Alternating and co-existing block in the divisions of the left bundle branch.

The tracings of two patients with acute myocardial infarction showing complete right bundle branch block and alternating block in the divisions of the left branch are presented. Both developed a type II (Mobitz) block which was attributed to a simultaneous interruption of conduction in the three main intraventricular tracts (trifascicular block). Left divisional blocks produced specific changes in the initial QRS vectors. Septal activation was considered to be a function of both divisions, rather than of the left bundle branch, a view departing from conventional electrocardiographic theory.