Online medical calculator to measure the QT corrected (QTc) interval using Bazett equation. QTc Bazett calculator is mainly used for clinical calculations.
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:
Corrected QT Interval Bazett’s formula online calculator
- Bazett’s formula The formula should be used in patients with a heart rate of 60 to 100 beats per minute. In cases of tachycardia or bradycardia, the values may not be correct.
- For heart rate below 60 or above 100 beats per minute, the adjusted QT interval should be calculated by the Framingham formula.
Normal values of Corrected QT: 320-430 for men and 320-450 for women.
This calculator is intended for use by health care providers. The results should not be used alone to determine medical treatment. This tool is a statistical model and is not a substitute for an individual treatment plan developed by a health care provider with personal knowledge of a specific patient. Factors such as medical history and the health care provider’s experience, knowledge, and training must also be considered. Results should be discussed with patients when presenting prognoses or treatment recommendations.
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.
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.
How to calculate QTc?
To get your patient’s QTc with our QTc calculator, you need to:
1) 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.
2) 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).
3) 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 mode button 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.
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 check a drug to 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 link.
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.
© Kacper Pawlik, MD and Dominika Śmiałek, MD, PhD candidate
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!
How does this QTc calculator work?
This is a handy health tool that can estimate the QT corrected interval by using the heart rate expressed in beats per minute and QT interval expressed either in seconds or milliseconds. The QT values can be obtained from the ECG test.
This QTc calculator is designed to show the QT corrected interval for heart rate extremes because it returns the estimations by 4 different equations as presented below:
QT corrected interval:
- by Bazett’s formula: QTc = QT/√(RR in seconds)
- by Fridericia’s formula: QTc = QT/(RR^0.33)
- by Framingham’s formula: QTc = QT + 0.154(1-RR)
- by Hodges’s formula: QTc = QT + 1.75(HR - 60)
RR interval = 60 / HR
HR = Heart rate in beats per minute.
Moreover it returns the QT corrected interval expressed in both seconds and milliseconds.
As agreed upon by ACC / HRS the normal QTc interval is below 450 milliseconds for men and below 460 milliseconds for women.
No gender specific, any QTc greater than 500 milliseconds is considered highly abnormal, while any value of QTc smaller than 340 milliseconds may indicate short QT syndrome.
Please remember that this QTc calculator should NOT be considered as a substitute for any medical professional service.
- For a heart rate/ Pulse of 72 beats per minute and a QT interval of 0.42 seconds the result is:
- QTc Interval by Bazett’s method = 0.460 sec OR 460 msec
- QTc Interval by Fridericia’s equation = 0.446 sec OR 446 msec
- QTc Interval by Framingham’s algorithm = 0.446 sec OR 446 msec
- QTc Interval by Hodges’s equation = 0.441 sec OR 441 msec
- RR Interval = 0.833 sec OR 833 msec
What is the short QT syndrome?
This is a condition that can cause arrhythmia which is a disruption in the heart’s normal rhythm because the QT interval shortening means that the heart takes less time to recharge/ relax between beats but there is no underlying structural anomaly of the heart.
This is a relatively new discovery of the 21st century medicine and there haven’t been numerous cases documented. It can be detected through EKG (electrocardiogram) that measures the electrical activity of the heart.
This condition that appears at any age, if left untreated leads to syncope which is fainting, feelings of dizziness and even to cardiac arrest and sudden death. On the other hand there are people, generally healthy that have shortened QT but don’t display any symptoms.
What if the QT is higher than normal?
The long QT syndrome is an uncommon condition, also put under arrhythmias and can pose a serious threat as the electrical activity of the heart is disrupted. There are also some individuals that have QT intervals longer but don’t develop serious arrhythmia while others experience moments in which their heart suddenly beats faster for no particular reason and this disruption of rhythm leads to the brain not being oxygenated properly and then fainting.
It is discussed that there is an inheritance pattern for this anomaly and that there are higher chances for it to appear to individuals that have cases of heart disease in the family.
1) Bazett HC. (1920) An analysis of the time-relations of electrocardiograms. Heart 1920; (7): 353–37
2) Indik JH, Pearson EC, Fried K, Woosley RL. (2006) Bazett and Fridericia QT correction formulas interfere with measurement of drug-induced changes in QT interval. Heart Rhythm; 3(9) 1003-7.