What Do You Think 26

Second opinion on an ECG from an elderly male with a reported long QT interval.

What do you think?

Let us revisit the normal limits of QTc intervals.

For a male, 470 ms is well within the range of prolonged QTc.

How do we measure the QT interval?

For consistency and reproducibility, the “maximum slope intercept” or “tangent” method is recommended to define the termination of the T wave.

The QT interval (red arrow) is measured from the beginning of the QRS complex to the point of intersection between the tangent drawn through the maximum down slope (blue line) and the vertical line (green line), which transects it at the baseline.

Let us return to our case study and measure the QT.

I chose V2 for illustrative purposes only. We usually use lead II or V5, V6.  

The QTc I calculated was also long.

However, on review of the ECG, the extension of the T wave was not a U wave, but rather a P wave and the diagnosis was a long PR interval and not a long QTc.

Remeasuring the QT/QTc obtained a Bazett of 437 ms, which is in the normal range.

The first and still the most utilized correction formula was the one designed by Bazett over a 100- years ago. It is most accurate when the sinus rate lies between 60 and100 bpm. The Bazett formula both under-corrects at over-corrects when the heart rates are fast or slow. The Bazett correction is logarithmic, whereas theHodges is linear and more accurate over wider sinus rate changes.

Our case was interpreted as a U wave. This added wave always creates a dilemma in calculating QTc. When fused into the T wave it is usually incorporated into theQT measurement as it is part of repolarization and hence the T wave. However, a separate U wave would extend the T wave and result in erroneous measurements.By definition, this would  be a QU measurement. Thus, by convention in everyday practice, these separate waves are usually ignored.

Some authorities recommend that when a prominent U wave is present, it is preferable to define the end of the T wave at the point where the wave is lowest or on the baseline referred to as the TU nadir (green vertical line). As can be seen, this is slightly longer than the tangent method (red vertical line).

In summary, with QT measurements, beware of the added wave after the T wave. It may be a U wave, but you need to exclude a P wave. Perform manual measurements before concluding that there is a long QTc measurement.