Beat-to-Beat QT Variability: A Population Study of the QT Variability Index Composition

Background/Objectives: One of the topics of electrocardiographic risk factor studies is investigations of beat-to-beat QT interval variability. The seminal study that reported QT variability as a prognostic risk factor introduced the so-called QT variability index (QTVi). QTVi quantification relies not only on the variance of QT intervals but also on correction factors, including RR interval variance, heart rate, and overall QT interval duration. This study investigated the influence of all the measured factors on QTVi values. Methods: Long-term electrocardiograms (ECGs) were obtained from 251 healthy subjects (mean age 33.6 ± 9.1 years, 108 females) during repeated postural tests that involved supine, sitting, and standing positions maintained for 10 or 15 min. During each position, a 5-min ECG segment with a stable heart rate and without any ectopic disturbances was found. In these segments, standard deviations of normal-to-normal RR (NN) interval durations (SDNN) and of beat-to-beat QT interval durations (SDQT) were measured together with the means of NN and QT intervals. QTVi was subsequently calculated. For each subject, results obtained during each postural position were averaged. Results: In multivariable regression models, evaluated separately in female and male sex-subgroups of the population, QTVi values were significantly dependent on SDQT, SDNN, and mean NN intervals (all p < 0.001) but practically independent of mean QT interval durations. Conclusions: QTVi is significantly influenced by factors that are unrelated to the beat-to-beat changes in QT interval durations. This needs to be considered when interpreting QTVi values. In future studies, multivariable statistical models are needed to ensure that QTVi findings are independent of associated heart rate variability indices.