Diagnosis of Isolated and Combined Block

T HE HIS BUNDLE is a narrow, cylindrical, structure which connects the atrioventricular (A-V) node with the bundle branches (fig. 1) and is composed of a penetrating and a branching portion. The former begins in the A-V node and ends where it emits the posteroinferior fibers of the left bundle. The branching portion extends from the latter site to the origin of the right branch (which is also close to where the anterosuperior fibers of the left bundle are emitted). Conduction disturbances occurring in the specialized tissues located between His bundle and ventricles can produce A-V block as well as QRS abnormalities.' Full understanding of the morphological changes produced by isolated and combined block in the bundle branches and their divisions is important in view of their prognostic and therapeutical implications.2