How to Diagnose Incomplete RBBB on ECG
Incomplete RBBB is diagnosed when the QRS duration is between 110-120 ms in adults (90-100 ms in children 4-16 years, 86-90 ms in children <8 years) AND the same morphologic criteria for complete RBBB are present in the precordial leads. 1
Diagnostic Criteria for Adults
The diagnosis requires two components working together:
1. QRS Duration Threshold
- 110 to 120 ms (just below the 120 ms cutoff for complete RBBB)
- This is the critical distinguishing feature from complete RBBB 1
2. Morphologic Features (Same as Complete RBBB)
You must identify the characteristic right ventricular conduction delay pattern:
- rsr', rsR', or rSR' pattern in leads V1 or V2 (the R' or r' deflection is typically wider than the initial R wave)
- S wave duration > R wave duration OR S wave > 40 ms in leads I and V6
- Normal R peak time in V5 and V6, but > 50 ms in V1 1
Pediatric-Specific Criterion
In children, incomplete RBBB can also be diagnosed when the terminal rightward deflection is 20-40 ms (less than the 40 ms seen in complete RBBB but ≥20 ms) 1
Critical Diagnostic Pitfalls to Avoid
Don't Confuse with Normal Variants
- An rsr' pattern in V1-V2 with NORMAL QRS duration (<110 ms in adults) is a normal variant, not incomplete RBBB 1
- The guidelines explicitly state: "The terms rsr' and normal rsr' are not recommended to describe such patterns, because their meaning can be variously interpreted" 1
- This pattern is particularly common in children and young adults with normal hearts 1
Technical Factors That Mimic Incomplete RBBB
The ECG pattern may appear falsely positive when:
- Lead V1 is placed higher than standard position or too far right (especially if the terminal r' is <20 ms) 1
- Pectus excavatum is present (look for negative P waves as a clue) 2
Pathologic Patterns to Differentiate
Incomplete RBBB must be distinguished from:
- Type 2 Brugada pattern (critical for sudden death risk)
- Right ventricular enlargement
- Arrhythmogenic right ventricular cardiomyopathy
- Wolff-Parkinson-White syndrome
- Hyperkalemia 2
Clinical Context Matters
When Incomplete RBBB is Likely Benign
- Young patients or athletes with isolated finding
- No symptoms, normal physical exam, no family history
- QRS duration closer to 110 ms 2, 3
When Further Evaluation is Warranted
Look for these concerning features on ECG that suggest true conduction disease rather than a normal variant 4:
- Diminished S wave depth in V1-V2 (100% sensitive in progression to complete RBBB)
- Inverted S wave ratio (SV1 > SV2) (93% sensitive)
- Slurring of S wave downstroke or upstroke (27% prevalence)
- QRS duration ≥100 ms (73% prevalence in true conduction disease)
High-Risk Populations Requiring Attention
- Fixed splitting of S2 on auscultation → consider atrial septal defect 2
- Pulmonary hypertension or structural heart disease 3
- Older patients (mean age 60 vs 33 years for normal variants) 4
- Patients with atrial fibrillation (incomplete RBBB prevalence 4.4%, though not independently associated with recurrence after ablation) 5
Practical Diagnostic Algorithm
Measure QRS duration globally (earliest onset to latest offset across all leads)
- <110 ms → Normal variant (not incomplete RBBB)
- 110-120 ms → Proceed to step 2
- ≥120 ms → Complete RBBB
Examine V1-V2 for morphology
- Present: rsr', rsR', or rSR' pattern → Proceed to step 3
- Absent: Consider other diagnoses
Check lateral leads (I, V6)
- S wave duration > R wave OR S > 40 ms → Incomplete RBBB confirmed
- Absent: Reconsider diagnosis
Assess clinical context
- Check for S wave abnormalities (depth, ratio, slurring) suggesting true conduction disease
- Examine for concerning features requiring further workup
- Consider technical factors (lead placement, body habitus)
The key distinction is that incomplete RBBB requires both the characteristic morphology AND the specific QRS duration range—it is not simply any rsr' pattern with a narrow QRS 1.