For a better overview, the leads aVL and V2-V4 are not shown in this ECG. The basic rhythm is atrial fibrillation (no P waves or flutter waves visible, but fibrillation waves). When the conduction rate drops, the QRS complexes are narrow. Faster conduction results in wide QRS complexes with LBBB morphology. This is an example of phase 3 (acceleration dependant) LBBB.