Trifascicular Block

Definition of Trifascicular Block

Trifascicular block (TFB) refers to the presence of conducting disease in all three fascicles:

Right bundle branch (RBB)
Left anterior fascicle (LAF)
Left posterior fascicle (LPF)

Incomplete vs complete TFB

Trifascicular block can be incomplete or complete, depending on whether all three fascicles have completely failed or not.

Incomplete trifascicular block

Incomplete (“impending“) trifascicular block can be inferred from one of two electrocardiographic patterns:

Fixed block of two fascicles (i.e. bifascicular block) with delayed conduction in the remaining fascicle (i.e. 1st or 2nd degree AV block).
Fixed block of one fascicle (i.e. RBBB) with intermittent failure of the other two fascicles (i.e. alternating LAFB / LPFB).

Complete trifascicular block

Complete trifascicular block produces 3rd degree AV block with features of bifascicular block.
This is because the escape rhythm usually arises from the region of either the left anterior or left posterior fascicle (distal to the site of block), producing QRS complexes with the appearance of RBBB plus either LPFB or LAFB respectively.

Patterns of TFB

The most common pattern referred to as “trifascicular block” is the combination of bifascicular block with 1st degree AV block.

Incomplete trifascicular block

Bifascicular block + 1st degree AV block (most common)
Bifascicular block + 2nd degree AV block
RBBB + alternating LAFB / LPFB

Complete trifascicular block

Bifascicular block + 3rd degree AV block

NB. For patients with the combination of bifascicular block plus 1st or 2nd degree AV block it is usually impossible to tell from the surface ECG whether the AV block is at the level of the remaining fascicle (a “true” trifascicular block) or at the level of the AV node (i.e. not technically a trifasicular block).

Clinical Implications

Incomplete trifascicular block may progress to complete heart block, although the overall risk is low.
Patients who present with syncope and have an ECG showing incomplete trifascicular block usually need to be admitted for a cardiology work-up as it is possible that they are having episodes of complete heart block. Some of these patients will require insertion of a permanent pacemaker (class II indication).
Asymptomatic bifascicular block with first degree AV block is not an indication for pacing (class III).