Variability in the location of LV endocardial activation sites affects S-wave progression. (A) More apical coupling sites of the LV anterior fascicular branch were associated with increasingly more negative S-waves in leads II, III and aVF (blue arrows). (B) Similar trends were present for a more basal coupling of the LV posterior fascicular branch in precordial leads V5 and V6 (red arrows). Variability in LV stimulation also resulted in notched QRS complexes in precordial and limb leads (black arrows). (C) QRS complexes of two healthy individuals in the PTB Diagnostic ECG Database⁸ (subjects 237 and 117, respectively), exhibiting similar QRS features. Simulated and clinical ECG grid resolutions: 40 ms/0.1 mV.