Fascicular Blocks

Abstract

The concept of fascicular blocks is based on the predominantly trifascicular infrahisian ventricular conduction system, the right bundle branch on one side and the left anterior and left posterior fascicle forming the left bundle branch on the other side (Figure 9.1). The expression fascicular block is synonymous with the older expression hemiblock. This is reasonable because, in approximately 10% of the human intraventricular conduction system, there is a third left ventricular fascicle called medial fascicle of a plurifascicular system, and this impairs diagnostic accuracy. Generally, the lesion responsible for the block is localized very proximally in the fascicle, near the His bundle. Rarely, in some individuals, fascicular blocks progress to bifascicular/bilateral blocks: right bundle-branch block (RBBB) + left anterior fascicular block (LAFB) or RBBB + left posterior fascicular block (LPFB) (see Chapter 11) and then to complete atrioventricular block (see Chapter 12). Thus, fascicular blocks represent potential precursors of infrahisian complete atrioventricular block, which is often associated with syncope and premature, sudden death.