A paper by L. Sciarra et al (1) presents evidence that LBBB with left axis deviation ( frontal plane axis between -30 and -90 degrees) differs in terms of the conduction sequence and regional left ventricular contraction patterns. They argue that patterns differs from what is observed in LBBB with a normal axis and suggest that a different approach to CRT is indicated.
Normally, with intact left bundle branch conduction the interventricular septum is activated from left to right. In LBBB the septum is activated from right to left In typical LBBB there is early activation and contraction of the septum with bulging of the left basal portion of the left ventricle which then contracts late causing a bulging of the septum. Because the lateral LV wall is the last segment to contract it has been thought best to place the coronary sinus lead in a vein draining that area of the heart to optimize CRT.
However, in LBBB with LAD the last segment to contract is the anterior wall of the LV suggesting optimal CRT might be obtained by positioning the CS lead more anteriorly .
A sub analysis from the MADIT CRT trial found that LBBB plus LAD has an increase risk of Heart failure and death and a trend ( not stat significant ) toward less benefit from CRT.
Speculation - maybe CS leads were not in optimal location.
1) Sciarra, L et al Patients with left bundle branch block and left axis deviation show a specific left ventricular asynchrony pattern Implications for lft ventricular lead placement during CRT implantation.J Electrocardiology Mar-Apr 2018:51,(2) 175