What is the difference between complete and incomplete Right Bundle Branch Block (RBBB)?

Differences Between Complete and Incomplete Right Bundle Branch Block

The primary difference between complete and incomplete RBBB is the QRS duration, with complete RBBB having a QRS duration ≥120 ms in adults, while incomplete RBBB has a QRS duration between 110-119 ms, with both sharing similar morphological features. 1

Diagnostic Criteria

Complete RBBB

1. QRS duration:

   • ≥120 ms in adults

   • 100 ms in children 4-16 years

   • 90 ms in children <4 years 1

2. Morphological criteria:

   • rsr', rsR', or rSR' pattern in leads V1 or V2 (R' or r' deflection usually wider than initial R wave)
   • Wide and often notched R wave pattern may be seen in lead V1/V2
   • S wave of greater duration than R wave or >40 ms in leads I and V6
   • Normal R peak time in leads V5 and V6 but >50 ms in lead V1 1

Incomplete RBBB

1. QRS duration:

   • Between 110-119 ms in adults
   • Between 90-100 ms in children 4-16 years
   • Between 86-90 ms in children <4 years 1

2. Morphological criteria:

   • Same morphological criteria as complete RBBB
   • In children, incomplete RBBB may be diagnosed when terminal rightward deflection is <40 ms but ≥20 ms 1

Clinical Significance

Complete RBBB

• Detected in approximately 1% of the general population
• Prevalence of 0.5% to 2.5% in young adult athletes 1
• May represent a spectrum of structural and physiological cardiac remodeling characterized by RV dilation with resultant QRS prolongation 1
• Studies in collegiate athletes with complete RBBB showed larger right ventricular dimensions and lower right ventricular ejection fraction but preserved fractional area change compared to those with normal QRS complexes 1

Incomplete RBBB

• More common than complete RBBB
• Often a benign finding, particularly when:
  • Lead V1 is recorded higher than or to the right of normal position
  • r is less than 20 ms 1
• In children, an rsr' pattern in V1 and V2 with normal QRS duration is considered a normal variant 1
• May represent early development of right bundle branch block, with early signs including:
  • Diminution of S wave amplitude in lead V2
  • Slurring or notching of the upstroke of S wave in V2
  • Development of an r' deflection that becomes progressively taller 2

Differential Diagnosis

It's important to distinguish incomplete RBBB from:

1. Normal variant patterns:

   • CSV (crista supraventricularis) pattern: RSR' with QRS width <100 ms 3
   • Electrode placement variations (higher placement of V1/V2) 3
   • Athletic heart adaptations 3

2. Pathological conditions:

   • Brugada type 2 and 3 patterns (can be distinguished using angle measurements between the upslope of S-wave and downslope of r'-wave) 4
   • Right ventricular enlargement
   • Arrhythmogenic right ventricular cardiomyopathy
   • Ventricular preexcitation (WPW syndrome)
   • Hyperkalemia 3
   • Atrial septal defect (particularly important to evaluate splitting of S2) 3

Clinical Approach

When evaluating RBBB patterns:

• Assess for SV1/SV2 ratio >1.0, which may indicate true incomplete RBBB rather than a normal variant (associated with older age, S wave slurring, and QRS duration ≥0.10 s) 5
• Consider clinical context - incomplete RBBB in asymptomatic young individuals or athletes is often benign
• Evaluate for underlying structural heart disease if:
  • Abnormalities found on clinical exam
  • Family history of cardiac disease
  • Presence of symptoms
  • Evidence of left ventricular hypertrophy 3

Key Points to Remember

• Complete and incomplete RBBB share similar morphological features but differ primarily in QRS duration
• Incomplete RBBB is often a benign finding, especially in young individuals and athletes
• Complete RBBB may be associated with right ventricular remodeling but not necessarily pathological structural cardiac disease 1
• Clinical evaluation should focus on identifying any underlying cardiac pathology, particularly when RBBB is accompanied by symptoms or abnormal physical findings