Bundle Branch Blocks
Interruptions in electrical conduction through the branches of the Bundle of His,
affecting ventricular depolarization.
Understanding the anatomy and physiology of the conduction system is crucial for interpreting
electrocardiograms (ECGs) in the presence of these blocks.
Key Points
● Bundle of His: Divides into a right bundle branch and a left bundle branch.
● Right Bundle Branch: Stimulates the right ventricle and one-third of the
interventricular septum.
● Left Bundle Branch: Stimulates the left ventricle and the remaining two-thirds of the
interventricular septum.
● Left Bundle Branch Fascicles: The left bundle branch subdivides into an anterior
superior fascicle and a posterior inferior fascicle.
Anterior Superior Fascicle: Innervates the anterior superior region of the left ventricle.
Posterior Inferior Fascicle: Depolarizes the posterior inferior region of the left
ventricle.
Right Bundle Branch Block
Right bundle branch block occurs when electrical conduction through the right bundle branch
of the Bundle of His is interrupted. This alters the normal sequence of ventricular
depolarization, affecting the morphology of the QRS complex on the ECG.
Right bundle branch block (RBBB) is more common than left bundle branch block (LBBB) due
to the greater anatomical vulnerability of the right bundle branch.
Factors such as aging, heart diseases (hypertension, coronary artery disease, heart failure),
and its higher incidence in men contribute to this greater prevalence. Also, its more exposed
pathway makes it more susceptible to damage. While it can be a benign finding in healthy
hearts, RBBB can also be associated with underlying diseases, whereas LBBB is usually more
indicative of cardiac pathology.
, Key Points
● Conduction cannot follow the normal pathway through the blocked right bundle
branch.
● Left ventricular depolarization occurs normally through the left bundle branch.
● Right ventricular depolarization is delayed and propagates through muscle cells.
● Muscle cells are slower.
● QRS Widening: The QRS complex widens due to the slower ventricular
depolarization through muscle cells.
● Morphology in V1: In lead V1 (which faces the right ventricle), a small, rapid initial
deflection is observed (left ventricular depolarization), followed by a wider, delayed
deflection (right ventricular depolarization).
● "Rabbit Ears": The classic pattern in V1 can resemble "rabbit ears" (two separate R
waves), but this is not the defining criterion. The main criterion is QRS widening. This
signifies ventricular cells depolarizing slowly (the impulse had to travel from left to
right). With the right bundle branch blocked, ventricular cells are forced to depolarize
the right ventricle.
● T Wave Abnormality: Repolarization is also affected, resulting in an abnormal T
wave, typically negative in V1.
● Axis Deviation: The electrical axis may deviate to the right due to the delayed
depolarization of the right ventricle (end of depolarization finishes there), although the
axis can be normal (not a criterion).
Diagnostic Criteria
● Widened qRs in V1 and V2: rSR' (due to slower depolarization of muscle cells)
with inverted T waves (due to abnormal repolarization).
● Wide S waves in V5 and V6 (the QRS may appear normal but is widened).
● Normal T wave (no problems in repolarization of VI).
Right Bundle Branch Block: Complete vs. Incomplete
Interruptions in electrical conduction through the branches of the Bundle of His,
affecting ventricular depolarization.
Understanding the anatomy and physiology of the conduction system is crucial for interpreting
electrocardiograms (ECGs) in the presence of these blocks.
Key Points
● Bundle of His: Divides into a right bundle branch and a left bundle branch.
● Right Bundle Branch: Stimulates the right ventricle and one-third of the
interventricular septum.
● Left Bundle Branch: Stimulates the left ventricle and the remaining two-thirds of the
interventricular septum.
● Left Bundle Branch Fascicles: The left bundle branch subdivides into an anterior
superior fascicle and a posterior inferior fascicle.
Anterior Superior Fascicle: Innervates the anterior superior region of the left ventricle.
Posterior Inferior Fascicle: Depolarizes the posterior inferior region of the left
ventricle.
Right Bundle Branch Block
Right bundle branch block occurs when electrical conduction through the right bundle branch
of the Bundle of His is interrupted. This alters the normal sequence of ventricular
depolarization, affecting the morphology of the QRS complex on the ECG.
Right bundle branch block (RBBB) is more common than left bundle branch block (LBBB) due
to the greater anatomical vulnerability of the right bundle branch.
Factors such as aging, heart diseases (hypertension, coronary artery disease, heart failure),
and its higher incidence in men contribute to this greater prevalence. Also, its more exposed
pathway makes it more susceptible to damage. While it can be a benign finding in healthy
hearts, RBBB can also be associated with underlying diseases, whereas LBBB is usually more
indicative of cardiac pathology.
, Key Points
● Conduction cannot follow the normal pathway through the blocked right bundle
branch.
● Left ventricular depolarization occurs normally through the left bundle branch.
● Right ventricular depolarization is delayed and propagates through muscle cells.
● Muscle cells are slower.
● QRS Widening: The QRS complex widens due to the slower ventricular
depolarization through muscle cells.
● Morphology in V1: In lead V1 (which faces the right ventricle), a small, rapid initial
deflection is observed (left ventricular depolarization), followed by a wider, delayed
deflection (right ventricular depolarization).
● "Rabbit Ears": The classic pattern in V1 can resemble "rabbit ears" (two separate R
waves), but this is not the defining criterion. The main criterion is QRS widening. This
signifies ventricular cells depolarizing slowly (the impulse had to travel from left to
right). With the right bundle branch blocked, ventricular cells are forced to depolarize
the right ventricle.
● T Wave Abnormality: Repolarization is also affected, resulting in an abnormal T
wave, typically negative in V1.
● Axis Deviation: The electrical axis may deviate to the right due to the delayed
depolarization of the right ventricle (end of depolarization finishes there), although the
axis can be normal (not a criterion).
Diagnostic Criteria
● Widened qRs in V1 and V2: rSR' (due to slower depolarization of muscle cells)
with inverted T waves (due to abnormal repolarization).
● Wide S waves in V5 and V6 (the QRS may appear normal but is widened).
● Normal T wave (no problems in repolarization of VI).
Right Bundle Branch Block: Complete vs. Incomplete
