Cardiology: Interpreting QT Interval

Identifying the QT interval (QTI) is a very important part of 12-Lead EKG interpretation.  Here are some quick tips to help you quickly interpret the QTI in the field.

A long QT interval can lead to some very big problems.  So, the question is, how do we know what is long, and what is the proper way to measure the QT interval?  Generally speaking, the Corrected QT Interval (QTc) is prolonged in men if it is >440 ms, and prolonged in females if it is >460 ms.  If the QTc is >500 ms, you had better get your MgSO4 ready, because Torsades de Pointes is likely.  To find the QTc, you must first identify the QTI.

To identify the QTI, measure from the start of the Q wave to the end of the T wave in lead II, V5, or V6.  You don’t have to search around and guess where the end of the T wave is though, simply draw a line that matches the downslope of the T wave.  Where that line intersects the isoelectric line is your point of reference for the end of the T wave.

**An easy way to tell if a QTI is long is to compare it to the R-R interval, if the QTI is greater than half of the R-R, it is most likely prolonged!

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To determine the QTc, there are several formulas available.  I will show you the one I use below which is known as Bazett’s formula.

QTc= QT / √ RR

Where QT represents the QT interval and RR represents the R-R interval. * The RR should be given in seconds by using the formula RR= 60/ Heart rate.  So with the above strip from lead II, we see that the QTI is about 400 ms and the RR is about 800 ms.  The HR is 77 so…

60/77= .779

QTc= .400 / √ .779

QTc = .453 s

Prolongation of the QTI can be caused by any of the factors listed below:

  • Drugs
  • Hypomagnesemia
  • Hypokalemia
  • Hypocalcemia
  • Hypothermia
  • Increased ICP
  • Myocardial Ischemia

So next time you are checking all of your intervals, remember to check the QTI!  Also, check out the articles listed in my references below from LITFL and Dr. Smith’s ECG blog, for a much more in-depth explanation of QTI & QTC.

Thanks for reading!  Since I started this blog, I have gotten so much helpful feedback from everyone, and I am very thankful for it!  I certainly never expected I would have over 160 readers in such a short time.  Don’t forget to share this with your friends and colleagues!  Until next time, be safe and stay focused.

-Owen

References

Burns E. QT interval LITFL ECG Library Basics. LITFL: Life in the Fast Lane Medical Blog. [accessed 2016 Dec 17]. http://lifeinthefastlane.com/ecg-library/basics/qt_interval/

Smith S. Long QT: Do not trust the computerized QT interval when the QT is long. Long QT: Do not trust the computerized QT interval when the QT is long. [accessed 2016 Dec 17]. http://hqmeded-ecg.blogspot.com/2014/06/acquired-long-qt-do-not-trust.html

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