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Background— Analysis of repolarization instability, manifested by T-wave alternans (TWA), has proved useful for arrhythmia risk assessment. However, temporal relations between TWA and the spontaneous initiation of ventricular tachyarrhythmias (VTA) in humans are unknown. We examined continuous dynamics of repolarization in Holter electrocardiograms with spontaneous sustained (&gt;30 seconds) VTA.Methods and Results— Ambulatory electrocardiograms from 42 patients (79% with ischemic heart disease; left ventricular ejection fraction, 37±15%) were digitized, and the lead with the highest magnitude of the T wave was selected for analysis. TWA was examined by the modified moving average and intrabeat average analyses. To examine non-TWA (longer-period) oscillations in the repolarization segment, spectral energy of oscillations of consecutive T-wave amplitudes was calculated with the use of the short-time Fourier transform. Heart rate variability was assessed with the Fourier transform as well. TWA increased before the onset of VTA and reached a peak value of 23.6±11.7 μV 10 minutes before the event (P =0.0007). Spectral power of the oscillations of consecutive T-wave amplitudes increased nonuniformly, with the greatest increase in the respiratory range (2.6 μV2 ;P =0.005). In the TWA range, the change was smaller but highly pronounced relative to the 60- to 120-minute level (65%;P =0.003). The low-frequency and high-frequency heart rate variability power declined before the arrhythmia (P =0.04 andP =0.06, respectively).Conclusions— The magnitude of repolarization instability, manifested by TWA and beat-to-beat oscillations of T-wave amplitudes at other frequencies, increased before the onset of VTA. Tracking of these dynamics can facilitate timely detection of high-risk periods and may be useful for initiation of preventive treatments.

Upsurge in T-Wave Alternans and Nonalternating Repolarization Instability Precedes Spontaneous Initiation of Ventricular Tachyarrhythmias in Humans