Bundle Branch and Fascicular Blocks

Bundle branch and fascicular blocks occur when the normal electrical activity in the His-Purkinje system is interrupted. These blocks can be due to many etiologies that may affect the structure of the heart or the conduction system directly. The blocks are classified into right bundle branch block, left bundle branch block, left anterior fascicular block, and left posterior fascicular block depending on the location of the disruption. Most individuals are asymptomatic. ECG will provide the diagnosis. Some common ECG findings include a prolonged QRS interval, R-wave changes, axis deviation, and (in some cases) S-wave changes. No specific treatment is indicated.

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Classification and Epidemiology

Classification

Bundle branch and fascicular blocks are classified on the basis of where the disruption occurs within the His-Purkinje system.

  • Bundle branch blocks:
    • Right bundle branch block (RBBB)
    • Left bundle branch block (LBBB)
  • Fascicular blocks:
    • Left anterior fascicular block or hemiblock (LAFB)
    • Left posterior fascicular block or hemiblock (LPFB)
Bundle branch and fascicular blocks arise due to obstruction of electrical current

Bundle branch and fascicular blocks arise because of obstruction of electrical current through the His-Purkinje system and are named on the basis of the location of that disruption.

Image by Lecturio.

Epidemiology

  • RBBB:
    • Prevalence:
      • In the general population: < 1%
      • Increases with age
    • Can occur in young, healthy people
  • LBBB:
    • Prevalence:
      • In the general population: < 1%
      • Increases with age
    • Occurs infrequently in young, healthy people
  • LAFB:
    • Prevalence in the general population: 1%–2.5%
    • Prevalence increases with age
  • LPFB:
    • An isolated LPFB is rare.
    • Prevalence: 0.1%–0.6%
    • Often occurs in association with RBBB

Etiology

RBBB

  • Structural disease:
    • Includes conditions resulting in right ventricular:
      • Hypertrophy
      • ↑ Pressure
      • Stretch
      • Injury
      • Inflammation/infiltration
    • Examples:
      • Pulmonary hypertension and cor pulmonale
      • Pulmonary embolism
      • Myocardial ischemia or MI
      • Myocarditis
      • Cardiomyopathy
      • Valvular disease
      • Atrial septal defect
      • Chagas disease
  • Idiopathic cardiac conduction disease (Lenegre or Lev disease)
  • Iatrogenic trauma to the conduction system:
    • Right-heart catheter insertion
    • Septal reduction therapy with ethanol ablation
    • Cardiac surgery
  • Can also occur in those with no heart disease

LBBB

  • Structural disease
    • Includes conditions resulting in left ventricular:
      • Hypertrophy
      • ↑ Pressure
      • Stretch
      • Injury
      • Inflammation/infiltration
    • Examples:
      • Myocardial ischemia or MI (most common)
      • Cardiomyopathy
      • Hypertension
      • Endocarditis (particularly with abscess)
      • Myocarditis
      • Valvular disease
      • Congenital defects
  • Idiopathic cardiac conduction disease
  • Iatrogenic:
    • Septal myectomy
    • Transcatheter aortic valve implantation

LAFB and LPFB

These fascicular blocks can occur because of many of the same causes of RBBB or LBBB, most notably:

  • Aortic valvular disease (LAFB)
  • Myocardial ischemia or MI
  • Cardiomyopathy
  • Hypertension
  • Chagas disease
  • Infiltrative and inflammatory diseases
  • Congenital defects
  • Idiopathic cardiac conduction disease

Pathophysiology

Normal physiology

  • The cardiac impulse is generated by pacemaker cells in the sinoatrial (SA) node and moves through the atria → depolarization → atrial contraction
  • Transmitted to the atrioventricular (AV) node → His-Purkinje system → depolarization → ventricular contraction
  • His-Purkinje system:
    • Provides rapid electrical conduction for the ventricles → synchronized ventricular depolarization and contraction
    • Contains:
      • Bundle of His
      • The bundle splits in 2 → right and left bundles branches
      • Left bundle branch splits → left anterior fascicle and left posterior fascicle
      • Purkinje fiber

Bundle branch blocks

  • Impairment in a bundle causes disruption of downward cardiac impulse transmission.
  • This subsequently causes the impulse to be conducted through the opposite branch.
  • The contralateral ventricle will depolarize first.
  • The ipsilateral ventricle will depolarize later (electrical impulse travels slowly through the muscle, reaching the conduction system below the block).
  • This will appear as prolongation of the QRS interval on ECG.

