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.

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.

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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.

In the setting of atrial fibrillation (AF) or even significant sinus arrhythmia, there are several proposed approaches to deriving the QTc 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 adjustments proposed, then the calculator will be updates.

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].

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