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).

Main Causes

• Ischaemic heart disease
• Hypertension
• Aortic stenosis
• Anterior MI
• Primary degenerative disease of the conducting system (Lenègre-Lev disease)
• Congenital heart disease
• Hyperkalaemia (resolves with treatment)
• Digoxin toxicity

ECG Examples

Example 1

Incomplete Trifascicular Block:

• Right bundle branch block
• Left axis deviation (= left anterior fascicular block)
• First degree AV block

Example 2

Incomplete Trifascicular Block:

• Right bundle branch block
• Left axis deviation (= left anterior fascicular block)
• First degree AV block

Example 3

Complete Trifascicular Block:

• Right bundle branch block
• Left axis deviation (Left anterior fascicular block)
• Third degree heart block

Clinical Cases

• Top 100 ECG – Case 094

Related Topics

• Left bundle branch block LBBB
• Right Bundle Branch Block RBBB
• Left anterior fascicular block LAFB
• Left posterior fascicular block LPFB
• Interventricular Conduction Delay IVCD
• Bifascicular block
• AV block: 1st degree
• Complete Heart block CHB

LITFL Further Reading

• ECG Library Basics – Waves, Intervals, Segments and Clinical Interpretation
• ECG A to Z by diagnosis – ECG interpretation in clinical context
• ECG Exigency and Cardiovascular Curveball – ECG Clinical Cases
• 100 ECG Quiz – Self-assessment tool for examination practice
• ECG Reference SITES and BOOKS – the best of the rest

Advanced Reading

• Brady WJ, Truwit JD. Critical Decisions in Emergency and Acute Care Electrocardiography
• Surawicz B, Knilans T. Chou’s Electrocardiography in Clinical Practice: Adult and Pediatric
• Wagner GS. Marriott’s Practical Electrocardiography 12e
• Chan TC. ECG in Emergency Medicine and Acute Care
• Rawshani A. Clinical ECG Interpretation
• Mattu A. ECG’s for the Emergency Physician
• Hampton JR. The ECG In Practice, 6e