Fascicular blocks

  • The pathophysiology is similar to that of bundle branch blocks, though the bundle branch is not completely affected → depolarization of the left ventricle is dependent on the opposite fascicle
  • This causes axis deviation, but has less effect on QRS duration.
Left anterior and posterior fascicular blocks

Diagram of left anterior and posterior fascicular blocks:
In left anterior fascicular block or hemiblock (LAFB), the resultant electrical vector results in significant left axis deviation. In left posterior fascicular block or hemiblock (LPFB), the electrical vector is deviated a bit rightward but is not significantly displaced from the normal QRS axis range.

Image by Lecturio.

Clinical Presentation and Diagnosis

Clinical presentation

  • Usually asymptomatic
  • Most people are unaware.
  • Rarely, may cause syncope or presyncope (may have associated AV block)
  • A split S1 is common in RBBB.
  • A split S2 can also be noted: 
    • Persistent in RBBB → delayed pulmonic valve closure (due to delayed activation of the right ventricle)
    • Paradoxical in LBBB → delayed aortic valve closure (due to delayed activation of the left ventricle)
Persistent splitting of S2

A diagram of a persistent split S2, in which closure of the pulmonic valve is delayed further by inspiration (right). This can occur in a right bundle branch block.

Image by Lecturio.

Audio:

This audio clip is an example of a split S2 in the setting of an RBBB. The 2 sounds occurring during S2 result from delayed closure of the pulmonic valve in relation to the aortic valve.

Heart sound by The Regents of the University of Michigan. License: CC BY-SA 3.0
Paradoxical splitting of S2

A diagram of a paradoxical split in closure of the aortic valve which is delayed:
The name “paradoxical” is due to the fact that the split narrows inspiration (right). This can be heard in some individuals with a left bundle branch block.

Image by Lecturio.

Right bundle branch block on ECG

  • QRS duration ≥ 120 msec
  • Leads V1 and V2:
    • Rsr′, rsR’ or rSR’ (many variations)
    • The R’ or r’ deflection is usually wider than the initial R wave.
    • Appears as “rabbit ears”
  • Leads I and V6 will have an S wave that is:
    • Deep
    • Of longer duration
    • Slurred 
  • T waves tend to be discordant to the terminal QRS vector.

LBBB on ECG

  • QRS duration > 120 msec in adults
  • Broad notched (R,R’) or slurred R wave in leads I, aVL, V5, and V6
  • Absent Q waves in lateral leads
  • Large S wave in V1 and V2
  • ST segments and T waves are usually discordant to QRS complex.

Incomplete RBBB and LBBB

A bundle branch block may be considered incomplete if the usual RBBB or LBBB pattern is seen but the QRS duration is 110–119 msec.

Left anterior fascicular block on ECG

  • QRS duration < 120 msec
  • Left-axis deviation (approximately 45–90 degrees)
  • R-peak time ≥ 45 msec in lead aVL (measured from the start of the Q wave to the peak of the R wave)
  • qR complexes in leads I and aVL
  • rS complexes in leads II, III, and aVF
Left anterior fascicular block on ECG

ECG demonstrating left anterior fascicular block:
Here, the axis is deviated to –60 degrees and a small Q wave is noted in aVL. The QRS is slightly prolonged, but still < 120 msec.

Image by Lecturio.

