Heart Sounds in Right Bundle Branch Block
In RBBB, the characteristic heart sound finding is wide splitting of the second heart sound (S2) that persists during expiration, caused by delayed closure of the pulmonic valve due to delayed right ventricular depolarization and contraction. 1
Mechanism of S2 Splitting in RBBB
The delayed electrical activation of the right ventricle in RBBB leads to:
- Delayed right ventricular contraction and prolonged ejection time, which causes the pulmonic valve (P2) to close later than the aortic valve (A2) 1
- Wide splitting of S2 that remains audible during both inspiration and expiration (though it may widen further with inspiration) 2
- The split is best heard at the left upper sternal border (pulmonic area), where P2 is most prominent 2
First Heart Sound (S1) Characteristics
The first heart sound in RBBB remains essentially normal in timing and quality:
- All three components of S1 maintain normal timing and intervals in RBBB, unlike in left bundle branch block where they are delayed 1
- No additional right-sided components precede S1, confirming that the audible first heart sound originates from left heart structures (mitral valve closure and aortic valve opening) 1
- Apparent wide splitting of S1 may occasionally be recorded due to the first and third components being captured, but this is a recording artifact rather than a true pathological finding 1
Clinical Examination Pearls
When examining a patient with suspected RBBB:
- Listen carefully at the left upper sternal border for the characteristic wide, persistent S2 splitting 2
- Fixed splitting of S2 (no respiratory variation) should raise concern for atrial septal defect rather than isolated RBBB, as this is a common associated finding with ostium secundum ASD 2
- The presence of wide S2 splitting helps distinguish complete RBBB from incomplete RBBB, where splitting may be less pronounced 2
Important Clinical Caveat
Examiners must be particularly alert to differentiate the wide splitting of RBBB from the fixed splitting seen in atrial septal defect, as RBBB is a common ECG finding in ostium secundum ASD and the two conditions frequently coexist 2. In isolated RBBB, the S2 splitting should still show some respiratory variation (widening with inspiration), whereas in ASD the splitting remains fixed throughout the respiratory cycle.