Bundle Branch Block, Animation.


Bundle branch blocks happen when there is
an obstruction in one of the bundle branches. The names “left bundle branch block” and
“right bundle branch block” indicate the side that is affected. In a normal heart, the two ventricles are
depolarized simultaneously by the two bundles and contract at the same time. In bundle branch blocks, the UN-affected ventricle
depolarizes first. The electrical impulses THEN move through
the myocardium to the other side. This results in a DELAYED and SLOWED depolarization
of the affected ventricle, hence a broader QRS complex – typically longer than 120
milliseconds; and a loss in ventricular synchrony. Left and right bundle branch blocks are diagnosed
and differentiated by looking at ECG recordings obtained from the CHEST leads, which register
signal movements in a horizontal plane. Of these, the most useful are leads V1 and
V6 as they are best located to detect impulses moving between the left and right ventricles. Activation of the ventricles starts with the
interventricular septum. In normal conduction, depolarization of the
septum is initiated from the left bundle going to the right, TOWARD V1 and AWAY from V6. This results in a small positive deflection
in V1 and a negative deflection in V6. The signals then move both directions to the
two ventricles, but as the left ventricle is usually much larger, the NET movement is
to the left, AWAY from V1, TOWARD V6. This corresponds to a negative wave in V1
and a positive wave in V6. In RIGHT bundle branch block the initial septal
activation is unchanged. The left ventricle depolarizes NORMALLY toward
V6, away from V1, producing a positive deflection in V6, negative in V1. The impulses then REVERSE the direction spreading
to the right ventricle, hence a subsequent negative wave in V6, positive in V1. Lead V1 gives a characteristic M shape with
a terminal R wave, while V6 sees a broader S wave. In LEFT bundle branch block septal depolarization
is REVERSED, from right to left, giving a negative wave in V1. The right ventricle activates first, with
the signals moving to the right, generating a small upward deflection. Depolarization then spreads to the larger
left ventricle, resulting in a large downward deflection. Lead V6 sees the opposite, producing a wide,
characteristic “bunny ears” QRS complex with two R waves. In some cases, right ventricular depolarization
may not be visible. Some people with bundle branch blocks are
born with this condition. They usually do not have any symptoms and
do not require treatments. Others acquire it as a consequence of another
heart disease. These patients need monitoring, and in severe
cases, a pacemaker may be required to restore ventricular synchrony.

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