Trifascicular Block

Diagnostic criteria

The ventricular 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.

Impending trifascicular block

Other rare indicators of trifascicular block include:

• Normal sinus rhythm with alternating LBBB/RBBB
• RBBB with alternating fascicular blocks on a beat-to-beat basis

These herald impending failure of all three fascicles and associated 3rd degree AV block.

A clinical misnomer

The term “trifascicular block” is most commonly used clinically to describe bifascicular block associated with 1st degree AV block or 2nd degree AV block.

• This term is inaccurate, as the conduction delay resulting in PR interval prolongation usually occurs in the AV node, not in the third remaining fascicle
• There is no difference in progression to complete heart block in patients with bifascicular block versus those with bifascicular block and associated 1st degree AV block or 2nd degree Mobitz I block (1% risk per year)
• The American Heart Association/American College of Cardiology Foundation/Heart Rhythm Society (AHA/ACCF/HRS) recommends against the use of the term “trifascicular block”, as it does not have a unique anatomical association and does not accurately reflect the underlying anatomical disease process

Clinical Implications

• Patients with true trifascicular block (associated third degree AV block) will require pacemaker insertion
• Patients with bifascicular block and syncope or presyncope should be admitted for monitoring and likely pacemaker insertion (class II)
• Asymptomatic bifascicular block with first degree AV block is not an indication for pacing (class III)

Main Causes

Causes are similiar to those of bifascicular block, related to degeneration or conduction impairment in fascicles below the AV node:

• Ischaemic heart disease (40-60%)
• Structural heart disease (50-80% association)
• Aortic stenosis
• Anterior MI
• Lenègre-Lev disease
• Congenital heart disease
• Hyperkalaemia (resolves with treatment)
• Digoxin toxicity

ECG Examples

Example 1

True Trifascicular Block:

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

Example 2

Example 3 (previously termed “trifascicular block”)

Bifascicular block + first degree AV block

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

Commonly referred to clinically as “trifascicular block”, this represents conducting disease of two fascicles, with delayed conduction through either the AV node or the remaining third fascicle. It is not possible to differentiate between the two based on the ECG alone

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

Further reading

Online

• Wiesbauer F, Kühn P. ECG Yellow Belt online course: Become an ECG expert. Medmastery
• Wiesbauer F, Kühn P. ECG Blue Belt online course: Learn to diagnose any rhythm problem. Medmastery
• Rawshani A. Clinical ECG Interpretation ECG Waves
• Smith SW. Dr Smith’s ECG blog.

Textbooks

• Mattu A, Tabas JA, Brady WJ. Electrocardiography in Emergency, Acute, and Critical Care. 2e, 2019
• Brady WJ, Lipinski MJ et al. Electrocardiogram in Clinical Medicine. 1e, 2020
• Straus DG, Schocken DD. Marriott’s Practical Electrocardiography 13e, 2021
• Hampton J. The ECG Made Practical 7e, 2019
• Grauer K. ECG Pocket Brain (Expanded) 6e, 2014
• Brady WJ, Truwit JD. Critical Decisions in Emergency and Acute Care Electrocardiography 1e, 2009
• Surawicz B, Knilans T. Chou’s Electrocardiography in Clinical Practice: Adult and Pediatric 6e, 2008
• Mattu A, Brady W. ECG’s for the Emergency Physician Part I 1e, 2003 and Part II
• Chan TC. ECG in Emergency Medicine and Acute Care 1e, 2004