QT Interval & QTc: Normal Values, Correction & Prolonged QT

The QT interval measures how long the ventricles take to depolarise and recover. Corrected for heart rate, it becomes the QTc — and a long QTc is one of the ECG findings you cannot afford to miss.

What is the QT interval?

The QT interval is the time from the start of the QRS complex to the end of the T wave — the full span of ventricular depolarisation and repolarisation. Because it shortens as heart rate rises, it is almost always reported as the rate-corrected value, the QTc.

Normal QT and QTc values

QTc	Interpretation
< 440 ms (men) / < 460 ms (women)	Normal
440–460 ms (men) / 460–480 ms (women)	Borderline
> 480 ms	Prolonged
> 500 ms	Markedly prolonged — high torsades risk

A useful eyeball rule: the QT should be less than half the R-R interval at normal heart rates.

How the QTc is corrected

Several formulas correct the QT for heart rate, using the R-R interval in seconds:

• Bazett: QTc = QT ÷ √RR — the most common, but it over-corrects at fast rates and under-corrects at slow ones.
• Fridericia: QTc = QT ÷ RR^(1/3) — more accurate at heart-rate extremes.
• Framingham and Hodges — linear alternatives.

Get both Bazett and Fridericia instantly with our QTc calculator.

What prolongs the QT interval?

• Medications — many antiarrhythmics (sotalol, amiodarone), some antibiotics (macrolides, fluoroquinolones), antipsychotics, and antiemetics. The list is long; checking a QT-drug database is standard practice.
• Electrolyte disturbances — low potassium, magnesium, or calcium.
• Congenital long QT syndrome — inherited ion-channel disorders.
• Bradycardia, ischaemia, and hypothermia.

Why a long QT matters

A prolonged QTc reflects delayed repolarisation, which predisposes to torsades de pointes — a polymorphic ventricular tachycardia that can degenerate into ventricular fibrillation. A QTc over 500 ms, or an increase of more than 60 ms from baseline, is a recognised danger threshold. Management centres on stopping offending drugs, correcting electrolytes (magnesium first for torsades), and monitoring. Review the emergency rhythms in our ECG rhythm guide.

Key takeaways

• The QT interval spans ventricular depolarisation and repolarisation.
• Report the QTc; normal is under 440 ms (men) / 460 ms (women).
• Bazett is standard; Fridericia is better at heart-rate extremes.
• Drugs and low electrolytes are the common reversible causes.
• A QTc over 500 ms markedly raises torsades de pointes risk.

Frequently asked questions

What is a normal QT interval?

Reported as the rate-corrected QTc, normal is under 440 ms in men and under 460 ms in women; a QTc over 480 ms is prolonged.

How is the QT interval corrected for heart rate?

With formulas using the R-R interval — most commonly Bazett (QT ÷ √RR), and Fridericia (QT ÷ RR^⅓) which is more accurate at fast or slow rates.

What is a dangerous QTc?

A QTc over 500 ms, or an increase of more than 60 ms from baseline, markedly raises the risk of torsades de pointes.

What causes a prolonged QT interval?

QT-prolonging medications, low potassium, magnesium, or calcium, congenital long QT syndrome, bradycardia, ischaemia, and hypothermia.

What is the difference between QT and QTc?

QT is the measured interval; QTc is that value corrected for heart rate so it can be compared across different rates.

Why is a long QT dangerous?

It reflects delayed repolarisation that can trigger torsades de pointes, a polymorphic ventricular tachycardia that may cause sudden cardiac arrest.

How do you treat torsades de pointes?

Stop offending drugs, correct electrolytes (intravenous magnesium is first-line), and provide monitoring and support; unstable patients may need defibrillation.