Overview

• Normally the septum is activated from left to right, producing small Q waves in the lateral leads.
• In LBBB, the normal direction of septal depolarization is reversed (becomes right to left), as the impulse spreads first to the RV via the right bundle branch and then to the left ventricle via the septum.
• This sequence of activation prolongs the QRS duration to ≥120 ms and eliminates the normal septal Q waves in the lateral leads.
• The overall direction of depolarization (from right to left) produces tall R waves in the lateral leads (I, V5-V6) and deep S waves in the right precordial leads (V1-V3), and usually leads to left axis deviation.
• As the ventricles are activated sequentially (right, then left) rather than simultaneously, this produces a broad or notched (‘M’-shaped) R wave in the lateral leads.

ECG Features

• Mean QRS duration ≥120 ms in adults, >100 ms in children 4-16 years of age, and >90 ms in children <4 years of age
• Late QRS complex forces should be negative (i.e. terminal S wave) in V1
• Broad notched or slurred R wave in I, aVL, V5, and V6, with an occasional RS pattern in V5-V6 attributed to displaced transition of QRS complex
• Absence of q waves in I, V5, and V6, but narrow q waves may be present in aVL in the absence of myocardial pathology
• Delayed onset of intrinsicoid deflection (>60 ms from beginning of QRS complex to peak of R wave) in V5-V6 but normal in V1-V3 when small initial r waves can be discerned in these leads
• ✓ Note: In the setting of a LBBB, ST and T waves are usually opposite in direction (discordant) to the QRS complex. Positive T waves in leads with upright QRS complexes may be normal (positive concordance). Depressed ST segments and/or negative T waves in leads with negative QRS complexes (negative concordance) are typically abnormal and may even represent myocardial injury. The appearance of a LBBB may change the mean QRS axis in the frontal plane rightward, leftward, or superiorly; this may occur in a rate-dependent manner.

Causes

• Aortic stenosis
• Ischemic heart disease
• Hypertension
• Dilated cardiomyopathy
• Anterior myocardial infarction
• Primary degenerative disease (fibrosis) of the conducting system (Lenegre disease)
• Hyperkalemia
• Digoxin toxicity