Objective:

To report a rare case of a pediatric patient who developed an unexpected right bundle branch block (RBBB) pattern during left bundle branch pacing (LBBP) after ventricular septal defect (VSD) repair, which is typically associated with LBBB.

Key Findings:

• The patient exhibited an RBBB pattern during initial pacing, which is rare and typically associated with procedural complications.
• The pacing lead's superficial contact with the right ventricular septum resulted in an unexpected RBBB morphology instead of the anticipated LBBB pattern.
• Surgical scarring and proximal conduction system block may have contributed to the observed pacing pattern.

Interpretation:

The RBBB pattern observed during LBBP may indicate irreversible structural impairment of the proximal conduction system due to surgical intervention, raising concerns for future pacing strategies.

Limitations:

• The exact underlying conduction mechanism remains speculative without detailed intracardiac activation mapping, which limits understanding of the pacing dynamics.
• The phenomenon is rare, and similar cases are not well-documented in the literature, making it difficult to draw broader conclusions.

Conclusion:

This case highlights a unique pacing pattern during LBBP in a pediatric patient, suggesting potential complications related to prior surgical repair and the need for careful monitoring in similar cases.