Relation between Beat-to-Beat QT Interval Variability and T-Wave Amplitude in Healthy Subjects

Objectives: Elevated beat-to-beat QT interval variability (QTV) has been associated with increased cardiovascular morbidity and mortality.The aim of this study was to investigate interlead differences in beat-to-beat QTV of 12-lead ECG and its relationship with the T wave amplitude. Methods: Short-term 12-lead ECGs of 72 healthy subjects (17 f, 38 +/- 14 years; 55 m, 39 +/- 13 years) were studied. Beat-to-beat QT intervals were extracted separately for each lead using a template matching algorithm. We calculated the standard deviation of beat-to-beat QT intervals as a marker of QTV as well as interlead correlation coefficients. In addition, we measured the median T-wave amplitude in each lead. Results: There was a significant difference in the standard deviation of beat-to-beat QT intervals between leads (minimum: lead V3 (2.58 +/- 1.36 ms), maximum: lead III (7.2 +/- 6.4 ms), ANOVA: P < 0.0001). Single measure intraclass correlation coefficients of beat-to-beat QT intervals were 0.27 +/- 0.18. Interlead correlation coefficients varied between 0.08 +/- 0.33 for lead III and lead V1 and 0.88 +/- 0.09 for lead II and lead aVR. QTV was negatively correlated with the T-wave amplitude (r =0.62, P < 0.0001). There was no significant affect of mean heart rate, age or gender on QT variability (ANOVA: P > 0.05). Conclusions: QTV varies considerably between leads in magnitude as well as temporal patterns. QTV is increased when the T wave is small.