Transient right bundle branch block unmasking anteroseptal infarction concealed by left posterior fascicular block.

A case of acute anteroseptal myocardial infarction (MI) associated with transient right bundle branch block (RBBB) and a probably persistent left posterior fascicular block is reported. When the RBBB disappeared, small initial r waves were present in the right precordial leads concealing the evidence of MI. Finally, the electrocardiographic pattern evolved to an atypical form of incomplete left bundle branch block. An hypothetic role of the middle septal fascicle in this electrocardiographic evolution is discussed.