Abstract
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.
