Heart blocks

Question 1

What is a true trifascicular heart block?

Answer 1

Conduction delay in all 3 fascicles below the AV node

Question 2

Name the 3 fascicles below the AV node

Answer 2

Right bundle branch

Left bundle branch (further subdivides into left anterior, left posterior and septal)

Question 3

How does a trifascicular block manifest? What are the ECG findings?

Answer 3

Bifascicular block & 3rd degree AV block

On ECG
- 3rd degree AV block + RBBB + LPFB

OR

• 3rd degree AV block + RBBB + LAFB

Question 4

In a trifascicular block, where does the escape ventricular rhythm normally arise from?

Answer 4

Left anterior or left posterior fascicle

Produces QRS complexes with the appearance of RBBB plus either LAFB or LPFB

Question 5

Name other rare indicators of trifascicular block

Answer 5

Normal sinus rhythm with alternating RBBB/LBBB

RBBB with alternating fascicular blocks on a beat-to-beat basis

Question 6

Which condition is the term trifascicular block commonly used to describe?

Answer 6

Bifascicular block associated with 1st degree or 2nd degree AV block

This is a clinical misnomer

Question 7

What are the clinical implications of a trifascicular block?

Answer 7

True trifascicular = pacemaker insertion

Bifascicular with syncope/pre-syncope = admission for monitoring and likely pacemaker insertion (class II)

Asymptomatic bifascicular with 1st degree AV block = not an indication for pacing

Question 8

What are the main causes of a trifascicular block?

Answer 8

Ischaemic heart disease (40-60% of cases)

Structural heart disease (50-80% of cases)

Aortic stenosis

Anterior MI

Lenegre-Lev disease

Congenital heart disease

Hyperkalaemia (resolves with Tx)

Digoxin toxicity