I had an MI 20 years ago which created a LBBB and required a triple bypass. After that surgery and cardiac rehab, my EF was 55.
Three years ago, my heart rate while sleeping was discovered to be in the low 20’s. Emergency implant of a two-lead pacemaker (right atrium and ventricle) solved the bradycardia issue, but PVC’s gradually increased over time, and my EF degraded to 30. The PVC’s (left ventricle not contracting simultaneously with the right) were increasing because my LBBB gradually became worse. The pacemaker was replaced with a bi-ventricular (3-lead) CRT-D device which synchronized my ventricles, and solved all the issues. EF now at 55. Life is good,…no restrictions. CRT-D will need to be replaced in 8 years. I expect to be around for that. I am now 82.
If your EF of 40 is being caused by PVC’s, a CRT-D would probably help. If the cause is otherwise, it probably would not. Try to see an electrophysiologist for an assessment. Good luck.