traveling along the BUNDLE OFHISor one of the
bundle branches, right or left, in the HEART. The
bundle branches focus and intensify the pacing
signals that originate in the SINOATRIAL(SA) NODE,
concentrating them enough to stimulate and syn-
chronize the powerful contractions the ventricles
need to eject BLOODfrom the heart. Various factors
can block this electrical pathway. Among the most
common are CORONARY ARTERY DISEASE(CAD), VA LV U-
LAR HEART DISEASE, HEART FAILURE, and CARDIOMYOPA-
THY. These conditions result in abnormal blood
supply to the myocardial cells, interfering with
their normal functions. BUNDLE BRANCHblock also
can exist without an identifiable cause in people
who have no apparent heart disease.
Bundle branch block typically shows up on an
ELECTROCARDIOGRAM(ECG) though often does not
cause symptoms. The heart continues to contract
and pump blood normally (unless other heart dis-
ease interferes) because factors other than electri-
cal stimulation contribute to heart function.
However, the slowed, delayed, or interrupted flow
of the electrical pacing impulse can cause a slow
heart rate (bradycardia) or other types of ARRHYTH-
MIA. The location of the blockage can disrupt the
synchronized contractions of the ventricles, caus-
ing one to contract before the other instead of
both contracting simultaneously. Often, the bun-
dle branch block requires only regular monitoring,
not treatment. The location and extent of the
block determines the approach. When the block is
fairly extensive, a PACEMAKERmay be necessary to
regulate the heart’s electrical activity. Bundle
branch block that coexists with other forms of
heart disease may require careful coordination of
therapeutic measures to preserve overall cardiac
function to the greatest extent possible.
See also SICK SINUS SYNDROME.bundle of His The bundle of NERVEfibers that
conveys the heart’s electrical pacing impulse from
theATRIOVENTRICULAR(AV) NODEto the ventricles,
also called the bundle branches. The short trunk
portion of the bundle before it splits into the right
BUNDLE BRANCH(right bundle of His) and left bun-
dle branch (left bundle of His) is the main bundle
of His. The German physician Wilhelm His (1863–
1934) discovered the bundle branches in 1893.
Doctors may use the terms bundle of His and bun-
dle branch interchangeably.
For further discussion of the bundle of His
within the context of cardiovascular structure and
function, please see the overview section “The
Cardiovascular System.”
See also ARRHYTHMIA; BUNDLE BRANCH BLOCK;
HEART; SICK SINUS SYNDROME.26 The Cardiovascular System