| dc.description | The concept of "reverse sequence of repolarization" is traditionally presented to account for the mainly concordant polarity of the T wave, with respect to the QRS complex in the body surface ECG during normal ventricular conduction. However, it is generally observed that in complete bundle branch blocks the T waves are discordant, i.e. of opposite polarity to the QRS complex, suggesting a concordant rather than reverse sequence of repolarization. The present investigation was initiated as a systematic effort to investigate the influence of spatial sequence of left ventricular excitation on the spatial/temporal sequence of repolarization in terms of concordance and discordance and its effects on the QT interval of the electrocardiogram. ECG data files of various North American population groups collected between the years 1976-1984 were used to construct vectorcardiograms and QRS-T correlation maps for the orthogonal X, Y and Z leads in normal ventricular conduction and in various categories of ventricular conduction defects. Several linear regression models were introduced to assess the contribution of the QRS duration to QT and JT intervals. The results indicate that the QRS duration has a small but significant influence on the QT interval in normal ventricular conduction, this influence becoming pronounced in complete bundle branch blocks. This finding suggests that even in normal conduction, repolarization is not universally reverse. It becomes prominently concordant in complete bundle branch blocks, particularly in left bundle branch block and complete bundle branch blocks of undetermined type. The local spatial direction of the excitation fronts may play an important role in determining excitation-repolarization relationships. It is suggested that in normal conduction, repolarization is mainly reverse. However, repolarization is likely to change from reverse to at least partially concordance sequence during repolarization of the posterior basal left ventricular wall, and possibly in portions of the ventricular septum where there is a higher likelihood of excitation propagating in an apex to base direction. In the left bundle branch block the repolarization process becomes mainly concordant, as regional/temporal sequence of excitation is mainly in a apex to base direction. A large fraction of the observed total QT interval variability remains unexplained. It is suggested that local and global differences in action potential duration gradients with regional and global contractility and other factors in addition to heart rate to be carefully considered in future development of more advanced repolarization models. | en_US |
