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J Electrocardiol. 2014 Sep-Oct;47(5):708-15. doi: 10.1016/j.jelectrocard.2014.06.003. Epub 2014 Jun 9.

Repolarization lability measured on 10-second ECG by spatial TT' angle: reproducibility and agreement with QT variability.

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Whiting School of Engineering, Johns Hopkins University, Baltimore, MD, USA.
The Division of Cardiology, Department of Medicine, Johns Hopkins Hospital, Baltimore, MD, USA; The Knight Cardiovascular Institute, Oregon Health & Science University, Portland, OR, USA. Electronic address:



Reproducibility of spatial TT' angle on the 10-second ECG and its agreement with QT variability has not been previously studied.


We analyzed 2 randomly selected 10-second segments within 3-minute resting orthogonal ECG in 172 healthy IDEAL study participants (age 38.1±15.2years, 50% male, 94% white). Repolarization lability was measured by the QT variance (QTV), short-term QT variability (STV(QT)), and spatial TT' angle. Bland-Altman analysis was used to assess the agreement between different log-transformed metrics of repolarization lability, and to assess the reproducibility.


The heart rate showed a very high reproducibility (bias 0.14%, Lin's rho_c=0.99). As expected, noise suppression by averaging improves reproducibility. Agreement between two 10-second LogQTV was poor (bias -0.04; 95% limits of agreement [-1.89; 1.81]), while LogSTV(QT) (0.04 [-1.01; 1.10]), and especially LogTT' angle (-0.009 [-0.84; 0.82]) was better.


TT' angle is a satisfactory reproducible metric of repolarization lability on the 10-second ECG.


ECG; Repolarization; Spatial TT′ angle

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