["The Journal of Physiology, Volume 604, Issue 13, Page 5378-5390, 1 July 2026. ", "\nAbstract figure legend This study examines the relationships between activation and repolarization times in epicardial reconstructed unipolar electrograms derived from ECG‐imaging in patients with normal and delayed ventricular activation. In patients with a narrow QRS‐complex, ECG‐imaging reveals significant variability in the activation–repolarization time relationship at the epicardium. The physiological assumption that late‐activated regions have a shorter repolarization interval may thus not always be valid. Patients with delayed ventricular activation consistently demonstrate a positive activation–repolarization time relationship on ECG‐imaging: regions of early activation correspond to shorter repolarization intervals, whereas regions of late activation correspond to longer repolarization intervals.\n\n\n\n\n\n\n\n\n\nAbstract\nIn healthy human hearts, the T‐wave is concordant with the QRS‐complex, which is often explained by the negative relationship between the activation (AT) and repolarization time (RT). Delayed ventricular conduction is typically accompanied by discordant T‐waves, but the link to regional repolarization is unknown. The present study aimed to compare AT‐RT relationships in patients with narrow QRS‐complexes (nQRS) vs. patients with left bundle branch block (LBBB) or intraventricular conduction disturbances (IVCD) using electrocardiographic imaging to reconstruct epicardial unipolar electrograms. Evaluations included the slopes of AT‐RT relationships, maximum (max) AT and RT, ventricular electrical uncoupling (VEU; the difference of mean AT or RT between the left and right ventricle), and the standard deviation of AT (SDAT) and RT (SDRT). In nQRS patients, the AT‐RT slope was highly variable (−0.370 ± 1.125, P = 0.193 ± 0.212), whereas it was consistently positive in IVCD and LBBB patients (1.220 ± 0.907 and 0.968 ± 0.325, P = 0.0172 ± 0.0254 and P = 0.0219 ± 0.0657 respectively). Max AT, max RT, AT‐VEU, RT‐VEU, SDAT and SDRT were similar for IVCD and LBBB patients and significantly higher compared to nQRS patients. In patients with delayed ventricular activation, there was a positive association between AT‐VEU and RT‐VEU (r = 0.549, P = 0.0149). The present study demonstrates that, in nQRS rhythms with concordant T‐waves, the physiological assumption that sites of late activation have shorter repolarization intervals might not be consistently present at the epicardium. Furthermore, delayed ventricular activation is associated with delayed completion of repolarization and a dyssynchronous activation is accompanied by a dyssynchronous repolarization.\n\n\n\n\n\n\n\n\n\nKey points\n\nThis study examines the relationships between activation and repolarization times in epicardial reconstructed unipolar electrograms derived from ECG‐imaging in patients with normal and delayed ventricular activation.\nIn patients with a narrow QRS‐complex and concordant T‐waves on a 12‐lead ECG, ECG‐imaging reveals significant variability in the activation–repolarization time relationship. The physiological assumption that late‐activated regions have a shorter repolarization interval may thus not always be valid on the epicardium.\nPatients with delayed ventricular activation typically exhibit discordant T‐wave polarity on a 12‐lead ECG and consistently demonstrate a positive activation–repolarization time relationship on ECG‐imaging: regions of early activation correspond to shorter repolarization intervals, whereas regions of late activation correspond to longer repolarization intervals.\n\n\n"]