1. Bundle Branch Blocks and Hemiblocks

2. Take Home Points
Bundle branch blocks are best characterized on ECG by:
prolongation of the QRS > 0.12 sec
Region of slowed conduction determines the orientation of the terminal QRS vector and localizes the block
Hemiblocks do not typically show QRS prolongation, rather they reveal axis deviation corresponding to the QRS terminal vector
Ventricular conduction blocks are common and can represent a spectrum from normal variant to serious pathology

3. Normal Anatomy of the Cardiac Conduction System
SA Node
AV Node
Bundle of His
Left and R Bundle Branches
Septal Fascicle
LAF
LPF
Purkinje fibers
Myocardium

4. Anatomy of the Ventricular His-purkinje Conduction System
Left bundle branch
Right bundle branch
Left anterior fascicle
Septal fascicle
Left posterior fascicle

5. Bundle Branch Blocks
Intrinsic impairment of conduction in the left or right bundle branches
Can be chronic or intermittent
Can be rate dependant
Can be present without cardiac disease
Most often due to CAD or HTN causing ischemic or degenerative changes

6. EKG Manifestations of BBBs
Widened QRS complex > 0.12 sec
Results from slow myocyte-to-myocyte conduction causing prolonged ventricular depolarization
incomplete ( s) or complete (>0.12s)
Terminal QRS is sluggish and reflects the electric forces in the affected ventricle.
LBBB = terminal QRS is superior, posterior, and left
RBBB = terminal QRS in inferior, anterior, and right
Repolarization abnormalities
ST and T-wave oriented opposite of the terminal QRS

7. Right Bundle Branch Block
Left bundle branch
Right bundle branch
Left anterior fascicle
Septal fascicle
Left posterior fascicle

8. Phase 1: Initial QRS in RBBB
The initial QRS deflection represents the septum depolarizing from left to right. This conduction is mediated by the septal fascicle arising from the left bundle branch
The initial QRS is unchanged in RBBB because conduction is independent of the right bundle branch

9. Phase Two: RBBB
Following the initial QRS, the left ventricle rapidly depolarizes via conduction from the LAF and LPF.
The right ventricle is delayed.
This produces a QRS vector oriented to the left and posterior.
The left ventricle is normally dominant during the QRS and thus the EKG still shows no abnormally

10. Phase 3: Terminal QRS in RBBB
Following left ventricular depolarization, there continues to be delayed depolarization of the right ventricle via slow myocyte-to-myocyte spread.
The right ventricle is now the dominate electrical force and therefore the terminal QRS vector is oriented to the right and anterior. This shows on an ECG as an additional tall upright R’ wave in V1 and a deep S wave in V6.

11. RBBB Initial QRS remains normal Middle portion of QRS remains normal
Terminal QRS is sluggish and dominated by right ventricular forces

12. ECG findings in RBBB QRS duration > 0.12 sec RSR’ pattern in V1, V2
R’ is recorded in the right precordial leads demonstrating the period of RV electrical dominance
Wide S wave in leads I, V5, V6
Reflects slow RV conduction oriented towards the right and anterior. This is the left lateral lead equivalent finding of the RSR’ in the right lateral leads
Normal Axis of initial sec of QRS
qRS, qrS, or RS in I, V6; rsR' or qR in V1 - Reflects anterior mid-temporal right ventricular enlargement and left anterior fascicular block
Secondary ST-T wave changes
ST depression and T inversion in right precordial leads, upright in left precordial and limb leads - ST-T vectors are discordant to terminal mean QRS spatial vector

13. Right Bundle Branch Block
Notice the wide RSR' complex in lead V1 and the QRS complex in lead V6. Inverted T waves in the right precordial leads (in this case V1 to V3) are common with right bundle branch block and are called secondary T wave inversions.

