ECG – Diagnostic criteria

Bifascicular block can present as one of the following two ECG patterns⁷:

1. RBBB + LAFB

This is the most common of the two patterns due to the fact that the anterior fascicle is supplied by a single coronary artery, the LAD. Other possible factors are its relationship to the LV outflow tract, resulting in mechanotrauma to the fascicle.

ECG criteria: 

• RBBB – widened QRS with prominent R wave in V1, RSR pattern in V1-V3 and slurred S wave in V5-V6 
• Left axis deviation (LAD) 
• R waves with deep S waves in inferior leads 
• Small Q waves with tall R waves in lead I and aVL

2. RBBB + LPFB

The LPF is shorter and broader than the LAF and has a dual supply of blood from the right and left circumflex arteries. As such, block of the LPF is rare.

ECG criteria:

• RBBB 
• Right axis deviation (RAD) 
• R waves with deep S waves in I and aVL 
• Small Q waves with tall R waves in inferior leads

Learning bite

As a rule, RBBB has a normal axis on ECG. Whenever RBBB occurs with axis deviation, LAFB or LPFB should be looked for to diagnose bifascicular block.