How does His (bundle) pacing proximal to a right bundle branch block (RBBB) bypass the block?

His Bundle Pacing for Right Bundle Branch Block Correction

His bundle pacing (HBP) can correct right bundle branch block (RBBB) by delivering electrical stimulation proximal to the site of conduction block, allowing activation of the His bundle fibers destined for both the left and right bundle branches, thus bypassing the diseased right bundle branch and restoring physiologic ventricular activation. 1

Mechanism of RBBB Correction with His Bundle Pacing

HBP works through two primary mechanisms to correct RBBB:

1. Proximal Capture: When pacing is performed proximal to the site of block in the right bundle branch, the electrical impulse can bypass the block and travel through the native conduction system

2. Types of His Bundle Capture:

   • Selective His capture: Directly activates His bundle fibers without capturing surrounding myocardium
   • Non-selective His capture: Captures both the His bundle and adjacent ventricular myocardium 1

Evidence for HBP Effectiveness in RBBB

HBP has demonstrated significant benefits in patients with RBBB:

• In patients with RBBB and reduced left ventricular ejection fraction (LVEF), HBP was successful in 95% of cases with narrowing of RBBB in 78% of patients 2
• HBP resulted in significant QRS narrowing from 158±24 ms to 127±17 ms (p=0.0001) 2
• LVEF improved from 31±10% to 39±13% (p=0.004) with HBP 2
• Functional status (NYHA class) improved from 2.8±0.6 to 2±0.7 (p=0.0001) 2

Clinical Considerations for HBP in RBBB

The ACC/AHA guidelines support the use of physiologic pacing methods like HBP over right ventricular pacing in patients with atrioventricular block who have an LVEF between 36% and 50% and are expected to require ventricular pacing more than 40% of the time (Class IIa, Level of Evidence: B-NR) 3.

Limitations and Challenges:

• HBP typically requires higher pacing outputs compared to conventional RV pacing 1
• HBP may be less effective if the RBBB occurs very distal in the conduction system 1, 4
• In a study of RBBB patients, selective HBP resolved RBBB in only 1/21 patients when the block was distal to the pacing site 4
• Non-selective HBP was more effective, completely correcting RBBB in 22/35 patients at higher pacing voltage 4

Alternative Approaches for RBBB Correction

When HBP is not feasible or effective (particularly in distal RBBB):

• Left Bundle Branch Area Pacing (LBBAP): Can be effective in RBBB patients when combined with anodal capture of the RV septum myocardium 5, 6
  • LBBAP was successful in 88% of patients with RBBB and reduced LVEF 6
  • Resulted in QRS narrowing (156±20 ms to 150±24 ms, p=0.01) 6
  • Improved LVEF from 35±9% to 43±12% (p<0.01) 6

Clinical Decision-Making for RBBB Patients

For patients with RBBB requiring pacing:

1. First-line approach: Consider HBP if the RBBB is likely proximal to the His bundle
2. If HBP unsuccessful or high thresholds: Consider non-selective HBP at higher output voltages
3. For distal RBBB: LBBAP with RV septal anodal capture may be more effective
4. For patients with severely slow conduction in RV His-Purkinje system: Conventional CRT may be considered 5

In summary, HBP can effectively correct RBBB by pacing proximal to the site of block, restoring physiologic ventricular activation and improving cardiac function. The success depends on the location of the block relative to the pacing site and the type of His bundle capture achieved.