14. RBBB
RBBB

15. RBBB Incidence increases with age Can occur as a normal variant
Most common cause is CAD (LAD)
Other causes include both structural and functional causes
Structural: anything causing RV dilatation or hypertrophy, (acute PE, cor pulmonale, DCM, ect), trauma (right heart cath, steering wheel, CABG, ablation)
Functional: rate-related bundle branch block

16. Differential Diagnosis of RBBB
Myocardial infarction
Pulmonary embolism
Chronic obstructive lung disease/cor pulmonale
Pulmonary hypertension (primary or secondary)
Hypertensive heart disease
Degenerative disease of the conduction system
Brugada syndrome
Arrhythmogenic RV dysplasia
Cardiomyopathy
Chagas disease
Congenital heart disease (eg, Ebstein anomaly)

17. RBBB: prognosis and treatment
Prognosis depends on underlying etiology
Worse prognosis for patients with type II second degree atrioventricular (AV) block or multifascicular block
Generally good prognosis for patients without underlying heart disease
NO treatment necessary for isolated asymptomatic RBBB
Pacing may be necessary for symptomatic patients or those with other AV or multifascicular block

18. RBBB and MI
T-wave inversions may be seen in leads V1-V3, II, III, and aVF secondary to the additional rSR’-type complex (changes to QRS axis)
ST and T wave changes in the same direction as the terminal QRS complex should be considered primary abnormalities (ventricular gradient not between 0º and 90º in the frontal plane)
Since RBBB does not alter initial phase of ventricular depolarization, RBBB does not mask abnormal Q waves seen in MI

19. Left Bundle Branch Block
Right bundle branch
Left anterior fascicle
Septal fascicle
Left posterior fascicle

20. Phase one: Initial QRS in LBBB
Normally the LBB begins the initial QRS with left-to-right septal depolarization
In LBBB the first phase is altered. The initial QRS vector remains slightly anterior and oriented right-to-left.

21. Phase two: LBBB
The right ventricle is stimulated first via the intact RBB while the left ventricle proceeds by slow slow cell-to-cell depolarization
The RV is not seen on ECG since it is overwhelmed by larger forces from the LV
This slow depolarization through the LV creates a widened QRS complex with upward deflexions in the lateral leads.
In LBBB all phases of ventricular depolarization are effected

22. LBBB Initial QRS orientation is right-to-left
Middle and terminal QRS forces are dominated by the larger left ventricular forces
Slow conduction causes long QRS deflection

23. ECG Findings in LBBB QRS duration 0.12 sec Intrinsicoid deflection
>0.05 sec in lead V6
Common pattern
Loss of q wave and wide slurred monophasic R in I, aVL, and V6
RS or QS in V1
ST depression and T wave inversion in leftward leads
ST elevation and upright T waves in right precordial leads
These changes reflect relentlessly leftward vectorial forces. V1 morphology depends on initial leftward or rarely rightward forces. ST and T vectors are usually 180º discordant to the QRS vector.
Variant pattern
Terminal S wave in V6 but not usually in I.
Mean electrical axis superior and to the left (about -55º).
ST depression and T wave inversion in leftward leads; ST elevation and upright T waves in right precordial leads
These changes reflect superior and posterior displacement of loop with a terminal, often rightward, conduction delay. ST and T vectors are discordant to the QRS vector.

24. LBBB

25. LBBB

26. LBBB

27. LBBB and MI
T-waves in leads with tall R waves (left precordial leads) are inverted secondary to changes in QRS in LBBB
T-wave inversions in the right precordial leads cannot be explained by LBBB
MI can be difficult to diagnose on ECG in LBBB because Q waves/ ST elevations can be masked
Exercise stress testing is of limited value in CAD
The 2002 American College of Cardiology/American Heart Association (ACC/AHA) guidelines concluded that there is no level of ST segment depression that confers diagnostic significance during exercise ECG testing in patients with LBBB
Can use myocardial perfusion imaging (MPI)

28. LBBB Incidence increases with age
Almost always associated with underlying heart disease
Most common causes include CAD, HTN, dilated CM (hypertrophy/dilatation of LV)
Other causes include valvular diseases-rheumatic heart disease, aortic stenosis, ect
Rarely results from primary degeneration of the conduction system
Rate- or acceleration-dependent LBBB

