Here is an attempt for a brief answer that starts with query.
1.The mechanism of LBBB in DCM is
1.Discrete proximal LBBB
2.Diffuse distal LBBB
3. Myocardial LBBB (It is just a LBBB morphology in ECG, due to delayed conduction and not a true LBBB )
4. It is always a combination of any of the above three components.
Answer :Either 3 or 4 should be the answer I think .It need to be reminded, the etiology of cardiomyopathy plays a crucial role in determining the cause of LBBB. In ischemic DCM, it could be localized necrosis and scarring . In infiltrative disease like sarcoid or Amyloid it is patchy destruction of conduction system. In idiopathic or genetic DCM, it could be a defect in various forms of structural or functional Inopathy.
2.Does LBBB gets corrected by left bundle branch pacing ?
No, it doesn’t correct it in all . This fact, by itself tell us what could be the true mechanism of LBBB in dilated cardiomyopathy. In selected patients with discrete proximal LBBB the strength of the pacemaker stimuli might breakthrough the block. Though the concept longitudinal splitting of left bundle and the proximal pre-destined left bundle fibre disease is attractive, mind you, bulk of LBBBs are diffuse. LBB pacing can’t work in majority of these patients.
Hence, it is largely a desired assumption ,that LBB pacing could correct the LBBB and restore the defective synchronization . It is true pacing near the bundle branches carry better chance of synchronous contraction, provided distal circuits are perfect. (Which is more often, not the case) Unfortunately, while technology allow us to replace a defective valve or even transplant a entire heart. we can’t replace a defective ventricular electrical circuit board with a brand new printed Purkinje network in lieu of the diseased one. At best, we can try to stimulate the heart it two or three sites . (Mind you, the natural Purkinje network runs for few kilo-meters of indicate wiring)
Is LBB area pacing or CSP a real Innovation ?
May be yes. But, until the the mechanism of LBBB in a given patient with cardiac failure is completely understood , we can’t choose a right mode of pacing and re-synchronisation modality. Unfortunately ,there appears to be a herd mentality, gradually creeping in many of us, to jump over from traditional RV /RA pacing to the bundle branch area pacing , as an alternative to CRT or even regular bradycardia pacing.
There is less compelling academic reasons to change, than we think. There are lot of, ‘if and buts” in this Innovation.
It is worth-while to wait, watch and think before we adopt this modality for mass consumption in heart failure with wide qrs complex.
Dr.S.Venkatesan MD 