QTc is the QT interval corrected for heart rate. Because the QT interval shortens as heart rate rises, it is corrected (QTc) so it can be compared against a baseline to judge whether it is genuinely prolonged.

What is QTc? Bazett, Fridericia & other formulas

Q: What is the difference between Bazett and Fridericia?

Bazett (QTc = QT / square root of RR) is the most traditional and widely used, but it overcorrects at high heart rates and can overestimate Long QT. Fridericia (QTc = QT / cube root of RR) is less sensitive to heart-rate extremes and is often preferred for better accuracy.

ECG+ · 3 min read

The QT interval has an inverse relationship with heart rate: it shortens as the heart speeds up and lengthens as it slows. To judge whether a QT is genuinely prolonged, it's first corrected for heart rate, giving the QTc. Several formulas exist for this correction, each with its own strengths and weaknesses. Here's how the main ones compare. For what a prolonged result means clinically, see Long QT on Apple Watch ECG.

Bazett

The Bazett formula (QTc = QT / √RR) is the most commonly used, thanks to its simplicity. However, a growing body of research questions its reliability: it significantly overestimates Long QT, especially at high heart rates.

Fridericia

The Fridericia formula (QTc = QT / ∛RR) is preferred for better accuracy across varying heart rates. It's less sensitive to heart-rate extremes and more reliable than Bazett at detecting Long QT.

Hodges

The Hodges formula (QTc = QT + 1.75 × (HR − 60)) is designed to be accurate across a broader range of heart rates, but it has limited clinical acceptance, and its correction factor (1.75) is an approximation that may not suit every case.

Framingham

The Framingham formula (QTc = QT + 0.154 × (1 − RR)) applies a linear correction considered more stable across different heart rates, though it can still be inaccurate at the extremes.

Rautaharju

The Rautaharju formula (QTc = QT × (120 + HR) / 180) is a newer method that minimizes the inaccurate corrections other formulas show at heart-rate extremes. It's less commonly used and may be less familiar to some practitioners.

Summary

Bazett and Fridericia are the most common, Bazett more traditional, Fridericia often preferred for accuracy across varying heart rates.
Framingham and Hodges use linear corrections considered more stable, but are less widely adopted.
Rautaharju is a newer alternative with specific use cases and advantages.

ECG+ reports QTc from your Apple Watch ECG and lets you see the measurement behind the headline, so you can have an informed conversation with your doctor about which correction applies to your reading.

Frequently asked questions

What is QTc?

QTc is the QT interval corrected for heart rate. Because the QT interval shortens as heart rate rises, it's corrected (QTc) so it can be compared against a baseline to judge whether it's genuinely prolonged.

What is the difference between Bazett and Fridericia?

Bazett (QTc = QT / √RR) is the most traditional and widely used, but it overcorrects at high heart rates and can overestimate Long QT. Fridericia (QTc = QT / ∛RR) is less sensitive to heart-rate extremes and is often preferred for better accuracy.