BEAT-TO-BEAT QT INTERVAL VARIABILITY - NOVEL EVIDENCE FOR REPOLARIZATION LABILITY IN ISCHEMIC AND NONISCHEMIC DILATED CARDIOMYOPATHY

Background Dilated cardiomyopathy (DCM) is associated with a high inci dence of malignant ventricular arrhythmias and sudden death. Abnormali ties in repolarization of ventricular myocardium have been implicated in the development of these arrhythmias. Spatial heterogeneity in repo larization has been studied in DCM, but temporal fluctuations in repol arization in this setting have been largely ignored. We sought to test the hypothesis that beat-to-beat QT interval variability is increased in DCM patients compared with control subjects. Methods and Results E ighty-three patients with ischemic and nonischemic DCM and 60 control subjects served as the study population. Beat-to-beat QT interval vari ability was measured by automated analysis on the basis of 256-second records of the surface EGG. A QT variability index (QTVI) was calculat ed for each subject as the logarithm of the ratio of normalized QT var iance to heart rate variance. The coherence between heart rate and QT interval fluctuations was determined by spectral analysis. In patients , ejection fractions were assessed by echocardiography or ventriculogr aphy, and spatial QT dispersion was determined from the standard 12-le ad EGG. DCM patients had greater QT variance than control subjects (60 .4+/-63.1 versus 25.7+/-24.8 ms(2), P<.0001) despite reduced heart rat e variance (6.7+/-7.8 versus 10.5+/-10.4 bpm(2), P=.01). The QTVI was higher in DCM patients than in control subjects, with a high degree of significance (-0.43+/-0.71 versus -1.29+/-0.51, P<10(-12)). QTVI did not correlate with ejection fraction or spatial QT dispersion but did depend on New York Heart Association functional class. QTVI did not di ffer between DCM patients with ischemic and those with nonischemic ori gin. Coherence between heart rate and QT interval fluctuations at phys iological frequencies was lower in DCM patients compared with control subjects (0.28+/-0.14 versus 0.39+/-0.18, P<.0001). Conclusions DCM is associated with beat-to-beat fluctuations in QT interval that are lar ger than normal and uncoupled from variations in heart rate. QT interv al variability increases with worsening functional class but is indepe ndent of ejection fraction. These data indicate that DCM leads to temp oral lability in ventricular repolarization.