An unusual ECG pattern: left posterior fascicular block obscuring a right ventricular conduction defect?

Area of Science:

• Cardiology
• Electrophysiology
• Medical Diagnostics

Background:

• A 23-year-old male presented with palpitations and a syncopal episode.
• Physical examination yielded normal results.

Observation:

• Electrocardiogram (ECG) showed significant right axis deviation, prolonged QRS duration (0.13 sec), and an rS pattern from V1 to V6.
• High right precordial leads displayed an rsR' pattern.
• Vectorcardiography (VCG) indicated rightward QRS vector displacement, clockwise QRS loop inscription, and slowed terminal vectors.
• Cardiac catheterization ruled out intracardiac shunts and pressure abnormalities.
• Basal His bundle electrogram demonstrated H-V prolongation.

Findings:

• Atrial pacing progressively increased QRS duration and induced an R wave in V1 without axis deviation.
• The observed electrocardiographic pattern suggests a left posterior fascicular block.
• This fascicular block may be masking an underlying right ventricular conduction defect in standard right precordial leads.

Implications:

• This case highlights a complex electrocardiographic presentation requiring careful interpretation.
• Understanding fascicular blocks and associated conduction defects is crucial for accurate cardiac diagnosis.
• Further investigation may be warranted to fully elucidate the underlying cardiac condition.