So how does cardiac conduction system really work?

 Using recording catheters in the left ventricle, Upadhyway et al (Circulation 2019) demonstrated that the location of the blockage or slowing of the electricity  in patients with surface EKG findings of LBBB was not predictable from the morphology of the surface EKG pattern of left bundle branch block (LBBB). This was the case regardless which criteria for the EKG diagnosis of LBBB was used, i.e. either the Strauss criteria or those recommended by the ACC/AHA/HRS criteria. In some cases there was no detectable blockage in the Purkinje system and therefore in those cases conduction system pacing would not correct the abnormal EKG pattern nor any associated  cardiac functional impairment in those patients. 

Dr. R Mahmud and S, Jamal studied the effect of nonselective His Bundle Pacing ( NS HBP) in 41 patients with LBBB. In 23 of those NS HBP decreased the prolonged PLWAT ( peak lateral wall activation time) normalized the QRS axis and increased Lead I voltage and normalized the Left axis deviation and resolved the mid ORS notch.  

The theory of longitudinal dissociation has become the standard explanation for how pacing in the His bundle could correct the conduction defect thought to exist in patients with the EKG pattern of LBBB.If the blockage was actually anatomically  present in the left bundle  how could pacing in the Bundle of  His  which is proximal to the  blockage in Left Bundle normalize the EKG. The secret is said to be that the fibers that are predestined to be those in the Left Bundle are already anatomically distinct within the His bundle and a lesion or a something that slows electrons  a happen in the His and the electrode is placed distal to that problem site and viola the impulse travels down The distal HIS and to the Left bundle .

It is now thought that the majority of LBBB cases are due to blockage or slowing of electricity in the His Bundle 

Mahmud has also demonstrated that NS HBNP can correct  the pattern of RBBB by " activating early the delayed right ventricular free wall depolarization." NS HBP produces a fusion beat with the fusion of the activation  early anterior septal wall with varying degree of His Bundle activation.