With our QTc calculator, you will be able to perform a correction of QT interval for your patient’s heart rate. In the text below, you will learn how to calculate QTc using Bazett’s formula and other equations (e.g., Fridericia, Framingham). We also explain what a regular QT interval is, the risks of QT prolongation, and which QT-prolonging drugs you should avoid in treating patients with a prolonged QT interval.
Remember that using this calculator is by no means equivalent to a consultation with a specialist. If the result you obtained is troubling you, be sure to visit your physician!
QT interval is a part of an electrocardiographic representation of an electrical function of a heart. It starts at the beginning of the QRS complex (caused by the depolarization of ventricles) and ends at the end of the T wave (the repolarization of ventricles).
Cardiologists often measure the QT interval since its prolongation correlates with a prevalence of ventricular arrhythmias such as torsades de pointes and is a risk factor for sudden cardiac death. The duration of the QT interval is highly influenced by heart rate: the faster the rhythm, the shorter the QT interval is. Because of that, a correction of the QT interval for heart rate should be made. It can be performed with, Bazett’s formula.
The most common equation used for QT interval correction all around the world is a formula presented by physiologist Henry Cuthbert Bazett in the article “An Analysis of the Time-Relations of Electrocardiograms,” published in 1920 in the Heart Journal. Bazett’s formula looks like this:
QTc = QT / √RR,
- QTc = corrected QT;
- QT = duration of QT interval; and
- RR = duration of RR interval.
Although very popular, Bazett’s formula has its limitations – it is known for overcorrecting at high heart rates and undercorrecting at low heart rates.
Several other equations have been proposed:
A formula by Louis Sigurd Fridericia:
QTc = QT / ∛RR
This method is also thought to give more consistent results at faster heart rates.
The Framingham correction based on the Framingham Heart Study:
QTc = QT + 0.154(1-RR)
An equation by M Hodges:
QTc = QT + 1.75(Heart_Rate - 60)
And by Pentti M Rautaharju:
QTc = QT - 185 × ((60 / Heart_Rate) - 1) + sex_coefficient
The sex coefficient equals 6 ms for men and 0 ms for women.
To see the QTc calculated with these formulas, click on the
advanced mode button.
In a recentpublished in the Journal of the American Heart Association, authors compared previously listed methods of QT correction and concluded that Framingham and Fridericia equations provided the best rate correction and mortality prediction.
How to calculate QTc?
To get your patient’s QTc with our QTc calculator, you need to:
- Determine the length of the QT interval:
- Find the beginning of the Q wave (and QRS complex at the same time).
- Localize the end of the T wave.
- Measure the distance between these two points on X-axis. You can use a ruler or a caliper.
- Transform the length of the QT interval from millimeters or boxes to milliseconds. With a paper speed of 25 mm/s, one small box (1 mm) lasts 0.04 s, and one big box (5 mm) – 0.2 s.
- Type the result in the QT interval field of our QTc calculator.
- In the heart rate section, type the number of beats per minute of your patient’s heart rate. Alternatively, you can measure the RR interval proceeding already evaluated QT interval – use the same technique as in the determination of QT interval length but start at the peak of one R wave and assess the distance to the peak of the next R wave (it should be a part of the QRS complex mentioned in the first point of this list).
- Read the result from the QTc (Bazett) field. This is the corrected QT calculated with Bazett’s formula by our calculator! Now, you only need to verify whether it is the normal QTc for your patient's sex. You can also click on the
advanced modebutton to discover the results of other correcting QT interval formulas.
It is often assumed that a normal QTc lasts (after correction) under 430 ms for an adult male and 450 ms for an adult female. We can diagnose QT prolongation with QTc of over 450 ms for the adult male and 470 ms for the adult female.
Do you want to find an easy way to get your patient’s heart rate from his ECG? Check our ECG heart rate calculator.
QT prolongation increases the risk of a premature action potential developing during the late phase of depolarization – before the repolarization is fully completed. This can lead to severe arrhythmias:
- torsades de pointes - a polymorphic ventricular tachyarrhythmia, with characteristic morphology of QRS complexes changing their amplitude and twisting around the isoelectric line on ECG,
- ventricular fibrillation- a state of complete disorganization of the heart's electrical activity with a loss of its pumping function.
Usually, cardiomyocytes manage to repolarize entirely before the next wave of depolarization reaches them through the heart's electrical conduction system and other cardiomyocytes. However, when repolarization is happening longer than physiologically (as in QT prolongation), or a premature depolarization develops, the depolarization wave may reach cardiomyocytes before they can fully repolarize. Such an event may cause serious electrical disturbances resulting in previously mentioned tachyarrhythmias.
Many factors may create QT prolongation, including:
- Genetic disorders – an inherited Long QT Syndrome, for example in Romano-Ward syndrome or Jervell and Lange-Nielsen syndrome;
- QT prolonging drugs;
- Low blood potassium – hypokalemia;
- Low blood calcium– hypocalcemia;
- Low blood magnesium - hypomagnesemia;
- Heart diseases;
- Low heart rate – bradycardia;
- Central nervous system pathologies;
- Low body temperature - hypothermia; and
- Insufficient function of the thyroid gland – hypothyroidism.
You may want to checkto learn whether it's associated with prolonged QTc.
QT prolonging drugs
There are many drugs with prolongation of QT as one of their side effects. It can be a result of treatment with anti-arrhythmic drugs, such as amiodarone and sotalol, antibiotics like macrolides (erythromycin) and quinolones (levofloxacin), antihistamines, a number of pharmaceuticals used in psychiatry and many others. A list of QT prolonging drugs can be found under this.
Before making a decision to include a QT prolonging drug in a patient’s therapy, a risk-benefit analysis should be performed, especially when there are other risk factors present.