An Evidence Based Approach to QT Interval Measurement

With the growing and concerning COVID-19 pandemic and no targeted proven treatments, physicians are turning to off label use and FDA Emergency Use Authorization for various medications that are available currently. While their therapeutic efficacy remains to be established, a potential side effect of several of these medications (chloroquine, hydroxychloroquine, azithromycin, and lopinavir/ritonavir)        is drug-induced QT interval prolongation with subsequent increased risk for potentially life threating arrhythmias (drug-induced torsade de pointes, DI-TdP), or worse, drug-induced sudden cardiac death (DI-SCD).


[1, 2, 3]

Know Your Patient’s QTc Value

Ideally, before starting your patient on any of these potential QT-prolonging therapies, the patient’s QTc value should be determined by either a 12-lead electrocardiogram (ECG), from telemetry (if already an inpatient being monitored, or from a smartphone-enabled mobile ECG device like AliveCor’s KardiaMobile 6L which received FDA Emergency Approval for QTc monitoring on 3/20/2020). Although the risk of lethal cardiac arrhythmias or sudden cardiac death with these medications is low, it is not zero, and on a global scale, this risk is real especially for the 5-10% of patients whose baseline QTc is already elevated and the 1% of patients whose baseline QTc is already at the potentially torsadogenic threshold of > 500 ms.

In an already difficult time with resources stretched, the ability to quickly and meticulously measure the QT interval and derive the heart-rate corrected QTc value in patients with COVID-19 (or any other disease for that matter where FDA-approved, potentially QT-prolonging medications are being used) is critical.

This free online resource aims to provide you with an evidence-based approach for calculation of your patient’s QTc. Below, there is guidance on how to measure the QT interval, an easy and intuitive QTc calculator (with several heart-rate correction formulas), as well as frequently asked questions (FAQs). 

Depending on the patient’s QTc value, you may be able to proceed with a

Green Light Go”  (90% of patients)

Yellow Light Pause” (9%)

Red Light Stop” (1%)


Please see the Mayo Clinic Proceedings article        and the QTc Algorithm below for further guidance. This algorithm is based upon the Bazett QTc. 

COVID-19-QTc algorhythm

QT Interval Measurement

QTc Calculator


Need guidance on how to accurately calculate the QT interval? This section will take you through step by step.

A go-to reference for finding answers regarding measurement of the QT interval and calculation of the QTc value

This easy to use calculator will calculate the QTc value using multiple formulas taking into account patient characteristics and rhythm.


Measuring the QT Interval and Calculating the QTc Value

The QT interval is measured on the surface 12-lead ECG (or telemetry or smartphone-enabled mobile ECG) from the beginning of the QRS complex to the end of the T wave. The QT interval is influenced by heart rate; with a longer QT interval at slower heart rates and shorter QT interval at faster heart rates. Therefore, the QT interval must be corrected for the patient’s heart rate to generate the patient’s QTc value.

The diagram below provides a step-by-step guide to using the Windland Smith Rice Genetic Heart Rhythm Clinic’s QTc Calculator.


After measuring/verifying the patient’s QT interval, heart rate, and their QRS duration (if it is > 120 ms), then simply use the calculator to calculate your patient’s QTc value.



 QTc Calculator



Input data below to obtain the QTc. 

There are literally one hundred possible heart rate-correction formulas to derive a heart rate corrected QTc value with the most common formulas:

1) Bazett   – QTcB=QT/RR
2) Fridericia   – QTcFri=QT/RR

3) Hodges   – QTcH=QT+0.00175 ([60/RR]−60)
4) Framingham   – QTcFra=QT+0.154 (1−RR)

Depending on your institution, the formula used in the 12-lead ECG laboratory varies.  For example, at Mayo Clinic, the QTc is reported using Bazett’s formula.  For Thorough QT Drug Studies, the FDA prefers Fridericia.


In general, among otherwise healthy people, the average Bazett’s-derived QTc is about 410 ms pre-puberty, 410 ms in post-pubertal males, and 420 ms in post-pubertal females.  In contrast, in the Mayo Clinic Windland Smith Rice Genetic Heart Rhythm Clinic, the average QTc among patients with genetically-confirmed long QT syndrome (LQTS) is 465 ms.  Among patients being seen at Mayo Clinic as either an outpatient or inpatient where an ECG was ordered, 1% of those patients had a QTc > 500 ms with less than 10% stemming from patients with congenital LQTS.  As shown in the Algorithm, we estimate that 90% of patients will have a baseline QTc less than their respective age- and gender-predicted 99th percentile values: < 460 ms before puberty, < 470 ms in men, and < 480 ms in women.  In contrast, 1% of patients will have a QTc > 500 ms which is regarded as the established torsadogenic threshold.






