1. Fast & Easy ECGs, 2nd E – A Self-Paced Learning Program19
Bundle Branch Block
Fast & Easy ECGs, 2nd E – A Self-Paced Learning Program

2. Bundle Branches
Bundle of His divides into right and left bundle branches
Left bundle branch further divides into septal, anterior and posterior fascicles
Instructional point:
The first and smallest fascicle innervates the interventricular septum and for this reason it is called the septal fascicle. The anterior fascicle carries the impulse to the anterior and superior portion of the left ventricle. The posterior fascicle carries the impulse to the posterior and inferior portion of the left ventricle. I

3. Normal QRS Complex Narrow – 0.06 to 0.10 seconds in duration
Electrical axis is between 0° and +90°

4. Bundle Branch Block
Leads to one or both bundle branches failing to conduct impulses
Produces delay in depolarization of the ventricle it supplies
Instructional point:
Interruption of normal impulse conduction through either bundle is referred to as a bundle branch block. If one or more of the fascicles is blocked we call this a hemiblock.
1 refers to the conduction that continues uninterrupted through the unaffected bundle branch.
2 refers to the impulse that activates the intraventricular septum.
3 refers to the impulse that activates the other ventricle. I

5. Bundle Branch Block Key characteristic are widened QRS complexes
0.12 seconds or greater in duration
As such, the first step to identifying bundle branch block is to analyze the QRS complex width
Next, look at the appearance of the QRS complexes
They may be notched, slurred or M shaped in the chest leads
Instructional point:
The shape of the QRS complex is determined by which bundle branch is blocked. I

6. Bundle Branch Block
Only occurs in supraventricular rhythms because the impulse originates above the ventricles and travels down through the bundle branches in these rhythms

7. Bundle Branch Block
Possible sites of block within the ventricular conduction system include:
Right bundle branch (right bundle branch block)
Left bundle branch (left bundle branch block)
Left anterior fascicle (left anterior fascicular block), also called left anterior hemiblock
Left posterior fascicle (left posterior fascicular block), also called left posterior hemiblock
Any combination of these or along with prolongation of the PR interval (first degree AV block)

8. Right Bundle Branch Block (RBBB)
Conduction through the right bundle branch is blocked causing delayed right ventricular depolarization
Look for wide, tall, and notched QRS complexes in leads V1, V2 and slurred S waves in leads I, V5 and V6
Instructional Point:
In right bundle branch block conduction does not start until the left ventricle is almost fully depolarized. I

9. Right Bundle Branch Block

10. Causes of Right Bundle Branch Block
Occurs with anterior wall MI, coronary artery disease, hypertension, scar tissue that develops after heart surgery, viral or bacterial myocarditis and pulmonary embolism
May also be caused by drug toxicity or be due to a congenital heart abnormality such as atrial septal defect

11. Left Bundle Branch Block
Conduction through the left bundle branch is blocked, causing depolarization of the left ventricle to be delayed
Look for wide, tall, notched or slurred QRS complexes in leads V5, V6 and wide, largely negative rS complexes or entirely negative QS complexes in V1 and V2

12. Left Bundle Branch Block

13. Causes of Left Bundle Branch Block
Anterior wall MI, hypertensive heart disease, aortic stenosis, degenerative changes of the conduction system or cardiomyopathy

14. Repolarization Changes
Look for wide, tall, notched or slurred QRS complexes in leads V5, V6
In RBBB, ST segment depression and T-wave inversion is seen in leads V1 and V2
In LBBB, ST segment depression and T-wave inversion may be seen in most leads, particularly leads V5 and V6

15. Incomplete Bundle Branch Block
Sometimes either the right or left bundle branch conducts the electrical impulse more slowly but it is not completely blocked
Results in QRS complexes with a similar appearance to bundle branch block and slightly wider than normal but not as wide as with complete bundle branch block
duration is no greater 0.11 seconds

16. Hemiblocks Occur when one or more of the LBB fascicles is blocked
While the ECG appearance of anterior and posterior hemiblocks differs from that of bundle branch blocks, the mechanism is essentially the same
Minimal prolongation with hemiblock, it is not enough to widen the QRS complex to any real degree but the morphology of the QRS complex does change
Instructional points:
The most common hemiblocks occur in either the anterior or posterior fasicles.
Because the area of impulse blockade is relatively small the QRS duration is generally not affected in either left anterior or left posterior hemiblock. I

17. Hemiblocks Axis deviation is the key ECG characteristic of hemiblocks
It occurs because when one fascicle is blocked, the electrical current travels down the other to stimulate the heart. This causes the axis to shift accordingly
When diagnosing hemiblock, be sure to rule out other causes of axis deviation, such as ventricular hypertrophy

