The QT interval on the ECG reflects the total time of excitation and recovery (repolarization) of the ventricles. Because it depends on heart rate, doctors use the corrected value QTc.
Prolongation of the QTc increases the risk of a dangerous arrhythmia — torsades de pointes ventricular tachycardia.
Reference ranges
The QTc is calculated using Bazett's formula (QT divided by the square root of the RR interval). Reference points:
| Normal, men | < 440 ms |
| Normal, women | < 460 ms |
| Prolonged QTc | > 450 (men) / > 470 (women) |
| High arrhythmia risk | > 500 ms |
What an elevated level means
A prolonged QTc can be congenital (long QT syndrome) or acquired: due to drugs (antiarrhythmics, some antibiotics, antipsychotics, antiemetics), or low potassium, magnesium or calcium. The longer the QTc, the higher the risk of torsades de pointes and sudden cardiac arrest.
What a low level means
Values within the normal range reflect proper repolarization. Too short a QTc occurs in short QT syndrome, hypercalcemia, and with digoxin use. Isolated borderline prolongation without symptoms is often safe but requires monitoring.
How the test is done
The QT is measured from the start of the QRS to the end of the T wave and corrected for heart rate (Bazett's formula is accurate at a pulse of 60–100). Before prescribing drugs that prolong the QT, an ECG is recorded at baseline and over time.
What affects the value
- Tachycardia and bradycardia distort the Bazett QTc calculation.
- The level of potassium, magnesium and calcium in the blood.
- Many drugs prolong the interval — tell your doctor what you are taking.
Calculate using this marker
Calculators where QT interval is used directly:
Frequently asked questions
What QTc is considered dangerous?
A QTc above 500 ms substantially increases the risk of torsades de pointes ventricular tachycardia. Values of 450–500 ms require finding the cause and monitoring.
Why use QTc rather than plain QT?
The QT itself shortens with a fast pulse and lengthens with a slow one. Correcting for rate (QTc) allows values to be compared between people and over time.