Left bundle branch block is a cardiac conduction abnormality characterized by delayed activation of the left ventricle due to blocked or slowed impulses in the left bundle. It alters the normal sequence of ventricular depolarization and can obscure or mimic other cardiac pathology.
EKG
- QRS duration ≥ 120 ms
- Broad, notched or slurred R waves in leads I, aVL, V5, V6
- Absence of Q waves in leads I, V5, V6
- Dominant S waves in V1–V3
- Discordant ST-T changes (ST depression and T wave inversion in lateral leads)
Causes
- Structural heart disease (cardiomyopathy, MI)
- Hypertension
- Aortic valve disease
- Age-related conduction system fibrosis
- Idiopathic
Clinical Implications
- New LBBB with chest pain may indicate acute myocardial infarction and warrants urgent evaluation
- May mask STEMI criteria due to altered repolarization patterns
- Associated with increased mortality in patients with underlying heart disease
- Often suggests underlying LV dysfunction
Treatment
- Treat underlying cause (control HTN, manage CAD).
- Cardiac resynchronization therapy (CRT) indicated if:
- QRS ≥ 150 ms with LBBB morphology
- LVEF ≤ 35%
- NYHA class II–IV symptoms despite optimal medical therapy
Sgarbossa criteria
- Sgarbossa Criteria may help identify MI in presence of LBBB, if score > 3, suggests STEMI
- Do not rely on traditional ST-elevation criteria in LBBB, use clinical context and imaging