All concepts, explanations, trials, and studies have been
re-written in plain English and may contain errors. I am
not a doctor
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Bundle branch block by John T. Lohr

Bundle branch block (BBB) is a disruption in the normal flow of
electrical pulses that drive the heart beat.

Bundle branch block belongs to a group of heart problems called
Intraventricular Conduction Defects (IVCD). There are two bundle
branches, right and left.

The right bundle carries nerve impulses that cause contraction
of the right ventricle (the lower chamber of the heart) and the
left bundle carries nerve impulses that cause contraction of the
left ventricle.

The two bundles start together at a junction called the bundle
of His. Nerve impulses come through the sinus node of the heart
to the bundle of His and then move into the right and left
bundle branches. Bundle branch block is a slowing or
interruption of nerve impulses. A problem may exist in any of
the three bundles.

Patients with BBB are generally without symptoms unless the
disease is severe enough to cause a complete infranodal A-V block
and very slow heart rate.

In patients with right bundle branch block (RBBB), the nerve
impulse is conducted slowly or not at all. The right ventricle
finally receives the impulse through muscle-to-muscle spread,
outside the regular nerve pathway. This method of electrical
transmission is slow and results in a delayed contraction of the
right ventricle.

There are several types of left bundle branch block (LBBB), each
different. In every case, the nerve impulse is blocked or
delayed. Patients with LBBB may have left ventricular disease
or cardiomyopathy.

Left bundle branch block is usually caused by another disease, 
such as clogged arteries, rheumatic fever, congenital heart
disease, myocarditis, or heart attack. Right bundle branch
block happens less often from an underlying heart disease.

Detection of BBB usually happens during a normal physical
exam. The block shows up as a widening of the second heart
sound. BBB is then confirmed through electrocardiogram (EKG).
The EKG will show pulses in the heart beat and their length.
A QRS lasting longer than 110 milliseconds indicates BBB.
There is a unique EKG pattern for blocks in each of the 3
bundles.

There is no specific therapy for BBB. Patients are usually
treated for associated heart diseases.

Prognosis for a person with blockage in any of the 3 bundle
branches depends on the associated heart disease. The
associated diseases determine the outcome of the patient's
health.

Occasionally, disruptions in bundle branches lead to complete
infranodal A-V block, a more serious blockage of nerve
impulses. About 2% of patients with BBB develop infranodal
A-V blockage and these patients often require implanted
pacemakers.

Definition:

QRS: A pattern seen in an electrocardiogram that indicates the
pulses in a heart beat and their duration. Variations from a
normal QRS pattern indicate heart disease.

Source: 1999 Gale Research, Inc.