In the study “Relation of new permanent right or left bundle branch block on short- and long-term mortality,” the investigators present a study of 5,570 patients who were diagnosed with acute myocardial infarction (MI), of which 964 had documented bundle branch block (BBB; either preexisting, unknown, new permanent, or new transient). They reported that patients with BBB had higher 30-day and long-term mortality than patients without evidence of BBB, particular those with new permanent right or left BBBs. New left BBB (LBBB) reportedly showed the greatest 1-year and long-term mortality in the groups. However, we wonder whether the presence of LBBB was actually the result of acute ischemia or whether its presence was entirely coincidental, reflecting the already diseased conduction system of these patients.

The fact that right BBB occurs far more frequently in the setting of acute MI compared with LBBB has been established for several decades. A study published in 1970 by Norris et al describes a series of patients diagnosed with BBB in the setting of acute MI. Autopsy findings revealed that most patients discovered to have proximal left anterior descending artery occlusion also had electrocardiographic evidence of right BBB before death. Those who had LBBB were more likely to have diffuse coronary atheroma and less likely to have primary left anterior descending artery or left main occlusion on autopsy. Given what we know of the vast structure and coronary perfusion of the left bundle conduction system, it would conceivably require a catastrophic ischemic event across multiple coronary artery territories to knockout this conduction pathway.

The investigators in the present study admit that “new LBBB had an anterior location MI less frequently” which is consistent with the findings of previous studies. We would go further to propose that the development of “presumably new” LBBB, whether transient or permanent, in the setting of acute infarction in those patients had little, if anything to do with their ischemic event, and were solely a manifestation of their diffuse, underlying conduction system disease that was present before their infarction.