Left posterior fascicular block on ECG

  • QRS duration < 120 msec
  • Right-axis deviation (90–180 degrees)
  • qR complexes in leads II, III, and aVF
  • rS complexes in leads I and aVL
Left posterior fascicular block on ECG

ECG demonstrating a left posterior fascicular block:
There is right axis deviation (+ 100 degrees), small Q waves in II, III, and aVF, rS complexes in I and aVL. The QRS complex duration is also < 120 msec.

Image by Lecturio.

Management

  • Most individuals are asymptomatic and require no treatment.
  • If an underlying cause is present (e.g., myocardial ischemia), treatment is based on that.
  • Pacemaker may be considered in individuals with: 
    • Another conduction disturbance (e.g., high-degree AV block)
    • LBBB and syncope
  • Cardiac resynchronization therapy:
    • Cardiac pacing method used to resynchronize cardiac contraction
    • Indications include LBBB with:
      • Left ventricular ejection fraction ≤ 35% in sinus rhythm
      • Prolonged QRS interval ≥ 150 msec

Differential Diagnosis

  • Hyperkalemia: increased serum potassium concentration due to abnormal movement out of cells, decreased renal excretion, or increased intake. Hyperkalemia may present with muscle weakness and dangerous cardiac toxicity, such as ventricular fibrillation or asystole (if severe). Diagnosis is by serum potassium measurement. Appearance on ECG can be similar to RBBB or LBBB because of depressed conduction in the His-Purkinje system. Acute management includes insulin, sodium bicarbonate, albuterol, calcium gluconate, cation exchangers, and/or dialysis.
  • Wolff-Parkinson-White syndrome: congenital preexcitation condition in which antegrade conduction occurs over an accessory pathway. Individuals may present with tachypnea, chest pain, palpitations, and difficulty in breathing due to tachyarrhythmia. Appearance on ECG can be similar to LBBB, but with a shortened PR interval. Treatment is by radiofrequency ablation and antiarrhythmic medications.
  • Ventricular tachycardia: ≥ 3 ventricular beats at a rate of ≥ 120 beats/min. Symptoms of ventricular tachycardia are duration-dependent and vary from no symptoms to palpitations to hemodynamic instability and death. Diagnosis is by ECG, which can appear similar to LBBB or RBBB because of the QRS widening. However, there will be AV dissociation and extreme axis deviation. Treatment of acute episodes is with cardioversion or antiarrhythmic drugs. Long-term treatment is with an implantable cardioverter-defibrillator. 
  • Ventricular pacing: due to an implanted cardiac pacemaker. ECG can appear similar to LBBB (rarely RBBB). However, pacemaker spikes are usually also seen preceding the QRS complex.

References

  1. Goldberger, A.L. (2019). Basic approach to delayed intraventricular conduction. UpToDate. Retrieved July 29, 2021, from https://www.uptodate.com/contents/basic-approach-to-delayed-intraventricular-conduction
  2. Sauer, W.H. (2020). Right bundle branch block. Retrieved July 29, 2021, from https://www.uptodate.com/contents/right-bundle-branch-block
  3. Sauer, W.H. (2020). Left bundle branch block. UpToDate. Retrieved July 29, 2021, from https://www.uptodate.com/contents/left-bundle-branch-block
  4. Sauer, W.H. (2019). Left anterior fascicular block. UpToDate. Retrieved July 29, 2021, from https://www.uptodate.com/contents/left-anterior-fascicular-block
  5. Sauer, W.H. (2020). Left posterior fascicular block. UpToDate. Retrieved July 29, 2021, from https://www.uptodate.com/contents/left-posterior-fascicular-block
  6. Mitchell, L.B. (2021). Bundle branch block and fascicular block. MSD Manual Professional Version. Retrieved July 29, 2021, from https://www.msdmanuals.com/professional/cardiovascular-disorders/arrhythmias-and-conduction-disorders/bundle-branch-block-and-fascicular-block
  7. Lederer, E., et al. (2020). Hyperkalemia. Retrieved July 30, 2021, from https://emedicine.medscape.com/article/240903-overview

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