No, RBBB is NOT a 2nd or 3rd Degree Heart Block
Right bundle branch block (RBBB) is a distinct conduction abnormality affecting the right bundle branch of the His-Purkinje system and is completely separate from atrioventricular (AV) blocks, which involve impaired conduction between the atria and ventricles. 1
Understanding the Distinction
What RBBB Actually Is
RBBB is an intraventricular conduction delay characterized by:
- QRS prolongation >120 ms
- RR' or rR' pattern in V1
- Wide S waves in left-sided leads (I, aVL, V5-V6)
- Delayed intrinsic deflection >80 ms in right precordial leads 2
This represents delayed activation of the right ventricle due to impaired conduction through the right bundle branch, but atrial impulses still reach the ventricles - the hallmark difference from AV block.
What AV Blocks Are
- 2nd degree AV block: Some atrial impulses fail to conduct to the ventricles (intermittent block at the AV node or His-Purkinje system)
- 3rd degree AV block: Complete dissociation where no atrial impulses reach the ventricles 3
Critical Clinical Caveat
RBBB can coexist with or progress to AV block, but they are separate entities:
- RBBB with left anterior fascicular block (bifascicular block) carries higher risk of progression to complete AV block (42% vs 14% for isolated bundle branch blocks) 4
- In acute MI with RBBB, third-degree AV block occurs in 11.1% of cases 5
- Progression from isolated RBBB to complete AV block is rare in the absence of underlying cardiac disease 2
When to Worry About Progression
Watch for these high-risk features in patients with RBBB:
- Bifascicular block (RBBB + left anterior hemiblock)
- HV interval ≥100 ms on electrophysiology study
- Syncope with bundle branch block
- Acute MI setting (especially anterior wall) 6, 4, 7
The presence of RBBB alone does not indicate AV block, but it may signal increased risk for future development of complete heart block in specific clinical contexts.