Dr. Saer Abu-Alrub et al (1) offer one explanation for the occurrence of left axis deviation (LAD) in some cases of left bundle branch block (LBBB).
Twenty nine patients with non-ischemic cardiomyopathy were studied with non invasive cardiac mapping, CT and MR imaging .Sixteen had a normal QRS axis while 13 had LAD .The LAD group demonstrated delayed activation of the basal anterolateral region.The LAD group demonstrated an apex to base activation pattern versus a circumferential pattern observed in the normal axis group.
Imaging studies demonstrated no differences in cardiac structure between the two groups so the investigators concluded that LBBB with LAD was a "purely electrical phenomenon".
Some LBBB patients with LAD treated with CRT ( either by BIV or conduction system pacing)have normalization of the QRS duration and of the electrical axis while others have only the QRS normalized with the left axis unchanged. I have been unable to find any explanation of that.
1) Abu-Alrub,S et al Left axis deviation in patients with non ischemic heart failure and left bundle branch block is a purely electrical phenomenon. Heart Rhythm, 2021 April 33831543
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