Trifascicular Block

Definition

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 evidence of 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

Incomplete trifascicular block

Complete trifascicular block

The most common pattern referred to as “trifascicular block” is the combination of bifascicular block with 1st 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 (Lenegre’s / Lev’s disease)
  • Congenital heart disease
  • Hyperkalaemia (resolves with treatment)
  • Digoxin toxicity

 

Example ECGs

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

 

Related Topics

Further Reading

Author Credits

References

  • Hampton, JR. The ECG in Practice (5th edition), Churchill Livingstone 2008.
  • Surawicz B, Knilans T. Chou’s Electrocardiography in Clinical Practice (6th edition), Saunders 2008.
  • Wagner, GS. Marriott’s Practical Electrocardiography (11th edition), Lippincott Williams & Wilkins 2007.
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Comments

  1. says

    Dear Edward Burns
    thanks for nice article I am bit confused regarding third degree AV block and trifescular block because Browanld says it is delayed conduction rather complete failure of impulse to go down
    can clarify this

    Dr Umesh R Bilagi

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