What is Incomplete Right Bundle Branch Block (Incomplete RBBB)?
Incomplete RBBB is an ECG pattern characterized by a QRS duration between 110-120 ms in adults (90-100 ms in children 4-16 years; 86-90 ms in children <8 years) with the same morphologic features as complete RBBB—specifically an rsr', rsR', or rSR' pattern in leads V1 or V2. 1
Diagnostic Criteria
According to the AHA/ACCF/HRS standardization guidelines, incomplete RBBB requires:
QRS Duration Thresholds:
- Adults: 110-120 ms (below 120 ms but above normal)
- Children 4-16 years: 90-100 ms
- Children <8 years: 86-90 ms
Morphologic Features (Same as Complete RBBB):
- rsr', rsR', or rSR' pattern in leads V1 or V2, where the terminal R or r' deflection is usually wider than the initial R wave
- S wave duration > R wave duration or >40 ms in leads I and V6 (adults)
- Normal R peak time in leads V5 and V6, but >50 ms in lead V1
Key Distinction from Complete RBBB
The only difference between incomplete and complete RBBB is the QRS duration—incomplete RBBB has a narrower QRS complex that doesn't quite reach the threshold for "complete" block (120 ms in adults). The electrical activation pattern through the right bundle is delayed but not as severely as in complete block.
Clinical Context and Pitfalls
Often Benign But Not Always:
- In healthy individuals and athletes: Incomplete RBBB is frequently a normal variant, particularly when the rsr' pattern in V1-V2 occurs with normal QRS duration 1, 2
- In children: An rsr' pattern in V1-V2 with normal QRS duration is considered a normal variant 1
Important Caveats:
- Lead placement matters: The pattern may appear if V1 is recorded higher than or to the right of normal position, especially when the terminal r' is <20 ms 1
- Avoid outdated terminology: Terms like "rsr'" or "normal rsr'" are not recommended as they can be variously interpreted 1
When to Investigate Further:
Recent evidence suggests incomplete RBBB should not be routinely dismissed as harmless 2, 3. Consider further evaluation when:
- Associated with symptoms (chest pain, dyspnea, syncope)
- Older patients with cardiovascular risk factors
- Signs of right ventricular strain: May reflect pulmonary hypertension, structural heart disease, or right ventricular overload 2, 3
- Atrial septal defect: Always auscultate for fixed splitting of S2, as incomplete RBBB can be an early sign of ASD 3, 4
- Family history of cardiomyopathy or sudden death
Differential Diagnosis:
Distinguish incomplete RBBB from:
- Type 2 Brugada pattern (critical—requires expert evaluation)
- Right ventricular enlargement
- Arrhythmogenic right ventricular cardiomyopathy
- Wolff-Parkinson-White syndrome
- Hyperkalemia
Prognostic Considerations
While incomplete RBBB is generally more benign than complete RBBB, it may:
- Reflect underlying right ventricular abnormalities in selected populations 2
- Be associated with atrial fibrillation predisposition 2
- Progress to complete RBBB over time, particularly if accompanied by diminishing S wave depth in V1-V2 or S wave slurring 5, 6
Bottom line: In asymptomatic young patients or athletes with isolated incomplete RBBB and normal physical exam, no further workup is typically needed. However, in older adults or those with cardiopulmonary symptoms, incomplete RBBB warrants clinical correlation and may require echocardiography to exclude structural heart disease.