Bundle branch block old and new in myocardial infarction.

When left bundle branch block (LBBB) is associated with acute myocardial infarction (AMI) as many as 39.2% to 46.9% of patients may have old LBBB, as opposed to new-onset LBBB. In a typical presentations of AMI, however, the prevalence of old LBBB relative to new-onset LBBB is known, and may well be higher, given the association of old LBBB and 3-vessel coronary heart disease even in the absence of AMI.