The QT variability index (QTVI) indicates temporal dispersion in myocardial repolarization, and a high QTVI is associated with a propensity for sudden death from malignant ventricular arrhythmias in subjects at high risk. In this study, the authors assessed the effects of free breathing, controlled breathing, and sympathetic stress (tilt) on the QTVI in patients with chronic heart failure (CHF) and healthy control subjects. The authors also examined the influence of age on the same variables. To obtain normative data, they calculated 95% confidence intervals for healthy subjects grouped according to age. Under all experimental conditions, the QTVI was larger in the CHF group overall and in the age subsets than in controls. In patients and controls, the QTVI increased significantly during tilt, although no differences were found between the QTVI measured during free and controlled breathing. In healthy controls, the following variables correlated significantly with the QTVI: age and baseline heart rate (P < 0.001). In patients with CHF, aging had no influence on the QTVI. Conclusion: Age, sympathetic stress, and CHF all tend to increase the QTVI and could potentially induce sudden death. Further studies should assess the usefulness of the QTVI as a marker predicting sudden cardiac death under the various conditions of risk.
"In the HF band, the HP power decreased as a function of tilt table angles, whereas the VRD power remained constant. Data suggest that VRD variability can be used to infer cardiac sympathetic control,31-34 especially when the sympathetic drive is high (ie, at the highest tilt table inclination)35 and when assessed in the LF band (the relation of LF power with tilt table angles is stronger than that of the variance). Conversely, the absolute amount of HP variability, especially when assessed in the HF band, is mainly related to vagal modulation36 being progressively reduced during a graded head-up tilt protocol.16,17,19,30 "
[Show abstract][Hide abstract] ABSTRACT: We test the hypothesis that the degree of correlation between ventricular repolarization duration (VRD) and heart period (HP) carries information on cardiac autonomic regulation. The degree of correlation was assessed in the frequency domain using squared coherence function during an experimental protocol known to gradually induce a shift of sympathovagal balance toward sympathetic predominance (ie, graded head-up tilt). We observed a progressive decrease of squared coherence with tilt table inclination, thus confirming the working hypothesis. The VRD-HP uncoupling occurs in the high-frequency band, centered on the respiratory rate, thus suggesting that vagal withdrawal is responsible for the VRD-HP uncoupling.
Journal of electrocardiology 09/2011; 44(6):662-8. DOI:10.1016/j.jelectrocard.2011.08.002 · 1.36 Impact Factor
[Show abstract][Hide abstract] ABSTRACT: The QT Variability Index (QTVI) is a non-invasive measure of repolarization lability that has been applied to a wide variety of subjects with cardiovascular disease. It is a ratio of normalized QT variability to normalized heart rate variability, and therefore includes an assessment of autonomic nervous system tone. The approach assesses beat-to-beat variability in the duration of the QT and U wave in conventional surface electrocardiographic recordings, as well as determines the heart rate variability (HRV) from the same recording. As opposed to T wave alternans, QTVI assesses variance in repolarization at all frequencies. Nineteen studies have published data on QTVI in healthy individuals, while 20 have evaluated its performance in cohorts with cardiovascular disease. Six studies have assessed the utility of QTVI in predicting VT/VF, cardiac arrest, or cardiovascular death. A prospective study utilizing QTVI to determine therapy allocation has not been performed, and therefore the final determination of the value of the metric awaits definitive exploration.
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