What is an incomplete right bundle branch block (IRBBB)?

Incomplete Right Bundle Branch Block (IRBBB)

Incomplete right bundle branch block (IRBBB) is defined as an electrocardiographic pattern with the same QRS morphology criteria as complete RBBB but with a QRS duration between 110 and 119 ms in adults. 1

Diagnostic Criteria

IRBBB is characterized by:

1. QRS duration between 110 and 119 ms in adults (90-100 ms in children 4-16 years, 86-90 ms in children <4 years)
2. rsr', rsR', or rSR' pattern in leads V1 or V2 (the R' or r' deflection is usually wider than the initial R wave)
3. S wave of greater duration than R wave or >40 ms in leads I and V6
4. Normal R peak time in leads V5 and V6 but >50 ms in lead V1 1

Clinical Significance

IRBBB is a common ECG finding that can represent:

• Normal variant: Particularly common in:

  • Young adults
  • Athletes (prevalence of 13.3% in young athletes) 2
  • Individuals with certain chest wall configurations

• Pathological conditions:

  • Early manifestation of conduction system disease
  • Associated with atrial septal defects
  • Can be seen in right ventricular enlargement
  • May be present in arrhythmogenic right ventricular cardiomyopathy
  • Occasionally seen in Brugada syndrome 3

Prognostic Implications

• IRBBB has been associated with a 2.24-fold increased risk of cardiovascular mortality in some population studies 4
• However, in most cases, especially in young healthy individuals and athletes, IRBBB is considered benign and does not require further evaluation 3

Differential Diagnosis

It's important to differentiate IRBBB from:

1. Crista supraventricularis pattern (CSV): Defined as QRS ≤100 ms with an RSR' pattern in V1. This is a normal variant related to the activation of the crista supraventricularis, a right ventricular structure 3, 2

2. Electrode placement artifact: Higher placement of V1 and V2 electrodes can produce an IRBBB-like pattern 3

3. Type 2 Brugada pattern: Can resemble IRBBB but has characteristic ST-segment abnormalities 3

4. Wolf-Parkinson-White syndrome: Has a short PR interval and delta wave 3

Management Approach

• Asymptomatic individuals with isolated IRBBB:

  • Generally requires no specific treatment or additional testing
  • No indication for permanent pacing 5

• IRBBB with concerning features:

  • If abnormalities are found on clinical exam or if there are symptoms (syncope, palpitations), further cardiac evaluation is warranted
  • Pay particular attention to splitting of the second heart sound, as IRBBB is common in atrial septal defects 3

• Monitoring:

  • Only 1-2% of patients with asymptomatic bundle branch block progress to AV block per year 5
  • Regular follow-up may be considered in patients with additional conduction abnormalities

In summary, IRBBB is often a benign finding, especially in young healthy individuals and athletes, but can occasionally be associated with underlying cardiac pathology that may require further evaluation.