Exercise-induced repolarization alternans heterogeneity in patients with an implanted cardiac defibrillator

Repolarization alternans (RA), generally recognized as a promising noninvasive index for risk stratification, is often measured under exercise conditions, since RA increases its amplitude with heart rate. Instead, the effect of exercise on the RA location along the JT interval is still unknown. Aim of the present study was to evaluate exercise-induced RA heterogeneity in terms of both amplitude and location. To this aim, we analyzed the ECG precordial leads of 36 patients with an implanted cardiac defibrillator (ICD) who underwent a bicycle ergometer test during which the working load was increased from zero (NoWL) to the patient´s maximum capacity (MaxWL). RA was analyzed using our heart-rate adaptive match filter method, which provides an RA parameterization in terms of its amplitude (RAA, μV) and location, the latter measured as time-delay (RAD, ms) with respect to the T-wave apex, so that positive values of RAD indicate RA occurring in the T-wave right side and vice versa. According to our results, during MaxWL, RAA was higher than during NoWL (34±21 μV vs. 16±10 μV; P<;10^-5), whereas RAD was shorter (26±29 ms vs. 69±45 ms; P<;10^-4). Thus, in ICD patients, exercise not only induced a significant increment of RA amplitude, but also caused RA location to move from very late in the repolarization segment toward the T-wave apex.