et al. What clinicians should know about the QT interval. JAMA. 2003;289:2120-2127.