Automated versus manual measurement of the QT interval and corrected QT interval

Abstract

Background: The International Conference on Harmonization E14 Guideline specifies detailed assessment of QT interval or corrected QT interval prolongation when developing new drugs. We recently devised new software to precisely measure the QT interval.

Methods and results: The QT intervals of all leads for a selected single heart beat were compared between automated measurement with the new software from Fukuda Denshi and manual measurement. With both automated and manual measurement, QT intervals obtained by the tangent method were shorter than those obtained by the differential threshold method, but the extent of correction was smaller. QT interval data obtained by the differential threshold method were more similar to values obtained by visual measurement than were data obtained by the tangent method, but the extent of correction was larger. Variability was related to the T-wave amplitude and to setting the baseline and tangent in the tangent method, while skeletal muscle potential noise affected the differential threshold method. Drift, low-amplitude recordings, and T-wave morphology were problems for both methods. Among the 12 leads, corrections were less frequent for leads II and V(3) -V(6) .

Conclusion: We conclude that, for a thorough assessment of the QT/QTc interval, the tangent method or the differential threshold method appears to be suitable because of smaller interreader differences and better reproducibility. Correction of data should be done by readers who are experienced in measuring the QT interval. It is also important for electrocardiograms to have little noise and for a suitable heart rate and appropriate leads to be selected.

Figure 1

Measurement by the tangent method. In the tangent method, the maximum slope of the descending limb of the T wave is identified from the maximum negative value of the differentiated waveform. A tangent is drawn from that point, and the distance from the point where the tangent crosses the baseline to the start of the QRS complex is measured.

Figure 2

Measurement by the differential threshold method. For the differential threshold method, the point where the differential waveform of the T wave returns to the background noise level is identified as the T‐wave terminus, and its distance from the start of the QRS complex is measured.

Figure 3

Number of nonmeasurable QT intervals per lead and sum of the “corrected values.” The “corrected values” of the QT interval were provided as the difference between automated and manual QT interval.

Figure 4

Compaison of the QT intervals of all subjects.

Figure 5

Comparison of the average QT intervals.

Figure 6

Comparison of the corrected values. The “corrected values” of the QT interval were provided as the difference between automated and manual QT interval.

Figure 7

Causes of variability in measurement—drift, skeletal muscle potential noise, and low amplitude.

Figure 8

Causes of variability in measurement—T‐wave shape.

Figure 9

Tangent width and QT interval obtained by automated or manual measurement.