18. Hemiblocks
The anterior fascicle is longer and thinner and has a more fragile blood supply than the posterior fascicle, so LAHB is far more common than LPHB
While LAHB can be seen in both normal and diseased hearts, LPHB is almost always associated with heart disease

19. Left Anterior Hemiblock
Conduction down the left anterior fascicle is blocked
Depolarization of the left ventricle occurs, progressing in an inferior-to-superior and right-to-left direction
The axis of ventricular depolarization is redirected upward and slightly to the left, producing tall positive R waves in the left lateral leads and deep S waves inferiorly resulting in left axis deviation
Instructional point:
When the left anterior fascicle is blocked the predominant depolarization of the left ventricle is from the anterior wall around the bottom of the heart and to the left and upwards. This shifts the axis to the left resulting in an axis between 0 and -90 degrees. Lead I will be positive and aVF will be negative. I

20. Left Posterior Hemiblock
Conduction down left posterior fascicle is blocked
Electrical impulse rushes down left anterior fascicle, resulting in ventricular myocardial depolarization occurring in a superior-to-inferior and left-to-right direction
Main electrical axis is directed downward and to the right, producing tall R waves inferiorly and deep S waves in left lateral leads resulting in right axis deviation
Instructional point:
When the left anterior fascicle is blocked the predominant depolarization of the left ventricle is from the anterior wall around the bottom of the heart and to the left and upwards. This shifts the axis to the left resulting in an axis between 0 and -90 degrees. Lead I will be positive and aVF will be negative. I

21. Combination of Blocks
Patients with any type of ventricular conduction block and especially those with a combination of blocks are at high risk of developing complete heart block

22. Bifascicular Block
Is a conduction disturbance in which two of the three main fascicles of the His/Purkinje system are blocked
Most often, it refers to a combination of RBBB and either LAHB (more commonly) or LPHB
Some include LBBB in the definition of bifascicular block because the block occurs above the bifurcation of the left anterior and left posterior fascicles of the left bundle branch

23. Trifascicular Block
Is a conduction disturbance in which there are three features seen on the ECG such as:
Prolongation of the PR interval (first degree AV block)
RBBB
Either LAFB or LPFB
Is uncommon

24. Practice Makes Perfect
Determine the condition
Junctional escape rhythm with right bundle branch block I

25. Practice Makes Perfect
Determine the condition
Answer: Sinus tachycardia with left bundle branch block I

26. Practice Makes Perfect
Determine the condition
Answer: Sinus bradycardia with left atrial enlargement and right bundle branch block I

27. Practice Makes Perfect
Determine the condition
Sinus tachycardia with right atrial enlargement, right bundle branch block and left posterior hemiblock (bifascicular block) I

28. Practice Makes Perfect
Determine the condition
Sinus rhythm with occasional PVCs, right bundle branch block and left anterior hemiblock (bifasicular block) and inferior infarct (age undetermined) I

29. Practice Makes Perfect
Determine the condition
Answer: Left anterior hemiblock. I

30. Practice Makes Perfect
Identify this condition
Sinus bradycardia with a short PR and incomplete right bundle branch block I

31. Summary
Bundle branch block is a disorder that leads to one or both of the bundle branches failing to conduct impulses. This produces a delay in the depolarization of the ventricle it supplies
In bundle branch block a widened QRS complex is seen in the chest leads. It may be tall and notched or slurred and/or have an “M” shaped appearance
To diagnose right bundle branch block look for wide, tall, and notched QRS complexes in leads V1, V2 and slurred S waves in leads I, V5 and V6

32. Summary
To diagnose left bundle branch block Look for wide, tall, notched or slurred QRS complexes in leads V5, V6
Sometimes an R, R’ wave is seen in a QRS complex of normal duration. This is called incomplete bundle branch block
Similar to the repolarization abnormalities seen in hypertrophy, so too can these changes be seen in both RBBB and LBBB
If only a portion of the left bundle branch is blocked, it is called a fascicular block
Hemiblocks cause axis deviation. LAHB results in left axis deviation. LPHB results in right axis deviation
Bundle branch block often does not require treatment

33. Summary
Bifascicular block is a conduction disturbance in which two of the three main fascicles of the His/Purkinje system are blocked
Most often, it refers to a combination of RBBB and either LAHB (more commonly) or LPHB
Trifascicular block is a conduction disturbance in which there are three features seen on the ECG:
Prolongation of the PR interval (first degree AV block)
RBBB
Either LAFB or LPFB