Left bundle branch block (LBB) has been defined by various EKG criteria. The pattern of inter ventricular and intraventricular desynchrony cannot be predicted nor the response to CRT based on the EKG..
A patient with a EKG pattern of LBBB may have a conduction block or delay within the His Bundle system ( intra Hissian block) or distal to the His bundle or some version of a diffuse intra-ventricular conducting defect (s) .
Various criteria have been proposed by the EKG diagnosis of LBBB but no criteria offers 100% predictive value regarding response to cardiac resynchronization therapy (CRT )but some appear better than others.
CRT is the treatment for heart failure refractory to the usual medical treatment in patients with LBBB. CRT in the form of bi-ventricular pacing is a "work around" essentially replacing one pattern of ventricular dyssynchrony with another less damaging form of non physiological pacing, while His bundle pacing is capable of correcting the abnormal conduction.
The physiological purity of His pacing is undeniable but there are problems which include technical difficulty in placing the electrode in the right place (the His bundle is small and surrounded by fibrous tissue), variably high capture thresholds and a certain percentage of cases which later on develop an undesirable high capture threshold.
Incentivized by the recognition that although His pacing was beautifully physiologic it had definite problems, innovatively adventurous EP cardiologists discovered they could advance the catheter further towards the cardiac apex and screw the lead deeply into the interventricular septum and then could pace the left bundle branch thus fixing the LBBB with the apparently minor physiological cost of inducing a right bundle branch block. Further there appeared to be more likelihood of success as precise localization into the His bundle was replaced by the much less technically demanding job of getting the catheter in the left bundle branch area, There was the concern and occasional occurrence of pushing too far and placing the lead tip into the left ventricular cavity .If recognized during the procedure the lead could be safely withdrawn.Besides EP cardiologists penetrate the atrial septum routinely performing ablations so poking a small whole in a septum did not appear to be a deal breaker and a some centers Left bundle branch area pacing hasr replaced His pacing.
The term "true LBBB" has been used to indicate the conduction defect that responds well to CRT ( either Bi V or conducting system pacing ( His pacing or left bundle branch area pacing). Further True LBBB is associated with right to left activation of the interventricular septum and a echocardiographic finding referred to as "septal flash"
An important question is which of the several EKG criteria of LBBB best predicts who has a true LBBB and therefore a greater likelihood of benefit from CRT
Calle et al compared the new,more stringent ESC criteria ( 2021) with the older 2013 ESC criteria.They suggest that certain other criteria should be considered;delayed R wave peak time,a septal flash,apcial rocking and a specific septal strain pattern.
Calle has published several papers on this general topic and seems to consider that the echo cardiographic finding of septal flash defines the " true electromechanical LBBB substrate and is predictive of complete reversible re-modeling.
1) Calle,S Defining left bundle branch block according to the new 2021 European Society of Cardiology Criteria.Neth Heart J. 03 May 2022 in their "point of view" section