The influence of delayed ventricular activation on cardiac repolarization: insights from electrocardiographic imaging

In the normal human heart, the polarity of the T-wave is concordant with the QRS-complex. This phenomenon is often explained by the negative relationship between the activation (AT) and repolarization time (RT). Patients with a delayed ventricular conduction often present with discordant T-waves, but the link to regional repolarization is unknown.

To compare the epicardial RT-AT relationship in patients with and without conduction disorders using non-invasive electrocardiographic imaging (ECGi).

Patients with narrow QRS complex (nQRS, n=13), left bundle branch block (LBBB, n=8) or intraventricular conduction delay (IVCD, n=10) were included. ECGi was performed using a non-commercial system utilizing ~200 chest-leads. Epicardial potentials were reconstructed and clustered into 24 epicardial segments using averaging (Figure 1A). RT-AT relationships were evaluated for these segments using linear regression analysis. Maximum (max) AT and RT were defined as the maximum AT or RT segment respectively. Results are presented as median ± IQR. Statistical evaluation was performed using the Kruskal Wallis (differences between groups) and Wilcoxon signed-rank test (differences between left (LV) and right ventricle (RV) within groups). Significance was defined as p≤0.05.

(Figure 1B) Max AT and RT differed significantly between nQRS, IVCD and LBBB patients. An interventricular difference in max AT and RT was present for nQRS, LBBB and IVCD patients with a longer max AT and RT in the LV compared to the RV. The nQRS group demonstrated an overall negative RT-AT slope with high variability and low correlation coefficient (r = 0.047, p = 0.41), while the slope was positive in the majority of LBBB (r = 0.69, p ≤ 0.001) and IVCD (r = 0.55, p ≤ 0.001) patients. The RT-AT slope was significantly different between groups. Furthermore, the RT-AT slope was similar in the LV and RV for nQRS and IVCD patients, but was significant increased in the RV compared to the LV for LBBB patients.

This study demonstrates that in nQRS rhythms with concordant T waves, the physiological assumption that sites of late activation repolarize first might not be consistently present. Furthermore, delayed ventricular activation (IVCD and LBBB) with discordant T waves on the surface ECG, is associated with delayed completion of repolarization.