Lbbb ecg
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LBBB left bundle branch block ECG
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Left Bundle Branch Block (LBBB) on ECG
Pathophysiology
In LBBB, the left bundle branch fails to conduct, so the right ventricle depolarizes first via the normal His-Purkinje pathway, then ventricular activation spreads slowly cell-to-cell leftward through the myocardium rather than the fast conduction system. This produces a wide, abnormal QRS and abnormal repolarization (ST-T discordance).
ECG Diagnostic Criteria
| Feature | Finding |
|---|---|
| QRS duration | ≥ 120 ms (≥ 0.12 sec) — mandatory |
| V1 morphology | rS or QS pattern — deep, broad S-waves (or entirely negative) |
| Lateral leads (I, aVL, V5–V6) | Broad, monophasic, notched R-wave — no septal Q wave |
| ST segments & T waves | Discordant in V1–V6 (opposite direction to QRS vector) |
| Axis | Variable |
Key mnemonic: "WiLLiaM" — in LBBB, W-shape in V1, M-shape in V5/V6.
The absence of a septal Q wave in leads I, aVL, and V6 is a defining criterion that distinguishes true LBBB from nonspecific intraventricular conduction delay (IVCD). — Fuster and Hurst's The Heart, 15th Ed.
ECG Examples
Classic LBBB — 12-lead ECG from a cardiac stress test:
Features: Sinus tachycardia, QRS >120 ms, deep S-waves V1–V3, broad monophasic R-waves in leads I/aVL/V5/V6, discordant ST-T changes in lateral leads.
LBBB from Goldman-Cecil Medicine textbook:
Etiology & Clinical Significance
Common causes:
- Ischemia or myocardial infarction
- Left ventricular hypertrophy / LV dilation
- Fibrosis of the conduction system (e.g., Lenegre's/Lev's disease)
- Severe cardiac trauma
- Cardiomyopathy
Prognostic significance:
- In young, asymptomatic patients — weak predictive value (Bayes' theorem)
- In older patients — ominous marker for increased risk of death, stroke, and CHF
- Incomplete LBBB and hemiblocks are usually NOT associated with cardiac disease — Pfenninger & Fowler's Procedures for Primary Care
LBBB and Acute MI — The Critical Overlap
LBBB masks ST-segment analysis and complicates diagnosis of acute MI. Use the Sgarbossa Criteria (GUSTO trial):
| Criterion | Threshold | Score | Specificity |
|---|---|---|---|
| ST elevation concordant with QRS | ≥ 1 mm | 5 | Highest |
| ST depression in V1–V3 | ≥ 1 mm | 3 | High |
| ST elevation discordant with QRS | ≥ 5 mm | 2 | Lower alone |
A score ≥ 3 achieves ~90% specificity for AMI. — Rosen's Emergency Medicine
Modified Sgarbossa criteria (more sensitive): replaces the third criterion with an ST/S ratio > 0.25 (excessive discordance). This improves sensitivity without meaningful loss of specificity.
Clinical rule: New or presumed new LBBB in a patient with ischemic chest pain = treat as STEMI — immediate reperfusion (PCI preferred, or thrombolysis if PCI unavailable). However, new LBBB alone without Sgarbossa-positive features has <10% AMI rate. — Tintinalli's Emergency Medicine; Goldman-Cecil Medicine
Comparison: LBBB vs RBBB
| Feature | LBBB | RBBB |
|---|---|---|
| QRS duration | ≥ 120 ms | ≥ 120 ms |
| V1 morphology | rS or QS (deep negative) | rsr'/rsR' ("rabbit ears") |
| Lateral leads | Broad R, no Q wave | Wide S wave |
| ST/T discordance | V1–V6 | V1–V2 only |
| Septal Q in I/V6 | Absent | Present |
Management Considerations
- New LBBB + chest pain → STEMI workup immediately (PCI/thrombolysis)
- New LBBB + CHF or cardiomegaly → Echocardiogram
- Asymptomatic LBBB → Standard resting/exercise ECG unreliable; use stress echocardiogram or nuclear perfusion imaging
- LBBB + HF with reduced EF (EF ≤35%) → Consider cardiac resynchronization therapy (CRT)
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