29. LBBB prognosis and treatment
Also depends on underlying cardiac conditions
Worse prognosis for patients with type II second degree atrioventricular (AV) block or multifascicular block
Prognosis depends on age- worse with increasing age
Isolated LBBB is more likely than RBBB to progress to high-degree AV block and has worse mortality
NO treatment for asymptomatic isolated LBBB is necessary
Pacing may be necessary for symptomatic patients or those with other AV or multifascicular block

30. Hemiblock / Fascicular Block
The Left Bundle Branch divides into 3 separate fascicles
Septal fascicle innervates the ventricular septum
The anterior fascicle runs toward the anteriolateral papillary muscle and projects anterior, leftward, and superior
The posterior fascicle runs toward the posterior papillary muscle and projects posterior, rightward, and inferiorly
Normal electrical activation of the LV spreads simultaneously from the base of the two papillary muscles
LBB
RBB
LAF SF
LPF

31. Hemiblock / Fascicular block cont.
A fascicular block occurs when you have impaired conduction down one division of the LBB
The block is manifested on EKG as QRS axis deviation
No / minimal QRS prolongation (0.09 – 0.11 sec)
Most commonly due to CAD and MI; also cardiomyopathies

32. Left Anterior Fascicular Block
With LAFB the LPF carries the initial depolarization inferior, posterior and to the right
Then the vector shifts unopposed to the left, anterior, and superior
Common cause of left axis deviation
LBB
RBB
LAF
LPF

33. LAFB: ECG findings QRS duration < 0.1 sec
Left axis deviation (Usually -45° to -90°)
No other identified cause of left axis deviation
Tall R waves in I and aVL
Big S waves in II, III, and aVF

34. LAFB
QRS has normal duration
Left axis deviation

35. LAFB QRS has normal duration Left axis deviation
Tall R waves in I and aVL
Deep S waves in II, III, and aVF

36. Left Posterior Fascicular Block
With LPFB the LAF carries the depolarization left, anterior and superior
THEN the vector shifts unopposed inferior, posterior, and to the right
Rare cause of right axis deviation
LPFB is less likely to be caused by ischemia because it receives dual blood supply from RCA and LAD
LBB
RBB
LAF
LPF

37. LPFB: ECG findings Normal duration of QRS < 0.1 sec
Right axis deviation (+90° to +180°)
No other identified cause of right axis deviation
Big S waves in I and aVL
Big R waves in II, III, and aVF

38. LPFB

39. Bifascicular blocks: RBBB and LAFB
QRS > 0.12s
RSR’ in V1
Wide and slurred S waves in I, V5, V6
First half (0.06s) of QRS having frontal plane axis of -30° to -90°
Initial R wave in inferior leads

40. Bifascicular blocks: RBBB and LPFB
QRS>0.12s
RSR’ in V1with R’ broad and slurred
Wide and slurred S waves in I, V5, V6
First half (0.06s) of QRS having frontal plane axis of +90° or further to the right with rS deflection in lead I and qR waves in II, III, and aVF

41. Unknown? QRS prolonged > 0.12 sec
Terminal forces are left and posterior
Dx: LBBB

42. Unknown? QRS Prolongation > 0.12 sec RSR’ in V1, V2
ST, T-wave abnormalities
Slurred terminal S wave in I, V3, V4, V5, V6
Dx: RBBB

43. Unknown?
QRS has normal duration
Right axis deviation
Dx: LAFB

44. Unknown? QRS Prolongation > 0.12 sec RSR’ in V1, V2, V3
ST, T-wave abnormalities
Slurred terminal S wave in I, aVL, V4, V5, V6
Dx: RBBB
askdrwiki.com/mediawiki/images/b/bb/RBBB.PNG

45. Unknown? QRS prolonged > 0.12 sec
Terminal forces are left and posterior
Dx: LBBB

46. Unknown? QRS has normal duration Left axis deviation Dx: LAFB

47. Take Home Points
Bundle branch blocks are best characterized on ECG by:
prolongation of the QRS > 0.12 sec
Region of slowed conduction determines the orientation of the terminal QRS vector and localizes the block
Hemiblocks do not typically show QRS prolongation, rather they reveal axis deviation corresponding to the QRS terminal vector
Ventricular conduction blocks are common and can represent a spectrum from normal variant to serious pathology