ECG Findings of Incomplete Right Bundle Branch Block
Incomplete RBBB is diagnosed by a QRS duration of 110-120 ms in adults (90-100 ms in children 4-16 years, 86-90 ms in children <8 years) with the same morphologic criteria as complete RBBB: an rsr', rsR', or rSR' pattern in leads V1 or V2, prolonged S waves in leads I and V6, and delayed R peak time in V1. 1
Diagnostic Criteria
The AHA/ACCF/HRS guidelines provide age-specific QRS duration thresholds for incomplete RBBB 1:
QRS Duration Requirements:
- Adults: 110-120 ms
- Children 4-16 years: 90-100 ms
- Children <8 years: 86-90 ms
Morphologic Features (Same as Complete RBBB):
In leads V1 or V2:
- rsr', rsR', or rSR' pattern (the terminal R' or r' deflection is typically wider than the initial R wave)
- Alternatively, a wide notched R wave may be present in a minority of cases
In lateral leads (I and V6):
- S wave duration greater than R wave duration OR S wave >40 ms in adults
R peak timing:
- Normal R peak time in leads V5 and V6
- R peak time >50 ms in lead V1
Special Considerations in Children
In pediatric populations, incomplete RBBB may be diagnosed when the terminal rightward deflection is 20-40 ms (less than the adult threshold but still significant) 1.
Important caveat: An rsr' pattern in V1 and V2 with normal QRS duration is a normal variant in children and should not be labeled as incomplete RBBB 1.
Common Diagnostic Pitfalls
Lead Placement Issues:
The ECG pattern of incomplete RBBB may appear in the absence of heart disease when lead V1 is recorded higher than or to the right of normal position, particularly if the terminal r' wave is <20 ms 1. Always verify proper lead placement before making the diagnosis.
Terminology to Avoid:
The terms "rsr'" and "normal rsr'" are not recommended as descriptive labels because their meaning can be variously interpreted 1. Use the formal diagnostic criteria instead.
Clinical Context Matters
While often benign, incomplete RBBB should not be routinely dismissed as harmless 2. Recent evidence suggests it may reflect:
- Right ventricular strain or pulmonary hypertension
- Predisposition to atrial fibrillation
- Underlying structural heart disease in selected populations
Key differential considerations include distinguishing incomplete RBBB from 3:
- Crista supraventricularis (CSV) pattern (QRS <100 ms, S wave <40 ms in I or V6)
- Type 2 Brugada pattern
- Right ventricular enlargement
- Arrhythmogenic right ventricular cardiomyopathy
- Atrial septal defect (listen for fixed split S2)
The diagnosis requires both the QRS duration criteria AND the morphologic features to be present together. When evaluating athletes or young patients, consider that CSV pattern (prevalence 13.3% in young athletes) may be misdiagnosed as incomplete RBBB 4.