An in-silico analysis of the influence of left bundle branch block and myocardial fibrosis on electrocardiographic QRS duration and morphology and vectorcardiographic QRS area

BACKGROUND AND AIMS: Vectorcardiographic (VCG) QRS area has shown great promise as a predictor of cardiac resynchronization therapy outcomes. However, underlying conduction factors influencing the genesis of QRS area and its relationship with myocardial fibrosis remain unexplored. This study aimed to investigate the impact of left bundle branch block (LBBB) and fibrosis on QRS duration and morphology in comparison to QRS area through an in-silico analysis. METHODS: ECGsim was used to simulate four scenarios: normal ventricular activation without (1) and with fibrosis (2), and LBBB without (3) and with (4) fibrosis. Conduction patterns were modified to represent LBBB, while regional conductivity was modified to model focal fibrosis. 12-lead electrocardiograms and reconstructed VCGs were used to calculate QRS duration and area, and to assess QRS morphology. RESULTS: Myocardial fibrosis induced slight QRS notching during normal ventricular activation, but did not alter QRS morphology during LBBB activation. Furthermore, inducing regional fibrosis during normal ventricular activation hardly changed QRS duration (90 to 96 ms), while QRS area slightly decreased (37 to 32 mV.ms). LBBB without fibrosis increased QRS area compared to normal ventricular activation (90 to 145 mV.ms). Notably, inducing fibrosis in the LBBB scenario had little effect on QRS duration (145 to 142 ms) but significantly reduced QRS area (146 to 105 mV.ms). CONCLUSION: A LBBB activation, and not myocardial fibrosis, induces a large QRS area. In the presence of LBBB, myocardial fibrosis cannot be clearly identified with QRS morphology and has a limited effect on QRS duration but significantly reduces QRS area.