The bundle of his divides into two
- Anterior fascicle
- Posterior fascicle
- Middle septal fascicle*
Middle fascicle * Many dispute it’s presence . One may wonder , how can anatomy be under dispute ? If you cut a heart you should be able to clear the dispute . But medicine is not that simple . . . What you do not see may be more important than what we see.
The anterior fascicular block (LAFB) is one of the common conduction disorder. It ‘s significance : Can be a benign or a dangerous entity depending upon the clinical situation .The problem with LAFB is , it is diagnosed primarily by the axis shift it inflicts on the QRS complex.
In a strict sense, it is not a ideal way .There is a tendency to label all significant left axis (> -60*) deviations as LAFB. This practice has made diagnosing LAFB very common in elderly, hypertensives, etc. In these situations it may not mean anything , except to suggest a delay in conduction in left anterior fascicle.
If we filter out all these benign axis shift ECGs , the true organic pathological LAFB may not be that common .
Organic , LAFB occurs in the following situations.
- Degenerative blocks (Part of Lev & Lenegre’s disease)
- Aortic valve disease .
- Hypertensive heart disease
- Post MI (Either alone or part of bifascicular or trifascicular block )
- In association with dilated cardiomyopathy
Even in degenerative , ischemic conduction defects LAFB is far more common than LPFB why ?
The traditional explanations are
- Anterior fascicle is relatively sub epicardial in location
- It is a long and thin structure prone to damage easily
- Exposed to the mechanical stress of LVOT **
- Anterior fascicle has only a single blood supply(LAD)
** Which experiences the peak LV pressure at > 100mhg and a dp/dt up to 2000mmhg (While, the posterior fasicle is located away in the inflow portion of LV , which is exposed to low pressure – at best 10mmhg filling pressure )