The block in left bundle branch block (LBBB)-and this can apply to right bundle as well-may not be actually the left bundle branch, as least as it has been traditionally described.And the "block" may not mean complete disruption of the electrical cardiac system perhaps just a delay.Further amazingly, at least sometimes,electrical stimulation of the His Bundle can "fix" LBBB obtaining a normal QRS complex.
The traditional text book electrical anatomical pathway is from sa node to av node to His bundle and then a division into left and right bundle branches and then to branching purkinje fibers out to the myocardium.
Anatomical work at least as early as 1971 and physiological studies in 1977-1978 demonstrated in both the canine and human heart that the right and left bundles were actually anatomically distinct within the bundle of His.This was described as longitudinal dissociation.
It has been known at least as early as 1978 that stimulation of the his bundle can normalize the ekg pattern of LBBB and RBBB,Presumably-according to the longitudinal dissociation model - in these situations the lesion or block in the bundle branch was in the His bundle and that stimulation distal to the lesions resulted in a normal ekg.
In other words the left and right bundles are organized longitudinally and separate within the His Bundle.So a lesion in the His bundle could cause LBBB and could be corrected by stimulation distal to the block.
The work done in the 1970s did not really apply to clinical situations until cardiac resynchronization (CRT) was proven effective in the treatment of heart failure in the 1990s. About 30% of patients did not seem to respond to CRT and it was soon realized that the presence of LBBB was a favorable prognostic factor and that , in a sense it was the impaired electrical conduction system which was the culprit that caused mechanical dyssynchrony that could markedly impair cardiac function and in some after a variable lag period lead to disadvantageous cardiac hypertrophy, chamber enlargement and remodeling and heart failure
Further it has been suggested ( Strauss et al ) that the traditional ekg criteria for LBBB need to be replaced with critieria that make the diagnosis much more specific and therefore useful in predicting response from CRT.The QRS duration should be 0.14 seconds for men and 0.13 for women and there should be a double humped QRS either in lead i and AVL or V5 and V6 ( i.e. 2 contiguous leads) .Using the old criteria,according to Strauss, lead to overdiagnosis of true LBBB and included patients who actually had LVH ( left ventricular hypertrophy) and left anterior hemiblock). It was suggested that patients with "true "LBBB" by EKG are the ones who are likely to benefit from cardiac resynchronization therapy (CRT).
However, results from Denmark and Pittsburgh by Dr. Niels Risum (2) indicated that the sine qua non of "true" LBBB is not necessarily the EKG pattern but rather the mechanical activation pattern ( basically delayed left ventricular activation )which they described in terms of 2D strain echocardiography and that neither the supposedly specific Strauss criteria nor the traditional criteria did not always indicate which patients would have that contraction pattern and would therefore be more likely respond to CRT.See end note # 1for Risum criteria:
Risum and colleagues content that all patients with LBBB diagnosed on EKG do not have delayed left ventricular activation which correspond to their criteria for "classic" LBBB activation pattern.
Risum's work offers an explanation for at least some of the widely quoted 30% CRT poor response rate .Interestingly, as early as 1979 several different patterns of septal movement were described on echocardiogram in LBBB.( Fujii et al )
Their data demonstrated that during a 4 year follow-up 40% of patients without the classic pattern had an adverse event (combined end point of death,need for LVAD,or heart transplant) versus 14 % in the group with the classic pattern.
It would not be expected, on mechanistic grounds, for Bi-Ventricular pacing to improve cardiac output in patients with right bundle branch block since in RBBB the electrical activation and the contraction pattern of the left ventricle would not be abnormal.There is considerable clinical data that CRT results in RBBB are definitely worse than in LBBB; However, there may be an exception.
Rosenbaum described an RBBB pattern in which there is left axis deviation and broad slurred r waves in I and Avl which was termed LBBB masquerading as RBBB in which an argument can be made that Bi-V pacing might be on benefit.(1)
1)Auriccho, A Does Cardiac Resynchronization therapy have a role in patients with right bundle branch block.irc. Arrhymia Electrophysiol 2014 pg 532
2)Risum,N et al, Identification of typical left bundle branch block contraction by strain echocardiography is additive to electrocardiography in prediction of long term outcomes after cardiac resynchronization therapy. Journal American College of Cardioogy 2015 vol 66 no 6 pg 632
3)Fujii, J K, et al mode and cross sectional echocardiographic study of left ventricular wall motions in complete left bundle branch block. Brit Heart Journal 1979 42 (3) 255
end note 1
Risum 2d strain echo criteria:
1)early shortening of at least one segment of the septal wall atnd early stretching of at least one segment of the lateral wall
2)early septal peak shortening
3)lateral wall peak shortening after aortic valve closure