Are there native pathways to activate the right ventricle (RV) when pacing proximal to a right bundle branch block (RBBB) blockage?

Multiple Pathways to Right Bundle Branch Activation in RBBB

Yes, there are multiple pathways to activate the right ventricle when pacing proximal to a right bundle branch block (RBBB), which explains why pacing above a blockage can still result in right ventricular activation through alternative native pathways that bypass the block.

Anatomical Basis for Alternative Conduction Pathways

• The cardiac conduction system has anatomical redundancies that allow for alternative routes of electrical activation when the main pathway is blocked 1
• In RBBB, the main right bundle is blocked, but electrical impulses can still reach the right ventricle through:
  • Septal myocardial conduction (cell-to-cell conduction)
  • Specialized parallel pathways within the interventricular septum
  • Left-to-right transseptal activation 2

Evidence for Alternative Pathways in RBBB

Pacing Studies

• His bundle pacing (HBP) studies provide strong evidence for alternative pathways:
  • Non-selective His bundle pacing (NS-HBP) can correct RBBB even when the block is distal to the pacing site 2
  • This correction is voltage-dependent, suggesting activation of parallel specialized pathways 2
  • Higher pacing voltage completely corrected RBBB in 22/35 patients compared to only 5/18 patients at lower voltage 2

Fusion Mechanisms

• Right ventricular septal pacing can normalize QRS complexes in patients with RBBB through fusion between:
  • Artificial right ventricular stimulation
  • Intrinsic conduction over the left bundle branch 3

Left Bundle Branch Pacing

• Left bundle branch pacing can correct RBBB by:
  • Capturing intra-Hisian left bundle branch fibers
  • Activating the right ventricle through transseptal conduction 4
  • Creating fusion with anodal capture of right ventricular septal myocardium 5

Clinical Implications

• Understanding these alternative pathways has important implications for cardiac resynchronization therapy (CRT) and pacemaker placement:

  • Right ventricular septal pacing alone can significantly narrow QRS duration in some RBBB patients 6
  • His bundle pacing is effective in RBBB patients with otherwise healthy myocardium and Purkinje system 5
  • Left bundle pacing combined with RV septal anodal capture improves electrical synchrony 5

• The effectiveness of different pacing strategies depends on:

  • Location of the block within the right bundle branch
  • Presence of additional conduction abnormalities (e.g., left anterior fascicular block)
  • Conduction velocity in the myocardium and His-Purkinje system 5

Anatomical Considerations

• The right bundle branch emerges from the His bundle and travels along the right side of the interventricular septum 7
• The membranous septum between the right coronary and noncoronary leaflets is a critical anatomical landmark:
  • The length of the membranous septum affects conduction pathways
  • Compression of this area is associated with higher pacemaker implantation rates 7

In conclusion, the cardiac conduction system has built-in redundancy with multiple pathways that can activate the right ventricle even when the main right bundle branch is blocked. This explains why pacing proximal to an RBBB can still result in right ventricular activation through alternative native pathways that bypass the block.