What is a left fascicular block in your heart. I have mitral valve prolapse.?

I have mitral valve prolapse and last month i had an EKG done and the doctor said that I have a left fascicular block. He said it was a normal thing, but I don't know what this block in my heart is. Should I be concerned.

Sometimes either the right or the left bundle branch conducts the electrical impulse more slowly than normal, but is not completely blocked. When this occurs, on the side of the slow conduction, the electrical impulse arrives in the ventricle slightly later than normal. As a result, the QRS complex is slightly wider than normal, but not as wide as it would be with a complete BBB. This slight widening of the QRS is often called incomplete BBB. (Another name for it is an 鈥渋ntraventricular conduction defect," or an IVCD.)
Incomplete bundle branch block sometimes indicates underlying heart disease. But, especially when it occurs on the right side (i.e., incomplete RBBB,) it often has no significance at all. So in general, incomplete bundle branch block should trigger a non-invasive search for underlying heart disease. If none is found, no further tests or treatment are needed.

The Hemiblocks
The left bundle branch itself divides into two major branches 鈥?the anterior and the posterior fascicles. Sometimes block can occur in just one of these fascicles. Such blockage is referred to as a hemiblock, or a fascicular block. (Note that the right bundle branch does not divide into discrete fascicles.)
When blockage is in the anterior fascicle, left anterior hemiblock (or left anterior fascicular block) is said to be present; whereas if the blockage occurs in the posterior fascicle, then left posterior hemiblock (or left posterior fascicular block) is said to be present.

Hemiblock generally does not widen the QRS complex, but instead changes the pattern of the QRS complex. (It alters the 鈥渁xis鈥?of the QRS complex. The axis is calculated by measuring the pattern of the QRS complex on several of the 12 leads acquired on a standard ECG.)

In general, finding a hemiblock should trigger a search for underlying heart disease. Often none is found, in which case the hemiblock can be ignored.