Left bundle branch block (LBBB) is an abnormality of the heart’s electrical conduction system in which the left bundle branch (the pathway carrying electrical impulses to the left ventricle) is blocked or non-functional. The left ventricle is forced to depolarise via a slower, indirect route, producing a distinctively wide QRS complex on the ECG.
Unlike right bundle branch block, LBBB is rarely seen in a structurally normal heart and usually indicates significant underlying cardiac disease. Common causes include coronary artery disease and prior heart attack, dilated cardiomyopathy, hypertensive heart disease, and aortic valve disease. New-onset LBBB in a patient with chest pain or breathlessness is treated as a potential heart attack until proven otherwise, because the wide QRS pattern can mask typical ST-elevation changes on the ECG.
LBBB disrupts the synchrony of ventricular contraction: the right ventricle activates before the left, reducing pumping efficiency. In patients with heart failure and a wide LBBB, Cardiac Resynchronisation Therapy (CRT) can significantly improve symptoms and outcomes by pacing both ventricles in coordination.
LBBB does not directly cause cardiac arrest, but the underlying structural heart disease that produces it carries increased arrhythmia risk. Its presence should always prompt thorough investigation of the underlying cause.
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