Is Right Fascicular Block (RFB) the same as incomplete Right Bundle Branch Block (RBBB)?

Right Fascicular Block vs. Incomplete Right Bundle Branch Block

No, Right Fascicular Block (RFB) is not the same as incomplete Right Bundle Branch Block (RBBB). According to the ACC/AHA/HRS guidelines, these are distinct electrocardiographic entities with different diagnostic criteria and morphological features 1, 2.

Definitions and Diagnostic Criteria

Incomplete RBBB:

• Has the same QRS morphology criteria as complete RBBB but with a QRS duration between 110-119 ms in adults 1
• Requires an rsr', rsR', or rSR' pattern in leads V1 or V2 1
• S wave of greater duration than R wave or >40 ms in leads I and V6 1
• Normal R peak time in leads V5 and V6 but >50 ms in lead V1 1

Right Fascicular Block:

• The ACC/AHA/HRS guidelines do not specifically define "Right Fascicular Block" as a distinct entity 1
• The guidelines do define left anterior and left posterior fascicular blocks with specific criteria, but do not include a comparable definition for right fascicular block 1

Key Differences

1. Anatomical Basis:

   • Incomplete RBBB represents partial conduction delay in the main right bundle branch
   • Fascicular blocks involve specific divisions (fascicles) of the bundle branch system

2. QRS Duration:

   • Incomplete RBBB: 110-119 ms in adults 1
   • Fascicular blocks: Generally <120 ms (as defined for left fascicular blocks) 1

3. Morphological Features:

   • Incomplete RBBB has specific morphological criteria including an rsr' pattern in V1/V2 1
   • Fascicular blocks have distinct axis deviation patterns (as seen in left fascicular blocks) 1

Clinical Significance

Incomplete RBBB is relatively common, detected in approximately 1% of the general population and 0.5-2.5% of young adult athletes 2. It may represent a spectrum of structural and physiological cardiac remodeling characterized by right ventricular dilation with resultant QRS prolongation 2.

In asymptomatic individuals, incomplete RBBB is often benign 2. However, clinical evaluation should focus on identifying any underlying cardiac pathology, particularly when accompanied by symptoms or abnormal physical findings 2.

Common Pitfalls in Diagnosis

1. Electrode Placement: The ECG pattern of incomplete RBBB may be present without heart disease, particularly when lead V1 is recorded higher than normal position 2

2. Normal Variants: In children, an rsr' pattern in V1 and V2 with normal QRS duration is considered a normal variant 2

3. Misdiagnosis: Incomplete RBBB must be differentiated from other conditions such as:

   • Type-2 Brugada ECG pattern
   • Right ventricular enlargement
   • Arrhythmogenic right ventricular cardiomyopathy
   • Ventricular preexcitation (WPW syndrome)
   • Hyperkalemia 3

4. Overlooking Associated Conditions: Examiners should be alert to the splitting of the second heart sound because RBBB is a common finding in ostium secundum atrial septal defect 3

In conclusion, while both involve conduction abnormalities in the right side of the heart's conduction system, incomplete RBBB and right fascicular block represent different entities with distinct diagnostic criteria and clinical implications.