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What are 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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