What is the typical QRS width for an incomplete Right Bundle Branch Block (RBBB)?

Incomplete RBBB QRS Width

In adults, incomplete right bundle branch block (RBBB) is defined by a QRS duration between 110 and 120 milliseconds, with the same morphologic features as complete RBBB but without meeting the ≥120 ms threshold. 1

Age-Specific QRS Duration Criteria for Incomplete RBBB

The diagnostic criteria vary significantly by age group, as established by the AHA/ACCF/HRS guidelines:

Adults

• QRS duration: 110-120 ms 1
• Must demonstrate the same morphologic criteria as complete RBBB (rsr', rsR', or rSR' pattern in V1/V2, prolonged S wave in leads I and V6) 1

Children Ages 4-16 Years

• QRS duration: 90-100 ms 1
• Alternative criterion: terminal rightward deflection between 20-40 ms (even with shorter QRS duration) 1

Children Under 8 Years

• QRS duration: 86-90 ms 1
• Terminal rightward deflection ≥20 ms but <40 ms can also establish the diagnosis 1

Critical Morphologic Requirements

Beyond QRS duration, incomplete RBBB requires the same morphologic features as complete RBBB 1:

• rsr', rsR', or rSR' pattern in leads V1 or V2 (the terminal R' or r' is typically wider than the initial R wave) 1
• S wave duration > R wave duration or >40 ms in leads I and V6 (in adults) 1
• Normal R peak time in V5/V6 but >50 ms in V1 1

Important Clinical Caveats

Normal Variants vs. Pathology

• An rsr' pattern in V1/V2 with normal QRS duration (<110 ms in adults) is a normal variant, particularly in children and young adults 1, 2
• The crista supraventricularis (CSV) pattern—RSR' with QRS <100 ms—represents late activation of the right ventricular crest and is typically benign 2
• Incomplete RBBB can appear artifactually with high placement of V1/V2 electrodes or in pectus excavatum 1, 2

When to Suspect Pathology

Research suggests that incomplete RBBB may represent true conduction disease (rather than a normal variant) when accompanied by 3:

• Diminished S wave depth in V1/V2 (100% of cases progressing to complete RBBB) 3
• Inverted S wave ratio (SV1 > SV2) (93% of progressive cases) 3
• S wave slurring (27% of cases) 3
• QRS duration ≥100 ms (73% of progressive cases) 3

Exclusions and Mimics

• These criteria do not apply to congenital heart disease with left-axis deviation in infancy 1
• Must differentiate from Brugada type 2 pattern, right ventricular enlargement, arrhythmogenic RV cardiomyopathy, and WPW syndrome 2
• The terms "rsr'" and "normal rsr'" are not recommended due to variable interpretation 1

Clinical Significance

Incomplete RBBB is generally benign and does not require further evaluation unless accompanied by 2:

• Abnormal physical examination findings (particularly fixed splitting of S2, suggesting atrial septal defect) 2
• Family history of sudden cardiac death 2
• Symptoms suggesting cardiac disease 2
• Left ventricular hypertrophy on ECG 2

The pattern may represent a spectrum from normal variant to early conduction disease, with the morphologic features beyond QRS duration helping distinguish these entities 4, 3.