Bundle Branch Block
Conduction delay or block in one of the bundle branches causing widened QRS (≥120 ms) with characteristic morphology. LBBB may indicate structural heart disease; RBBB can be a normal variant.
Key Facts
QRS duration ≥120 ms with characteristic morphology distinguishes complete BBB from incomplete (<120 ms) RBBB: RSR' pattern in V1-V2 ('M' shaped), wide slurred S wave in V1, I, V6; may be normal variant in young/healthy individuals LBBB: broad notched R wave in I, aVL, V5-V6 ('M' shaped), deep S wave in V1-V3; usually indicates structural heart disease New LBBB with chest pain should be treated as STEMI equivalent and activate PPCI pathway LBBB makes interpretation of ST segments unreliable — use Sgarbossa criteria for MI diagnosis with LBBB Bifascicular block (RBBB + left anterior or posterior hemiblock): risk of progression to CHB if acute LBBB is associated with increased cardiovascular mortality; RBBB in isolation is generally benign
Overview
Key Facts
Bundle branch block (BBB) occurs when conduction is delayed or blocked in one of the main bundle branches of the His-Purkinje system. This produces a wide QRS complex (≥120 ms) with a characteristic morphology depending on whether the right or left bundle is affected.
Epidemiology
- RBBB: prevalence ~0.8-2% in the general population; increases with age
- LBBB: prevalence ~0.5-1%; strongly associated with age and structural heart disease
- RBBB is more common than LBBB in the general population
- LBBB prevalence: ~5-10% in patients with heart failure
Aetiology
RBBB:
- Normal variant (especially in young adults)
- Right heart strain: PE, cor pulmonale, pulmonary hypertension
- Ischaemic heart disease
- Atrial septal defect (ASD)
- Myocarditis, cardiomyopathy
- Post-cardiac surgery, right heart catheterisation
LBBB:
- Hypertensive heart disease
- Ischaemic heart disease
- Dilated cardiomyopathy
- Aortic valve disease (especially aortic stenosis)
- Degenerative conduction disease
- Myocarditis
- Post-TAVI (up to 20-30%)
Pathophysiology
- Block in one bundle branch causes the ventricle on that side to depolarise late via slow cell-to-cell conduction from the contralateral ventricle
- This produces a widened QRS with a characteristic 'double-peak' pattern
- Dyssynchronous ventricular contraction in LBBB can impair cardiac output and contribute to heart failure
- LBBB can be an indication for cardiac resynchronisation therapy (CRT) in heart failure
Clinical Presentation
RBBB
- Usually asymptomatic if isolated
- Wide splitting of S2 (delayed closure of pulmonary valve)
- May be associated with symptoms of underlying condition (e.g., PE, ASD)
LBBB
- May be asymptomatic or associated with symptoms of underlying cardiac disease
- Reversed splitting of S2 (delayed closure of aortic valve)
- New LBBB with acute chest pain = STEMI equivalent
- Chronic LBBB may contribute to heart failure via dyssynchrony
Red Flags
- New LBBB with chest pain: treat as STEMI — activate PPCI pathway
- New BBB following syncope: risk of intermittent higher-degree block
- Alternating RBBB and LBBB: indicates bilateral bundle disease, high risk of CHB
- Bifascicular block with syncope: consider pacemaker
Differential Diagnosis
| Diagnosis | Key Features | Investigation |
|---|---|---|
| Ventricular pre-excitation (WPW) | Short PR, delta wave, wide QRS | ECG |
| Ventricular paced rhythm | Pacing spikes, LBBB morphology (RV pacing) | Device check |
| Hyperkalaemia | Broad QRS, peaked T waves | U&Es |
| Ventricular tachycardia | Wide complex tachycardia, AV dissociation | ECG, clinical context |
| Incomplete BBB | QRS 100-119 ms with BBB morphology | ECG |
| Na-channel blocker toxicity | Wide QRS, history of TCA/flecainide overdose | Drug history, levels |
Diagnosis / Investigation
Bedside
- 12-lead ECG: QRS morphology and width, axis, Sgarbossa criteria if LBBB
- Comparison with previous ECGs: to determine if BBB is new or old
Bloods
- Troponin: if new LBBB with symptoms
- BNP/NT-proBNP: heart failure assessment
- U&Es: electrolytes
- D-dimer/CTPA: if PE suspected with RBBB
Imaging
- Echocardiography: assess LV function, structural heart disease, dyssynchrony
- Cardiac MRI: if cardiomyopathy or myocarditis suspected
Special Tests
- Holter monitor: if symptoms suggest intermittent higher-degree block
- Exercise testing: new BBB during exercise may indicate coronary disease
- Electrophysiology study: if bifascicular block with syncope to assess HV interval
Management
Non-pharmacological
- Isolated RBBB in young healthy patient: no treatment needed, reassurance
- Investigate and treat underlying cause
Pharmacological
- No specific drug treatment for BBB itself
- Treat underlying condition: heart failure, ischaemia, hypertension
- New LBBB with chest pain: full ACS management pathway
Surgical/Interventional
- CRT (cardiac resynchronisation therapy): indicated for LBBB with QRS ≥150 ms and LVEF ≤35% with heart failure symptoms (NICE TA314)
- Biventricular pacing resynchronises contraction
- Landmark trials: COMPANION, CARE-HF, MADIT-CRT
- Pacemaker: for bifascicular/trifascicular block with syncope or evidence of intermittent CHB
- ICD ± CRT: if heart failure with reduced EF criteria met
Referral Criteria
- New LBBB with symptoms: urgent cardiology referral
- LBBB with heart failure: assess for CRT
- Bifascicular block with syncope: urgent pacemaker assessment
- Alternating BBB: urgent pacing consideration
Prognosis
- Isolated RBBB: benign prognosis, no excess mortality in absence of structural heart disease
- LBBB: associated with increased cardiovascular mortality (~2-3 fold), largely driven by underlying heart disease
- New LBBB: ~50% associated with significant cardiac pathology
- Bifascicular block: annual progression to CHB ~1-4%
- CRT in LBBB with HF: reduces mortality by ~35% (CARE-HF trial)
- Post-TAVI LBBB: may need permanent pacemaker in ~10-20%
Other Relevant Information
ECG Patterns Summary
| Feature | RBBB | LBBB |
|---|---|---|
| V1 morphology | RSR' (M-shaped) | Deep QS or rS |
| V6 morphology | Deep slurred S | Broad notched R (M-shaped) |
| QRS duration | ≥120 ms | ≥120 ms |
| Axis | Often RAD | Often LAD |
| ST/T changes | Discordant in V1-V3 | Discordant throughout |
Sgarbossa Criteria (MI in LBBB)
| Criterion | Points |
|---|---|
| Concordant ST elevation ≥1 mm | 5 |
| Concordant ST depression ≥1 mm in V1-V3 | 3 |
| Discordant ST elevation ≥5 mm | 2 |
| Score ≥3 suggests MI |