How to find qt interval on ecg?

The QT interval should be measured manually, preferably by using one of the limb leads that best shows the end of the T wave on a 12-lead ECG. The QT interval should be measured from the beginning of the QRS complex to the end of the T wave and averaged over 3 to 5 beats.

The QT interval is defined from the beginning of the QRS complex to the end of the T wave. The maximum slope intercept method defines the end of the T wave as the intercept between the isoelectric line with the tangent drawn through the maximum down slope of the T wave (left).

When notched T waves are present (right), the QT interval is measured from the beginning of the QRS complex to the intersection point between the isoelectric line and the tangent drawn from the maximum down slope of the second notch.

There are multiple formulas used to estimate QTc. It is not clear which formula is the most useful:

Note: The RR interval is given in seconds (RR interval = 60 / heart rate).

Fortunately, there are now multiple phone apps that will calculate QTc for you, for example MDCalc.com has a quick and easy QTc calculator that is free to use.

Digoxin produces a relative shortening of the QT interval, along with downward sloping ST segment depression in the lateral leads (‘reverse tick’ appearance), widespread T-wave flattening and inversion, and a multitude of arrhythmias (ventricular ectopy, atrial tachycardia with block, sinus bradycardia, regularized AF, any type of AV block).

Viskin (2009) proposes the use of a ‘QT interval scale’ to aid diagnosis of patients with short and long QT syndromes (once reversible causes have been excluded):

In the context of acute poisoning with QT-prolonging agents, the risk of TdP is better described by the absolute rather than corrected QT.

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