Intra- and interreader variability in QT interval measurement by tangent and threshold methods in a central electrocardiogram laboratory.
BACKGROUND
The QT interval can be measured by tangent (QT(Tan)) and threshold (QT(Thr)) methods; the better method is the one with lower reader variability.


METHODS
QT(Tan) and QT(Thr) were measured twice in 100 digital electrocardiograms (ECGs) by 8 experienced readers in a central laboratory. For QT(Thr), the end of the T wave was the point where the T wave reached the isoelectric baseline; for QT(Tan), it was the point where a line from the peak of the T wave through the steepest part of the descending limb intercepted the isoelectric baseline.


RESULTS
The average absolute intrareader variability ranged from 3.4 to 6.9 milliseconds for QT(Tan) and from 3.5 to 5.2 milliseconds for QT(Thr). By analysis of variance, intrareader SD of QT(Tan) was 7.0 and 7.5 milliseconds for QT(Thr); interreader SD was 13.1 milliseconds for QT(Tan) and 11.9 milliseconds for QT(Thr). QT(Tan) was shorter than QT(Thr) in 96 of the 100 ECGs, it exceeded QT(Thr) in 4 ECGs, which had prominent U waves.


CONCLUSIONS
For trained readers in a central ECG laboratory using sophisticated on-screen tools for QT measurement in high-quality digital ECGs, between- and within-reader variability are comparable for QT(Tan) and QT(Thr). However, QT(Tan) is consistently shorter than QT(Thr) by up to 10 milliseconds.
Panicker
Gopi Krishna
GK
Research Section, Quintiles ECG Services, Mumbai, India. gopi.panicker@quintiles.com
Karnad
Dilip R
DR
Natekar
Mili
M
Kothari
Snehal
S
Narula
Dhiraj
D
Lokhandwala
Yash
Y
D000465:Algorithms; D001145:Arrhythmias, Cardiac / Q000175:diagnosis*; D003936:Diagnosis, Computer-Assisted / Q000379:methods*; D004562:Electrocardiography / Q000379:methods*; D006801:Humans; D015588:Observer Variation; D015203:Reproducibility of Results; D012680:Sensitivity and Specificity