Frequently Asked Questions

What is the QT interval?

The QT interval represents the total time of ventricular depolarization and repolarization on the electrocardiogram. It is measured from the beginning of the QRS complex to the end of the T wave. Its prolongation is associated with risk of severe ventricular arrhythmias such as Torsades de Pointes.

What is the corrected QT interval (QTc)?

The corrected QT interval (QTc) is the QT interval value adjusted for heart rate. Since QT varies naturally with HR (shortens in tachycardia and lengthens in bradycardia), the correction allows comparing measurements in different clinical situations using formulas like Bazett, Fridericia, Framingham or Hodges.

How to calculate the QT interval on ECG?

To calculate the QT interval: 1) Choose lead DII or V5-V6; 2) Identify the beginning of the QRS complex; 3) Identify the end of the T wave using the tangent method; 4) Measure the distance between these points in milliseconds; 5) Calculate the RR interval to get heart rate; 6) Apply a correction formula to get QTc. QTc Calculator automates this process using ECG photos.

What is the difference between QT and QTc?

QT is the raw interval measured directly on ECG (in milliseconds). QTc is the QT interval corrected for heart rate. Correction is necessary because normal QT varies with HR: it's shorter when the heart beats faster and longer when it beats slower. QTc allows comparing values regardless of the patient's HR.

Which QT correction formulas are available?

QTc Calculator offers 5 validated formulas: Bazett (QTc = QT/√RR), Fridericia (QTc = QT/∛RR), Framingham, Hodges and Dmitrienko. For HR 60-100 bpm, all are acceptable. For HR > 100 bpm, prefer Framingham, Hodges or Fridericia. For HR < 60 bpm, prefer Framingham or Hodges.

What are normal QTc values?

Men: Normal ≤440ms, Borderline 441-460ms, Prolonged >460ms

Women: Normal ≤460ms, Borderline 461-480ms, Prolonged >480ms

What is the normal QT interval for men?

The normal QTc interval for men is considered up to 440ms. Values between 441-460ms are classified as borderline and deserve clinical attention. QTc above 460ms in men is considered prolonged and increases the risk of arrhythmias such as Torsades de Pointes. The most accepted reference considers QTc >450ms as abnormal in men in some guidelines.

What is the normal QT interval for women?

The normal QTc interval for women is considered up to 460ms - about 20ms longer than for men. Values between 461-480ms are borderline. QTc above 480ms in women is prolonged. Women naturally have longer QTc due to hormonal differences, especially estrogen influence on cardiac ion channels.

Why is QTc different between men and women?

The difference in QTc between men and women is mainly caused by sex hormones. Testosterone shortens the QT interval by increasing repolarizing potassium currents, while estrogen has the opposite effect. Therefore, adult men have shorter QTc (~440ms normal) and women have longer QTc (~460ms normal). This difference appears at puberty and decreases after menopause.

Is there a free online QTc calculator?

Yes! QTc Calculator (qtc.mocha.app) is a free QT interval calculator that allows measuring QTc directly from ECG photos. It offers 10 free measurements, supports 5 validated formulas, automatically classifies results as normal, borderline or prolonged, and was developed by an electrophysiologist.

What can cause prolonged QT?

Prolonged QT can be caused by: medications (antiarrhythmics like amiodarone, antibiotics like azithromycin, antipsychotics), electrolyte disturbances (hypokalemia, hypomagnesemia, hypocalcemia), congenital long QT syndrome (LQTS), severe bradycardia, hypothermia, and myocardial ischemia.

Does this calculator replace medical analysis?

No. This tool is a clinical decision support. Evaluation by a qualified professional is always required.