Fascicular blocks involve the trifascicular infra-Hisian conduction system. This system is composed of a network of Purkinje fibers that allows synchronous atrial activation and ventricular contraction. 

It is trifascicular, meaning there are three branches. The infra-Hisian conduction system originates at the bundle of His and includes the right bundle branch and both the left anterior and left posterior fascicles of the left bundle branch. (see Figure 1). 

Fascicular blocks are a precursor to complete AV block and may cause syncope and sudden premature death. The lesion is found near the bundle of His. Fascicular blocks and bundle-branch blocks may represent a delay in rather than a total absence of conduction.

A trifascicular block involves the right bundle branch, left anterior fascicle, and the left posterior fascicle. A bifascicular block involves the right bundle-branch and either the left anterior fascicle or the left posterior fascicle. It is also termed a hemiblock.