Right Fascicular Block vs. Right Bundle Branch Block
No, right fascicular block (RFB) is not the same as right bundle branch block (RBBB). While both are conduction disorders affecting the right side of the heart's electrical system, they involve different anatomical structures and have distinct electrocardiographic patterns.
Anatomical Differences
- Right Bundle Branch: The main trunk that carries electrical impulses to the right ventricle
- Right Fascicles: The right bundle branch divides into three fascicles:
- Anterior fascicle
- Middle (lateral) fascicle
- Posterior fascicle 1
Electrocardiographic Differences
Right Bundle Branch Block (RBBB)
- Complete block of the entire right bundle branch
- QRS duration >120 ms
- RSR' pattern in right precordial leads (V1-V3)
- Wide S waves in leads I, aVL, V5, and V6 2
Right Fascicular Block (RFB)
- Block of one or more fascicles of the right bundle branch
- Delayed R-peak time in aVR and V1
- Specific ventricular complex morphologies
- Right or left axis deviation exceeding +60° depending on which fascicle is affected 1
Clinical Significance
RBBB is well-recognized and commonly encountered in clinical practice, while right fascicular blocks are often overlooked during diagnostic processes 1. The 2019 ACC/AHA/HRS guideline on bradycardia and cardiac conduction delay clearly distinguishes between bundle branch blocks and fascicular blocks, noting that the His-Purkinje system consists of the division of the His bundle into left and right bundle branches, followed by division of the left bundle into anterior and posterior fascicles 2.
Diagnostic Considerations
When evaluating conduction disorders:
- A 12-lead ECG is essential for accurate diagnosis and differentiation
- RBBB reduces the amplitude of the S wave in the right precordial leads
- Ancillary features like left atrial abnormality and left axis deviation have enhanced value for diagnosing LVH in the presence of RBBB 2
Prognostic Implications
- Isolated right fascicular block generally has a good prognosis
- RBBB can be a predictor of mortality in heart failure patients, similar to LBBB 3
- The combination of RBBB with left fascicular blocks (bifascicular block) carries a higher risk of progression to complete heart block 4, 5
Clinical Management
For patients with conduction disorders:
- Echocardiography is recommended for newly detected LBBB and should be considered for other intraventricular conduction disorders if structural heart disease is suspected 2
- Ambulatory electrocardiographic monitoring is useful in symptomatic patients with conduction system disease in whom atrioventricular block is suspected 2
- More frequent follow-up (every 3-6 months) is recommended for patients with conduction abnormalities beyond isolated blocks 4
Understanding the distinction between right fascicular block and right bundle branch block is important for proper diagnosis, risk stratification, and management decisions in patients with conduction disorders.