Conventionally a new onset left bundle branch (LBBB) with acute myocardial infarction (MI) is associated with a massive MI
Proximal left anterior descending artery (LAD) septal perforators perfuse the right bundle branch and the anterior fascicle of the left bundle branch ~90% of cases
The right coronary artery (RCA) perfuses the posterior fascicle of the left bundle branch ~90% of cases
Given the anatomy, a LAD occlusion should cause RBBB and/or LAFB; both a proximal LAD and RCA occlusion would be required for MI to cause LBBB
A recent cohort study analyzed 233 patients to evaluate if RBBB or LBBB was associated with a large anteroseptal scar:
RBBB was associated with larger scar size (24% vs. 6.5%; p<0.0001)
RBBB was more indicative of ischemic heart disease (79% vs. 29%; p<0.0001)
Based on this preliminary data RBBB may have a stronger association with ischemia and anteroseptal scarring than LBBB (*limitations - small cohort of cardiomyopathy patients with an EF<35%, further study is required)
References
Strauss DG, Loring Z, Selvester RH, et al. Right, But Not Left, Bundle Branch Block Is Associated With Large Anteroseptal Scar. JACC. Sept 2013; 62(11): 959-967.