DISCLAIMER: All calculations must be confirmed before use. The authors make no claims of the accuracy of the information contained herein; and these measurements should only be used with clinical judgement and be discussed with medical specialists. 



What lead from the surface ECG should I use to measure the QT interval?

Lead II or V5/V6 are usually the best options for measurement. The reason for this is that the end of the T wave is often more clearly defined in these leads and U waves, if present, are less prominent.

What is the tangent method to define the end of the T-wave?

It is a method used to determine where the T wave ends, defining the end of the QT interval. To do this a tangent is drawn on the steepest end limb of the T wave. Where this tangent intersects with the baseline defines the end of the T wave. Please click here to read more information T wave tangent t wave end long qt

What about U waves?

The U wave can often add to confusion and uncertainty in defining the end of the T wave. If the U wave is distinct and smaller than the T wave it should NOT be included in the QT interval measurement.

How do I correct the QT interval in patients with atrial fibrillation?

In the setting of atrial fibrillation (AF) or even significant arrhythmia, there are several proposed approaches to deriving the QTc value in the setting of such variable RR intervals. Perhaps, the most rigorous is to measure the QT intervals and the preceding RR intervals for at least 10 beats, derive average values for both, and then enter those values into the various QTc formulas. This is impractical. In contrast, although easier, simply taking the longest QT interval from those 10+ beats and using that value along with the average RR interval will result in an overestimated QTc value.

For this online QTc calculator, we have used the approach where you simply enter the heart rate from the 12-lead ECG, telemetry monitor, or smartphone enabled device (since it does a great job of deriving the average heart rate from those varying RR intervals) into the calculator and then find and enter the shortest QT interval and the longest QT interval seen. The calculator will then determine the average of those two values and automatically enter that average value into the calculator. We are continuing to analyze the utility of this approach. If a better AF/sinus arrhythmia adjustment is proposed, then the calculator will be updated.

How do I correct the QT interval if there is a wide QRS (bundle branch block or ventricular pacing)?

The QT interval will be exaggerated in patients with a wide QRS and could lead to overestimation of the QT interval. Calculation of the JT interval (JTc = QTc - QRS duration) has been proposed as an option however it has largely fallen out of favor due to concern with residual heart rate confounding.

Instead, we propose a simple wide QRS QTc adjustment if the patient’s QRS is > 120 ms.

  • Wide QRS adjusted QTc = QTc – [QRS – 120].

What if the QTc value is in the “Yellow Light” or “Red Light” status?

The Mayo Clinic Proceedings article offers guidance and please see algorithm above.


  1. Centre for Disease Control and Prevention;, accessed March 25th 2020

  2. Giudicessi JR, Ackerman MJ. Azithromycin and risk of sudden cardiac death: guilty as charged or falsely accused?. Cleveland Clinic journal of medicine. 2013 Sep;80(9):539.

  3. Chen CY, Wang FL, Lin CC. Chronic hydroxychloroquine use associated with QT prolongation and refractory ventricular arrhythmia. Clinical Toxicology. 2006 Jan 1;44(2):173-5.

  4. Postema PG, De Jong JS, Van der Bilt IA, Wilde AA. Accurate electrocardiographic assessment of the QT interval: teach the tangent. Heart Rhythm. 2008 Jul 1;5(7):1015-8.

  5. Bazett HC. An analysis of the time relations of electrocardiograms. Heart. 1920;7:353-70.

  6. Fridericia LS. The systole duration in the electrocardiogram in normal people and in cardiac patients. Acta Medica Scandinavica. 1920 Jan 12; 53 (1): 469-86.

  7. Hodges M. Bazett's QT correction reviewed: evidence that a linear QT correction for heart rate is better. J Am Coll Cardiol. 1983;1:694.

  8. Sagie A, Larson MG, Goldberg RJ, Bengtson JR, Levy D. An improved method for adjusting the QT interval for heart rate (the Framingham Heart Study). The American journal of cardiology. 1992 Sep 15;70(7):797-801.

  9. Giudicessi JR, Noseworthy PA, Ackerman MJ. The QT Interval: An Emerging Vital Sign for the Precision Medicine Era?. Circulation. 2019 Jun 11;139(24):2711-3.

  10. Rautaharju PM, Surawicz B, Gettes LS. AHA/ACCF/HRS recommendations for the standardization and interpretation of the electrocardiogram: part IV: the ST segment, T and U waves, and the QT interval a scientific statement from the American Heart Association Electrocardiography and Arrhythmias Committee, Council on Clinical Cardiology; the American College of Cardiology Foundation; and the Heart Rhythm Society Endorsed by the International Society for Computerized Electrocardiology. Journal of the American College of Cardiology. 2009 Mar 17;53(11):982-91.