Bifascicular and trifascicular blocks keeps coming up in the college exams, for instance Question 15.1 from the second paper of 2017. It is a situation where both the right bundle branch AND one of the left bundle branch fascicles is not conducting. To make it trifascicular, one also needs to have a prolonged PR interval.
Here's a block from the author's own collection.
RBBB and LAFB
The ventricles depolarize from the single remaining fascicle.
- This is a sign of extensive conducting system disease
- The example above is RBBB + LAFB:
Small Q waves and tall R waves in Lead I and aVL
Small R waves and deep S waves in Lead II, Lead III and aVF
But wait! … Isnt Left Bundle Branch Block (LBBB) a bi-fascicular block? Both the anterior and posterior fascicles are blocked!
Yes. Yes it is. In fact the guidelines from the European Society of cardiology include LBBB in their guidelines for management of bifascicular block.
Its Bifascicular Block – with the important addition of a prolonged PR interval (1st degree AV block).
How does this endanger my patient?
It may progress to complete heart block, and kill them ( 1% per year progress this way).
- If this ECG presents with a history of syncope, most would argue in favour of a pacemaker.
- In fact, if there is no reversible cause, a pacemaker is ideal.
- If a pacemaker is needed, make it a dual chamber
Looking for fascicular block whenever you see right bundle branch block
Whenever one sees RBBB, one may wish to look for additional fascicular block.
RBBB should not have any axis changes.
If there is RBBB with a right axis deviation, the posterior fascicle may be blocked as well.
(Lead III will have tall R waves)
If there is RBBB with a left axis deviation, the anterior fascicle may be blocked as well.
(Lead III will have deep S waves)