The vast majority of patients with an EKG pattern of LBBB have structural heart disease,that is myocardial disease as opposed to disease only involving the conduction system which is found in a very small minority-usually estimated to be about 1 % of LBBB cases.
It has been recognized that some patients with isolated or lone LBBB will over a variable period of time develop cardiomyopathy.The few clinical reports of this condition suggests delays of up to over twenty years and often dramatic resolution of heart failure with cardiac resynchronization therapy. ( ref 2)
LBBB is associated with inter-ventricular and intra-ventricular loss of synchrony, the later being more deleterious to cardiac function. Normally the various areas of segments of the left ventricle contract in sync. In LBBB the typically described or stylized pattern of abnormal septal motion is : there is early contraction of the septal wall with concomitant bulging of the lateral left ventricular wall followed by late contraction of the lateral wall and bulging of the septum.The delayed ventricular contraction is followed by delayed relaxation of the left ventricle.
Han et al ( ref 1) using tagged cine cardiac MR described 2 different patterns of abnormal septal movement in patients with LBBB.Interestingly in 2 of the 10 patients there was no septal dyskinesia at all. In some cases ( 50%) of LBBB there was dyskinesia of the entire septum while in others only the anterior septum was dyskinetic.This may at least partially explain why some patients with LLLB have little if any exercise intolerance over many years while other notice a decrease in exercise capacity soon after the onset of LBBB as was the situation in my own case and two others with whom I have had blog level correspondence.
Also animal experiments inducing LBBB have shown a rapid and often significant decrease in left ventricular function soon after the onset of LBBB.Perhaps the key is the presence or absence of the normal left to right activation of the interventricular septum and the degree to which conduction across the septum is delayed.The abnormal septal movement is the basis for the echocardiographic finding of "septal flash"
Of course the real game changer element of LBBB story is the capability of His Bundle pacing to "fix" LBBB , at least in a significant number of patients with LBBB.
1) Han Y et al Circumferential myocardial strain in cardiomyopathy with and without Left bundle branch block. J. of cardiovascular magnetic resonance .Jan 5 2010, doi:101186/1532-42.9x-12.7
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2) Vaillent C et al Resolution of left bundle branch block induced cardiomyopathy by cardiac resynchronization therapy. J Am College Cardiology 2013 vol 61 1089