Session S93.6

Transmural Differences in Rate Adaptation of Repolarization Duration Quantified from ECG Repolarization Interval Dynamics

A Mincholé*, E Pueyo, P Laguna

Universidad de Zaragoza
Zaragoza, Spain

Rate adaptation of repolarization duration to heart rate (HR) changes has been shown to provide relevant information for arrhythmic risk stratification. In this work we investigate rate adaptation of ECG repolarization indices, including QT interval (QT), T wave width (Tw), and distance from peak to end of the T wave (TpTe), in response to stepwise HR changes induced during tilt test.
A database of 15 volunteers without any previous cardiovascular history was recorded while subjects were performing the head-up tilt test: 4 min. supine, 5 min. orthostatic at 70º and 4 min. back in supine position. Repolarization indices were measured during the three steady stages as well as during the transitions between them. When moving from supine to tilted stage, HR increased by 30% in mean, while mean shortenings of 9% for QT, 5% for Tw, and 6% for TpTe were observed. When the supine position was restored from the tilted stage, the repolarization features returned to previous basal values.
The evolution of the repolarization indices to stepwise HR changes was as follows: 1) QT and Tw presented a pronounced memory effect, with their rate adaptation being completed in two phases: a fast initial one and a subsequent slow accommodation (Time for QT to complete 90% of the change was 130±51s in mean±std when RR decreased and 174±25s when RR increased), in agreement with previous clinical studies in the literature; 2) TpTe did not present that type of adaptation, but only a fast change, practically synchronous with HR change.
Since TpTe is considered to be a measure of transmural dispersion of repolarization (time difference between completion of repolarization by epicardial and midmyocardial cells), our results indicate that transmural dispersion of repolarization is memoryless, as opposed to repolarization duration measurements. This phenomenon is consistent with previous theoretical studies which have shown that APD at different transmural ventricular layers have the same characteristic slow HR adaptation but different initial fast HR adaptation. Consequently, the unique differential manifestation that we observe in TpTe of the surface ECG is the one related to the fast adaptation. Based on this finding, we propose a method to indirectly compute differences in APD restitution slopes along the ventricular wall by making only use of the ECG: if alpha1 denotes the slope of the restitution curve for endo/epicardial cells, and alpha2 denotes the restitution slope for midmyocardial cells, both computed in a specific RR range, the difference alpha2-alpha1 can be quantified as: alpha2-alpha1= Delta _TpTe/Delta_RR, where Delta_TpTe and Delta_RR are variations in TpTe and RR, respectively, for the analyzed RR range. In our study, mean and std values of alpha1-alpha2 are 0.031±0.029, which are in agreement with theoretical studies in human ventricle (ten Tusscher 2006 model), where alpha2-alpha1 is 0.033 for the same RR range.

(Abstract Control Number: 8)