What Happens in the Heart During Right Bundle Branch Block (RBBB)
In RBBB, the electrical conduction through the right bundle branch is interrupted or delayed, causing the right ventricle to activate later than normal—after the left ventricle has already begun contracting—resulting in a characteristic wide QRS complex (>120 ms) on ECG. 1, 2
The Electrical Sequence in RBBB
Normal conduction pathway disruption:
- The electrical impulse travels normally from the sinus node → atrial muscle → AV node → His bundle, but then encounters a block in the right bundle branch 3
- Because the right bundle branch cannot conduct the impulse, the right ventricle must be activated indirectly through slower cell-to-cell conduction from the left ventricle 1, 2
- This delayed right ventricular activation creates the prolonged QRS duration (≥120 ms in adults) that defines complete RBBB 3
The mechanical consequence:
- The left ventricle contracts first (activated normally through the intact left bundle branch), followed by delayed right ventricular contraction 2
- This asynchronous ventricular activation produces the characteristic ECG pattern: an rSR' ("rabbit ears") pattern in leads V1-V2 and wide S waves in leads I and V6 3
What Causes This Conduction Block
The pathophysiology can be:
- Structural damage to the right bundle branch from ischemic heart disease (particularly anterior MI), hypertensive heart disease, or cardiomyopathies 1
- Degenerative processes affecting the specialized conducting tissue, which may be age-related 1
- Infiltrative diseases such as sarcoidosis, cardiac tumors, or amyloidosis 1
- Inflammatory/infectious causes including myocarditis or Chagas' disease 1
- Congenital conditions such as Ebstein's anomaly or post-surgical changes (especially after tetralogy of Fallot repair) 3
- Idiopathic/benign interruption in otherwise healthy individuals, though this is less common than previously thought 4, 3
Critical Clinical Implications
RBBB is NOT always benign, despite older teaching:
- Recent evidence shows RBBB in patients without known cardiovascular disease still carries increased risk of all-cause mortality (HR 1.5) and cardiovascular-related mortality (HR 1.7) 5
- The presence of RBBB may be an early marker of subclinical cardiovascular disease rather than a truly benign finding 5
Warning signs requiring immediate attention:
- Bifascicular block (RBBB plus left anterior or posterior hemiblock) indicates extensive conduction system disease and carries increased risk of progression to complete AV block 4, 1, 2
- Alternating bundle branch block (RBBB on one ECG, LBBB on another) indicates severe bilateral conduction system disease and rapid progression to complete heart block 4, 2
- Acute RBBB with QR pattern in V1 has high positive predictive value for massive pulmonary embolism causing cardiac arrest 6
Mandatory workup for new RBBB:
- Transthoracic echocardiography to evaluate for structural heart disease 1, 3
- Exercise testing and 24-hour ECG monitoring 4, 3
- Assessment for symptoms including syncope, presyncope, dizziness, fatigue, or exercise intolerance 1
- Evaluation of functional aerobic capacity, as RBBB patients show decreased exercise tolerance even without known CVD 5
Common Pitfalls to Avoid
Do not dismiss RBBB as benign without investigation:
- While isolated RBBB rarely causes symptoms directly, it frequently indicates underlying structural heart disease 1
- Complete RBBB is uncommon in healthy individuals (<2% in athletes, ~1% in general population) and warrants thorough evaluation 4, 3
ECG interpretation challenges:
- ST-segment depression in anterior precordial leads (V1-V3) is common at baseline with RBBB and increases with exercise even without coronary obstruction 4
- Apply usual ischemia diagnostic criteria only to inferolateral leads (not V1-V3) when RBBB is present 4
Progressive disease monitoring: