|Trifascicular heart block|
|Classification and external resources|
Trifascicular block is a term that describes a problem with the electrical conduction of the heart. Due to its multiple meanings, the term has lost favor with cardiologists and experts in electrophysiology.
Key to understanding these definitions is that the electrical conduction system contains three fascicles that carry electrical signals from the atrioventricular node to the ventricles: the right bundle branch, the left anterior fascicle and the left posterior fascicle. The left anterior fascicle and left posterior fascicle are together referred to as the left bundle branch. "Block" at any of these levels can cause an abnormality on an electrocardiogram.
The most literal meaning, therefore, of trifascicular block is complete heart block: all three fascicles are blocked.
A second, and clinically distinct, definition of trifascicular block is a circumstance in which right bundle branch block (RBBB) and left bundle branch block occur in the same patient, but at distinct points in time. For example, a patient that is found to have a RBBB one day and a LBBB another can be said to have "alternating bundle branch blocks". In this context, because all three fascicles show evidence of block at different points in time, the term trifascicular block is often used.
Finally, the third meaning of trifascicular block refers to a specific finding on an electrocardiogram in which bifascicular block is observed in a patient with a prolonged PR interval (first degree AV block).
So, as should be apparent by this point, the term trifascicular block should be discontinued from clinical use. The treatment of trifascicular block is highly dependent on which clinical entity (one of the three above) is being described.
An electrophysiology study of the conduction system can help discern the severity of conduction system disease. In an electrophysiology study, trifascicular block due to AV nodal disease is represented by a prolonged AH interval (denoting prolonged time from impulse generation in the atria and conduction to the bundle of His) with a relatively preserved HV interval (denoting normal conduction from the bundle of His to the ventricles). Trifascicular block due to distal conduction system disease is represented by a normal AH interval and a prolonged HV interval. In the absence of symptoms, a prolonged AH interval is likely benign while a prolonged HV interval is almost always pathologic.
An implantable cardiac pacemaker or permanent pacemaker is recommended in the following clinical circumstances. Class 1 recommendation is the strongest recommendation. Level A evidence is the highest level of evidence.
- Bifascicular block + complete heart block, even in the absence of symptoms (1b)
- Bifascicular block + 2nd degree AV Block Type 2, even in the absence of symptoms (1b)
- Alternating bundle branch blocks, even in the absence of symptoms (1c)
Class III (i.e. pacemaker not recommended)
- http://circ.ahajournals.org/content/117/21/e350 - old guidelines, in which trifascicular block terminology is used.
- http://circ.ahajournals.org/content/97/13/1325.long - new guidelines